Chapter 2 - SS7 Network

Chapter 2 SS7 Network

		The SS7 network is separate from the voice network, and is used solely for the purpose of switching data messages pertaining to the business of connecting telephone calls and maintaining the signaling network. Packet switching is the method used for transferring messages through the network.

		The telephone switches used in many exchange offices today perform a dual function. Besides connecting voice circuits to other exchanges and switching voice circuits from one exchange to another (as well as from one subscriber to another) they must also perform signaling functions. Often this is accomplished through adjunct computers, which are connected through digital links to other computers in the network.

		These computers are referred to as signaling points. There are three functions required of signaling points. The originator and receiver of all messages in the network is located at the end office. All messages are switched through the network using transfer points. These transfer points do not originate messages and are seldom the receivers of messages; they are used to through-switch the packets which are received from end offices.

		Another function is to provide access to databases. The front-end system receives packets destined to an addressed database and converts from the SS7 protocol to either an X.25 protocol or some other transport protocol (such as TCP/IP) carrying as its payload the primitives which the database can read directly. This front end must be capable of receiving messages, routing to the appropriate database (based on an address), and maintaining reliable transfer of messages from the SS7 network into the database environment.

 

All nodes in the SS7 network are called signaling points. A signaling point has the ability to perform message discrimination (read the address and determine if the message is for that node), as well as to route SS7 messages to another signaling point. There are three different types of signaling points:

		Service Switching Point (SSP)

		Signal Transfer Point (STP)

		Service Control Point (SCP)

		Signaling points provide access to the SS7 network, provide access to databases used by switches inside and outside of the network, and transfer SS7 messages to other signaling points within the network. Signaling points are deployed in pairs for redundancy and diversity. The secret to the SS7 network and to making sure the network is always operational is to provide alternate paths in the event of failures. These alternate paths provide the reliability needed in a network of this nature, and ensure that SS7 messages can always reach their destinations (see Figure 2.1).

		The facilities which link the signaling points to one another are also deployed in pairs. Data links are used between signaling points to provide the speed necessary for SS7 message delivery (56 or 64 kbps, 10 Mbps, 100 Mbps, or 1.536 Mbps). These data links are typically DS0s from existing DS1/DS3 facilities used for interexchange trunking, for the exception of the 10 Mbps, 100 Mbps, and 1.536 Mbps links. The 10/100 Mbps links are TCP/IP-based connections found in many of the next-generation networks. The 1.536 Mbps links are ATM facilities.

		Figure 2.1 This figure depicts a typical SS7 network, with multiple B links and A links.

 

		The network is deployed as two distinct levels, or planes. There is an international plane, using the ITU-TS standard of the SS7 protocol, and there is the national plane. The national plane uses whatever standard exists within the country in which it is deployed. For example, in the U.S., ANSI is the standard for the national plane. Telcordia standards are an extension of the ANSI protocol and ensure the reliability required to interwork with Bell Operating Companies' (BOCs) networks.

		In other nations, there may be one or several different versions of national protocols for SS7. Yet all countries are capable of communicating with one another through gateways which convert the national version of the SS7 protocol to the international version of the SS7 protocol. This ensures that all nations can interwork with one another, while still addressing the requirements of their own distinct networks. This requires protocol converters, or an integrated solution where the conversion takes place on an STP.

		Using these two planes, communications are possible among all nations in the world, allowing automatic circuit connections from one network to the next. Yet when SS7 was first deployed here in the U.S., it was used for a different purpose. The emphasis then was placed on database access. In fact, early implementation provided nothing more than just that: access to network databases. These databases could be accessed directly by end offices using data links connecting them directly to the database entity or to a signaling point (such as an STP), which would route their database query to the appropriate database (as is in today's network).

		Even today, the SS7 network provides two types of services circuit: circuit-related and non-circuit-related. Circuit-related signaling is used for the setup and teardown of central office trunks, as well as voice connections in packet-switched networks such as TCP/IP. Non-circuit related services are all other services provided by the network, such as database access for translations and subscriber information and network management.

		This book is based mostly on ANSI and Telcordia standards, with some ITU implementation included. International networks may have different objectives and, while similar, do have fundamental differences. An understanding of the requirements of Telcordia and ANSI will provide great insight to anyone working with SS7, regardless of the country and the version being used. The Telcordia standards provide an excellent model of what a data communications network should look like for high reliability.

 

Service Switching Point (SSP)

		The Service Switching Point (SSP) is the local exchange in the telephone network. An SSP can be a combination voice switch and SS7 switch, or an adjunct computer connected to the local exchange's voice switch. The SSP provides the functionality of communicating with the voice switch via the use of primitives and creating the packets, or signal units, needed for transmission in the SS7 network (see Figure 2.2).

		The SSP must convert signaling from the voice switch into SS7 signaling messages, which then can be sent to other exchanges through the SS7 network. The exchange will typically send messages related to its voice circuits to the exchanges with a direct connection to it.

		In the case of database access, the SSP will be sending database queries through the SS7 network to computer systems located centrally to the network (or regionally). This was the first usage of the SS7 network, as the need for 800 number lookup became necessary.

		The traffic mix found in most SS7 networks is still primarily circuit-related messages. With the implementation of applications such as Local Number Portability (LNP), the traffic mix is changing significantly, becoming predominantly non-circuit related messages.

		Figure 2.2 This figure shows the relationship of the SSP to the SS7 network.

 

These messages originate from SSPs and are used to connect voice circuits from one exchange to another exchange. The SSP does not use circuit-related messages exclusively, however.

		Before a switch can route a call, it must first be able to access information regarding the destination of the call. For most calls, the telephone number dialed is sufficient for routing. However, with 800 and 900 numbers, routing is impossible, because the dialed digits do not provide enough information about the destination.

		With the implementation of number portability, almost every call now requires checking a LNP database to determine which network (or carrier) now services the called number. In the past, telephone numbers were assigned to specific central offices to simplify routing. For example, the 919-460-xxxx range of numbers would be assigned to one office switch (or several switches) in a carrier's network. With number portability, that number may be moved to another carrier's switch. The original ''owner" of the number must flag the switch database to show that the number is now "ported." Porting is the movement of a number from one switch to another (regardless of geography) allowing subscribers to move or change primary telephone companies without changing their telephone numbers.

		If the calling switch determines that the called number has been "ported," it will generate a primitive to the SSP requesting the SSP send a query to an LNP database to determine to which exchange the ported number has been reassigned.

		For this reason, the SSP must access a remote database to learn the routing number assigned to the 800 or 900 number, or the new location of a ported number. Once this information has been retrieved, the SSP can then begin circuit connections based on the new routing number information.

		The SSP function is to use the information provided by the calling party (such as dialed digits) and determine how to connect the call. A routing table will identify which trunk circuit or TCP socket to use to connect the call, and which exchange this trunk terminates at. An SS7 message must be sent to this adjacent exchange requesting a circuit connection on the specified trunk or socket.

		The adjacent exchange grants permission to connect this trunk or socket by sending back an acknowledgment to the originating exchange. Using the called party information in the setup message, the adjacent exchange can determine how to connect the call to its final destination. This may require several connections between several adjacent exchanges. The SSP function manages these connections until the final destination is reached.

		Many SSP functions are accomplished by adding a computer adjunct to existing switches. This computer receives signals from the voice switch which are used to trigger the transmission of specific SS7 messages. The called and calling party address must be passed from the voice switch to the SSP for transfer across the network.

		Using adjuncts allows telephone companies to upgrade their SS7 signaling points without replacing expensive switches, providing a modular approach to networking. Upgrades are typically limited to software loads, since these computers require very little hardware. With the advent of packet telephony, this has become less of an issue because new generation switches are server-based and a fraction of the cost of legacy circuit switched equipment. The SSP function is now found in multiple devices throughout the packet switched network.

		There are very few SS7 features required of an SSP. The ability to send messages using the ISDN User Part (ISUP) protocol and the Transaction Capabilities Application Part (TCAP) protocol is the only requirement, other than the network management requirements defined in the Telcordia publications. Specific Telcordia requirements for an SSP can be found in Telcordia publication TR-TSY-000024, Service Switching Points (SSPs) Generic Requirements (this has since been updated, and the new document is known as GR-024-CORE).

		Signal Transfer Point (STP)

		All SS7 packets travel from one SSP to another through the services of a Signal Transfer Point (STP). The STP serves as a router in the SS7 network. Messages are not usually originated by an STP. The STP switches SS7 messages as received from the various SSPs through the network to their appropriate destinations (see Figure 2.3).

		The STP can be an adjunct to a voice switch. Many tandem switches provide the capability of voice switching through the switch and STP functionality through the use of an adjunct computer. Although several manufacturers provide STP equipment, very few provide a standalone STP. A standalone solution offers many benefits.

		By using a standalone STP, companies can centralize many important revenue-generating functions in their network. This includes services provided by SCPs and monitoring of the entire SS7 network. Updates to SS7 are much simpler and more cost-effective when standalone STPs are used because the entire switch must be upgraded when this function is integrated into a tandem switch. Many carriers have changed their network architecture, replacing circuit-switched-based STPs with newer standalone STPs for this very reason.

 

		Figure 2.3 This figure shows the relationship of the STP to the SS7 network.

		There are three levels of STPs:

		National Signal Transfer Point (STP)

		International Signal Transfer Point (STP)

		Gateway Signal Transfer Point (STP)

		The national STP exists within a national network and is capable of transferring messages using the same national standard of protocol. Messages may be passed to another level of STP, the international STP, but the national STP has no capability of converting messages into another version or format. Protocol converters are often used to interconnect the national STP with an international STP, providing the conversion from a national standard (such as ANSI) to an international standard (ITU-TS). At least one manufacturer provides the protocol conversion on the STP itself, eliminating the need for two STPs (one national and one international).

		The international STP functions the same as the national STP, but is used in the international network. The international network provides interconnection of all countries using the SS7 protocol, using the ITU-TS standards. This ensures interconnectivity between worldwide networks, despite the use of different point code structures and network management. All nodes connecting to the international STP must use the ITU-TS protocol standard.

		The gateway STP provides protocol conversion from a national standard to the ITU-TS standard, or some other national standard. A gateway STP is often used as an access to the international network, providing access and conversion of messages to the ITU-TS protocol standard. This eliminates the need for adjunct protocol converters in the network. Gateway STPs must be able to work using both the international standard of protocol and the national standard, depending on the location of the STP.

		Gateway STPs are also finding their way into the cellular network. Many cellular networks use X.25 as a transport protocol between their Mobile Switching Centers and databases. The X.25 networks are private networks and do not provide the support for accessing other cellular providers' networks.

		The X.25 network has other limitations that do not lend themselves well to the applications of the cellular network. All information transfers in X.25 networks require connection-oriented services. Trying to connect to multiple networks and entities in various locations can become somewhat cumbersome. Many messages originated in the cellular network or the SS7 network do not require connection-oriented services.

		For this reason, many cellular providers have changed their networks to include an STP and data links for the use of SS7 protocols within the cellular network. The MSCs then can exchange information about the location of mobile phones and update their databases using the Transaction Capabilities Application Part (TCAP) protocol, which is much better suited to this task.

		The gateway STP serves as the interface into another network. Long distance service providers may have access into the local telephone company's database for subscriber information, or the local service provider may need access into the long distance service provider's database. In any event, this access is accomplished through a gateway STP.

		Gateway STPs use screening features to maintain network security. Screening is the capability to examine all incoming and outgoing packets and allow only those which are authorized. This is determined through a series of gateway screening tables that must be configured by the service provider. Gateway screening also prevents messages from unstable networks that have not been approved by the service provider from entering into the network and causing service conflicts. This function only exists on STPs, and is extremely important in any network to prevent unathorized use of a carriers SS7 network.

		In the international network, gateway STPs may provide an additional function. International SS7 is based on ITU-TS standards, yet every country uses a national version that is not 100 percent compliant with the ITU-TS standard, For example, in the U.S., we are ITU compliant, yet the ITU-TS standards have been modified for usage here. The major difference between the two standards is in addressing and network management functions of the protocol.

		The gateway STPs that connect us to other countries do not deviate from the ITU-TS standards. They must be 100 percent compliant. They can perform a protocol conversion, allowing ITU-TS messages into the network, converting the messages into the national format before transferring them into the network. This is true gateway functionality, yet not all gateway STPs must perform protocol conversion.

		Yet another feature of the Signal Transfer Point (STP) is measurements. There are many types of measurements defined by the Telcordia and ITU-TS standards. Measurements can be divided into two basic functions: traffic measurements and usage measurements.

		Traffic measurements provide peg counts and statistical information regarding the type of messages entering and leaving the network. For maintenance purposes, network events are also recorded (such as link out-of-service duration, local processor outage, etc.). Maintenance measurements record events that may or may not affect service, but need to be monitored by service personnel. Because of the speed of the network and the quickness at which SS7 entities respond to problems, traffic measurements are the best way for maintenance personnel to keep track of what is happening in the network and preventing network failures from happening.

		Usage measurements are always peg counts and record the number of messages by message type that enter and leave the network. These peg counts then are aggregated by a computer and used to create billing records for other carriers. This is especially important in the hub provider business because these companies depend on these measurements to determine how much their customers are using the hub provider SS7 network, and what types of services they are accessing in the network.

		This billing feature can be used to bill all users of the network and helps offset the expense of deploying the SS7 network. The cellular providers and PCS service providers have also been seeking database access from this network. Their access is also through a gateway STP, with usage measurements recording the access.

 

Many carriers are now using SS7 to generate subscriber billing records. By monitoring the SS7 messages sent through the network, computers can determine when a call is being connected to a specific subscriber number, and determine the duration of that call. By using the SS7 network rather than the end office as the source for billing records, billing activities can be centralized within the network, providing better efficiency and cost savings to the telephone companies. This makes a standalone STP even more valuable, allowing for centralization of the billing function.

		Routing to databases is another important STP function. Although a carrier may only have one SCP, many services may be provided by this SCP (see Figure 2.4). For example, the SCP may be supporting calling cards, 800 routing, and calling name display. Each of these services in the SCP will have a unique address. The STP uses a function called Global Title Translation to determine which database is to receive the query generated by SSPs.

		It may seem easier for the SSP to know the specific address of each and every database in the network, but this increases the amount of administration that must be managed. Every time a database is added or changed, the address information would have to be added to every SSP in the network. By using an STP and global title translation, the SSPs need to know only the address of the STP. By using this type of architecture, SCPs can be changed or added, affecting only the STP.

		Figure 2.4 This figure shows the relationship of the SCP to the SS7 network.

 

		The SSP will send a database query to the local STP, with the destination address of the STP. The STP will look at the dialed digits (or global title digits, as they are called) and determine, through its own translation tables, the address of the database. This is referred to as global title translation. The global title translation provides the subsystem number (address) of the database and the point code of the SCP that interfaces to the database.

		This is important to hub providers as well. Imagine if you were a hub provider, supporting many SCPs, and having to notify hundreds of carriers to update the routing information in their SSPs to reflect the new addressing information. By using the global title feature in an STP, the changes to the hub provider network are completely transparent to the carriers.

		The STP is the most versatile of all the SS7 entities, providing a wide array of services to the users of the network. Whether it be gateway services or routing functions, the STP is a major component in the network, and the vehicle to deliver revenue generating services to subscribers.

		Another unique new application for STPs is data mining. A lot of information flows through the SS7 network that, when disseminated, can tell a carrier a lot about its customers. Because SS7 messages contain the telephone number of the calling subscriber, a carrier can determine which services in the network individual subscribers are using, what their calling patterns are, what areas they call the most, and whether the carrier needs to begin increasing the network capacity in anticipation of increased usage.

		The role of the STP in data mining is to provide a centralized interface to computers used to gather this information. The STP sends SS7 messages (or copies of these messages) to these computers, which are then responsible for processing the data. Many companies have implemented data mining in their SS7 networks to track new service offerings and determine what new services their customers need.

		An STP is the most important investment a carrier can make for their network. Carriers who use hub providers rather than investing in their own STPs are missing out on the opportunity to be more responsive to their subscribers and provide unique services not provided by hub providers.

 

For specific information regarding Telcordia requirements for an STP, refer to the Telcordia publication GR-082-CORE, Signal Transfer Point Generic Requirements.

		Service Control Point (SCP)

		The Service Control Point (SCP) serves as an interface to telephone company databases. These databases are used to store information about subscribers' services, routing of special service numbers (such as 800 and 900 numbers), calling card validation and fraud protection, and even Advanced Intelligent Network (AIN) services used when creating a service for a subscriber.

		The SCP is usually a computer used as a front end to the database system. Some new SCP applications are being implemented in STPs, providing an integrated solution. In all cases, the address of the SCP is a point code, while the address of the database is a subsystem number.

		The SCP function does not necessarily store all the data, but is the interface to the mainframe or minicomputer system that is used for the actual database. These computer systems are usually linked to the SCP through X.25 links. In integrated STP/SCP, the database is resident in the SCP.

		The SCP can perform protocol conversion from SS7 to X.25, or it can provide the capability of communicating with the computer database directly, through the use of primitives. A primitive is an interface which provides access from one level of the protocol to another level. In the case of the database, the database is considered an application entity, and the protocol used to access and interface to this application entity is the Transaction Capabilities Application Part (TCAP).

		The type of database depends on the network. Each service provider has different requirements, and their databases will differ. Telcordia has defined some basic models of databases for achieving the needs of RBOC networks. In addition to the RBOC networks, cellular providers also use databases for the storage of subscriber information. The databases most commonly used within either of these networks are listed as follows:

		Call Management Services Database (CMSDB)

		Local Number Portability (LNP)

		Line Information Database (LIDB)

		Calling Name (CNAM)

		Business Services Database (BSDB)

 

Home Location Register (HLR)

		Visitor Location Register (VLR)

		Each database contains information for a specific application. Each database is also given an address, called a subsystem number, for use in routing queries from Service Switching Points (SSPs) through the SS7 network to the actual database entity. The subsystem numbers are defined by the service provider and are fixed. The following databases are not absolutes; in other words, not every network must have these specific databases. These are the databases used within the Bell Operating Company's (BOCs) networks, and they are mentioned here as a model of database types.

		Call Management Services Database (CMSDB)

		The CMSDB provides information relating to call processing, network management, and call sampling (for traffic studies). The call-processing portion is what defines the routing instructions for special service numbers such as 800, 976, or 900 numbers. In addition to routing instructions, this database also provides billing information, such as billing address or third-party billing procedures.

		CMSDB also provides certain network management functions used to prevent congestion in the network. When congestion occurs in the SS7 network, this database can provide important routing instructions for rerouting messages around the congested node.

		Call sampling is used to create reports that indicate the types of telephone calls being made in the telephone network. These reports are then used in traffic studies to determine if additional facilities are needed to handle the voice traffic. The Service Management System (SMS) schedules reports for automatic printing and allows administration personnel to update the database records through a terminal interface.

		Local Number Portability (LNP)

		LNP is a new application mandated by the Communications Act of 1996. The purpose of LNP is to allow subscribers to change telephone companies without having to change their telephone numbers. This of course changes a lot of methods that have been used for years within the telephone network. The office code (NNX) portion of a telephone number can no longer be used to identify the destination exchange. The number may have been reassigned to a different exchange, with a different office code.

		For this reason, when a call is initiated, the originating exchange must first search its routing table to determine if the called number has been flagged as "ported." If the NNX has been flagged as ported, then a query must be sent to determine if the actual called number has been ported, and if so, how the call is to be routed. Even if only one telephone number within an NNX has been ported, the entire NNX is considered ported, and a query must be generated for each and every call with that ported NNX.

		The query is sent to an LNP database, where the called number is looked up. If the called number has been ported, then the database will identify the new terminating exchange by giving its Local Routing Number (LRN). The LRN works the same as the NNX code did, providing a unique identity for each and every exchange in the network. This information is then returned to the originating exchange so the call can be routed. If the telephone number has not been ported, then the call is routed as it normally would be.

		The implementation of LNP is now taking place in international networks. Already, Spain has implemented LNP in their networks, and many other European countries are quickly following. International number portability presents many new challenges to vendors because every country uses a different implementation, so what may work in Spain most likely will not work in Great Britain.

		Line Information Database (LIDB)

		The LIDB provides information regarding subscribers, such as calling card service, third-party billing instructions, and originating line number screening. Billing is the most important feature of this database. Third-party billing instructions, collect call service, and calling card service all determine how subscribers will be billed for their telephone calls in real time.

		In addition to billing instructions, the LIDB also provides calling card validation, preventing fraudulent use of calling cards. The user's personal identification number (PIN) is stored in this database for comparison when a user places a call.

		Originating line number screening provides information regarding custom calling features such as call forwarding and speed dialing. These features carry from network to network, as each service provider provides its own distinct calling services.

 

Calling Name (CNAM)

		SS7 automatically enables carriers to provide their subscribers with the calling party number because this information is carried in call setup messages. However, the name of that calling party is not included. This database provides the name of the calling party based on information from LIDB databases.

		The Regional Bell Operating Companies (RBOCs) are typically the owners of these databases because they have by far the most accurate information about subscribers in their networks. However, a number of companies are looking to provide their own databases, using many sources.

		Calling name is a very popular feature and a great revenue-generating service for any carrier. The subscriber must possess a specially equipped telephone or external display device to receive the information sent from the end office switch, but this equipment can be found in most any retail store that sells telephones for a very reasonable cost.

		Business Services Database (BSDB)

		This database is mentioned only as a model, since the last publication of Telcordia recommendations still has not defined the applications for this database. The purpose of this database is to allow subscribers to store call-processing instructions, network management procedures, and other data relevant only to their own private network. The telephone company would offer this database as an extra service, allowing large corporate customers to create their own private networks linking PBX equipment across the country. With their own proprietary databases, corporations can alter traffic routing by time of day or congestion modes without altering software in the PBX equipment.

		This type of database would be a valuable asset to companies using the flexibility of the Intelligent Network (IN) to define their own services through Service Creation Environments (SCEs). The network management functionality can provide special routing instructions for calls destined to congested PBXs, a feature popular with inbound call centers using automatic call distributors (ACDs).

		Home Location Register (HLR)

		The Home Location Register (HLR) is found in cellular networks and is used to store information regarding a cellular subscriber. The HLR stores information regarding billing, as well as services allowed. In addition to these, the current location of the cellular phone is stored in the HLR for retrieval by Mobile Switching Centers (MSC) (actually, the servicing MSC is indicated in the database, and not the cell by which the caller is currently being serviced).

		When a cellular telephone is activated, a cell site receives a signal containing the Mobile Identification Number (MIN) and other identification. This information is received by an MSC, which must determine which HLR the MIN belongs to. This is accomplished in the same way that normal POTS telephone calls are connected. The MIN is equivalent to a POTS number.

		Every few minutes, the cellular phone resends this signal. As the car moves from one cell site to another, the MSC monitors which MSC is servicing the subscriber, and if they move to an area serviced by another MSC, updates the location of the mobile in the HLR database. When a call is received into the network for a mobile telephone, the home MSC must determine which HLR to access to obtain the location of the cellular telephone. The HLR informs the MSC of the location, and the call is connected using voice circuits through the appropriate MSC to the cell site servicing the cellular subscriber at that time.

		Visitor Location Register (VLR)

		The Visitor Location Register (VLR) is used when a cellular telephone is not recognized by the local MSC. This is a dynamic database used to store cell site location of active subscribers in a network. When subscribers roam outside of their ''home" areas, the servicing MSC must keep track of their locations and be able to verify the validity of the MINs. This is done through accessing the HLR, using the Transaction Capabilities Application Part (TCAP). The VLR is used to store the current locations (cell sites servicing the subscriber). The VLR communicates this information to the home HLR as well, allowing the HLR to keep track of the subscribers' locations.

		The VLR may or may not be colocated with every MSC. In some networks, there is one VLR and one HLR. The only requirement is that all MSCs be able to access all HLR and VLR databases using the SS7 protocol.

		For specific Telcordia requirements relating to Service Control Points (SCPs), refer to the Telcordia publication TR-NWT-001244, Supplemental SCP.

 

		Figure 2.5 The OSS is typically located adjacent to an STP and accesses the network via a signaling link to the STP. Using TCAP and SCCP, the OSS is then capable of sending SS7 messages to any entity within its own network.

		Operations Support Systems (OSS)

		In the U.S., the Bell Operating Companies (BOCs) have established remote maintenance centers for the monitoring and management of their SS7 networks and voice networks (see Figure 2.5). With the capability of accessing all equipment today through digital interfaces, on-site personnel are no longer required.

		The OSS serves many functions. Only those related to SS7 are discussed here. All signaling points within the network can be monitored from the remote maintenance center. The remote maintenance center uses large projection screens and terminals to display information (both textual and graphic) regarding the status of all signaling points and their data links.

		Access to any of these signaling points and their components is accomplished through terminals. Telcordia has defined a standard set of commands for use in all of their network equipment, SS7 and otherwise, allowing their maintenance personnel to learn one set of commands for accessing all devices in the network (SEAS). This eliminates the need for training on specific equipment. This has proven to be an expensive alternative for most carriers, and has not been widely accepted.

		The maintenance personnel in these offices may have to interact with many types of equipment. The equipment may include multiplexers, digital cross-connects, and switches. Use of a common command set is most helpful in this type of application. Other vendors have developed GUI interfaces for monitoring and management systems that allow carriers to use diverse mixes of equipment, and access and provision the equipment through a GUI interface. This eliminates the need for a common command set, which limits the vendor choice for the carrier.

		To update the SCP databases and monitor the performance of the databases, a Service Management System (SMS) is used. The SMS is a standard interface consisting of a command set and graphical interface that can be used to administrate the database, monitor the status of the database, and retrieve measurements pertaining to performance from the database. The SMS also provides a central point for making updates to multiple databases. The database changes are made within the SMS, and then propagated to all the databases in the network. This eliminates the need to visit each and every database site to incorporate new changes and ensures consistent database updates.

		Signaling Data Links

		All SS7 signaling points are interconnected via signaling data links. These links are 56/64 kbps, 10/100 Mbps, and 1.536-Mbps data facilities (the exception to this rule is Japan, where 4.8-kbps links are used). Links are bidirectional, using both a transmit and receive pair for simultaneous transmission in both directions.

		There are three modes of signaling which can be used. These three modes depend on the relationship between the link and the entity it services. The simplest mode is referred to as associated signaling. In associated signaling, the link is directly parallel with the voice facility for which it is providing signaling. This, of course, is not the ideal, because it would require a signaling link from the end office to every other end office in the network. There do exist, however, some associated modes of signaling.

 

		Figure 2.6 Nonassociated signaling involves the use of STPs to reach the remote exchange. As depicted in this figure, to establish a trunk connection between the two exchanges,signaling messages would be sent via SS7 and STPs to the adjacent exchange.

		Figure 2.7 In quasi-associated signaling, both SSPs connect to the same STP. The signaling path is still through the STP to the adjacent SSP.

		Nonassociated signaling uses a separate logical path from the actual voice, as seen in see Figure 2.6. There are usually multiple nodes involved to reach the final destination, while the voice may be a direct path to the destination. Nonassociated signaling is a common occurrence in many SS7 networks.

		Quasi-associated signaling (see Figure 2.7) uses a minimal number of nodes to reach the final destination. This is the most favorable method of signaling, because each node introduces additional delay in signaling delivery. For this reason, SS7 networks favor quasi-associated signaling. Packet networks make the distinction between signaling types more difficult because of the nature of these networks. The concept is important to understand, so it is discussed here in relation to circuit-switched networks.

 

		Figure 2.8 In some cases, it may be better to directly connect two SSPs via a signaling link. All SS7 messages related to circuits connecting the two exchanges are sent through this link. A connection is still provided to the home STP using other links to support all other SS7 traffic.

		Figure 2.9 All signaling links are labeled according to their location in the network. There is no real physical difference between different links other than network management treatment.

		Signaling data links (see Figure 2.8) are labeled according to their function. There is no difference between the various links, only in the way the links are utilized during message transfer and how network management interacts with the links (see Figure 2.9).

		Links are placed into groups, called linksets. All the links in a linkset must have the same adjacent node. The switching equipment will alternate transmission across all the links in a linkset to ensure equal usage of all facilities. Up to 16 links can be assigned to one linkset.

		Routes

		In addition to linksets, a signaling point must define routes. A route is a collection of linksets used to reach a particular destination. A linkset can belong to more than one route. A collection of routes is known as a routeset.

		A routeset is assigned to a destination. Routesets are necessary because, if only a single route existed and that route were to become unavailable, an alternate route would not be defined and no signaling could be sent to that destination. A routeset provides alternate routes to the same destination in the event that any one route becomes unavailable.

		A destination is an address entered into the routing table of a remote signaling point. The destinations do not have to be directly adjacent to the signaling point, but they must be a point code which can be reached by the signaling point. A signaling point does not have to know all point codes in between itself and its destinations, only which link or linkset to use to reach its destination. A signaling point can have multiple addresses, if it is necessary to partition a signaling point into multiple functions. For example, a gateway STP used to enter the international network may have multiple point codes: one for the gateway function and another for global title translation services.

		It may sound as if an STP would have to know millions of point codes, but in actuality this is not the case. If a carrier is connecting to another carrier's network, the STP need only know the point codes in that network. It becomes the responsibility of the other carrier to make sure that connections are made in other networks.

		Links should always be terrestrial, although satellite links are supported in the standards. Satellite links are unfavorable because of the delay introduced. In the event that satellite links are used, the labeling and functions of the links remain the same. Network management procedures are the same except for the procedures used at level two of the protocol (link alignment, error detection/correction).

		Satellite links use a different method of error detection/correction than terrestrial links. Basic error detection/correction is used for all terrestrial links, and preventive cyclic retransmission (PCR) is used for satellite links. The difference between the two lies in the retransmission mechanism. In basic error detection/correction, an indicator bit is used to indicate retransmission. In PCR, if an acknowledgment is not received for transmitted signaling units within a specific time, all unacknowledged signaling units are retranslated.

		Link Implementation

		When a node has links to a mated STP pair, the links are assigned to two linksets, one linkset per node. Both linksets can then be configured as a combined linkset. A combined linkset contains links to both STP pairs, which means their adjacent signaling point address (point code) will be different. Combined linksets are used for load sharing, where the sending signaling point can send messages to both pairs, spreading the traffic load evenly across the links (see Figure 2.10).

		Alternate linksets are used to provide alternate paths for messages. An alternate linkset or link is defined in the signaling points' routing tables and used when congestion conditions occur over the primary links. Figure 2.11 illustrates a typical configuration using alternate links to other nodes in the network, providing diversity in the event of node congestion.

		Link Performance

		Links must remain available for SS7 traffic at all times, with minimal downtime. When a link fails, the other links within its linkset must take the traffic. Likewise, if an SS7 entity (such as an STP) should fail, its mate must now assume the load. This means links can suddenly be burdened with more traffic than they can handle. For this reason, SS7 entities are designed to send less than 40 percent traffic on any link.

		Figure 2.10 A combined linkset connects an adjacent pair. Each link in this figure connects to a different signaling point, but has the same destination.

 

		Figure 2.11 In this figure, the E and F links are alternate links and would be used when the primary links become unavailable or congested.

		In the event of a failure, any one link may suddenly be responsible for the failed link's traffic. At 40 percent capacity, there is plenty of room for this traffic. Even at 80 percent capacity, the links still have enough capacity to carry SS7 network management messages in addition to the extra traffic.

		You can calculate the number of messages a link will support by first determining the average length (in bytes) of each message. For example, if a DSO link is being used, the transmission rate of that link is 56 kbps. If you divide 56,000 bits into 1 byte (8 bits) you will see that a link can support 7000 bytes per second. Given that constant, you must now consider that the link is engineered to carry 40 percent traffic, which comes to 2800 bytes per second.

		If the average message length is 40 bytes (as is the case with most ISUP), your link can carry 70 ISUP messages per second (2800/40 = 70). You can use this simple formula to calculate capacity of any link, which becomes important when you are sizing your network.

		A maximum of 10 minutes downtime per year is allowed for any one linkset. This downtime relates to the ability to send SS7 messages to the destination using levels two and three of the protocol stack. These are stringent rules and are specified in the Telcordia publication GR-246-CORE.

		There are six different types of links used in SS7:

		Access (A) links

		Bridge (B) links

		Cross (C) links

 

Diagonal (D) links

		Extended (E) links

		Fully associated (F) links

		Access Links (A)

		Access (A) links (see Figure 2.12) are used between the SSP and the STP, or SCP and STP. These links provide access into the network and to databases through the STP. There are always at least two A links, one to each of the home STP pairs. In the event that STPs are not deployed in pairs, there can be one A link; however, this is highly unusual.

		The maximum number of A links to any one STP is 16. A links can be configured in a combined linkset, with 16 links to each STP, providing 32 links to the mated pair. When connecting switches in a network to hub providers, A links are used.

		When trying to determine how many A links are required, the easiest formula is to calculate the number of access lines supported by the switch. One A link can support 9600 lines.

		Figure 2.12 Access links connect end signaling points to the SS7 network.

 

		Figure 2.13 Bridge links connect a mated pair of STPs to another mated pair of STPs.

		Bridge Links (B)

		Bridge (B) links are used to connect mated STPs to other mated STPs at the same hierarchical level. Bridge links are deployed in a quad fashion, as seen in see Figure 2.13, which is why these are often referred to as quad links. A maximum of eight B links can be deployed between mated STPs. Although this practice is closely followed in North America, European networks do not use bridge links as depicted. Mated STPs are connected to another mated pair via one set of links, but each STP does not have a connection to each of the other mated STPs.

		Cross Links (C)

		Cross (C) links (see Figure 2.14) connect an STP to its mate STP. They are always deployed in pairs, to maintain redundancy in the network. Normal SS7 traffic is not routed over these links, except in congestion conditions. The only messages to travel between mated STPs during normal conditions are network management messages. If a node becomes isolated and the only available path is over the C links, then normal SS7 messages can be routed over these links. A maximum of eight C links can be deployed between STP pairs.

 

		Figure 2.14 Cross links connect an STP to its mate STP, creating a mated pair. Mated pairs are identical in function and configuration, and have the ability of assuming the traffic of their mate in the event the mate fails. The cross links are used to share network management messages, and when no other route is available, to signal traffic.

		Diagonal Links (D)

		Diagonal (D) links (see Figure 2.15) are used to connect mated STP pairs at a primary hierarchical level to another STP mated pair at a secondary hierarchical level. For example, a carrier may have STPs deployed in every LATA. They could then deploy STPs in regions, acting as concentrators. This would prevent the need to interconnect every STP to every other STP. The LATAs within a defined region would all connect to one STP, which would provide connections to the other regional STPs. This hierarchical approach would only be found in very large SS7 network.

		Not all networks deploy D links, since not all networks use a hierarchical network architecture. D links are deployed in a quad arrangement like B links. A maximum of eight D links may be used between two mated STP pairs.

 

		Figure 2.15 Diagonal links connect mated STPs to another mated pair of STPs that are deployed on a higher level in the network hierarchy. In this figure, the D links are used to connect to a pair of regional STPs, which provide access to a regionally located database.

		Extended Links (E)

		Extended (E) links (see Figure 2.16) are used to connect to remote STP pairs from an SSP. The SSP connects to its home STP pair but, for diversity, may be connected to a remote STP pair as well, using E links. Extended links then become the alternate route for SS7 messages in the event that congestion should occur within the home STP pairs. A maximum of 16 E links may be used between any remote STP pairs.

		Fully Associated Links (F)

		Fully associated (F) links (see Figure 2.17) are used when a large amount of traffic may exist between two SSPs, or when an SSP cannot be connected directly to an STP. F links allow SSPs to use the SS7 protocol and access SS7 databases even when it is not economical to provide a direct connection to an STP pair.

		When traffic is particularly heavy between two end offices, the STP may be bypassed altogether, providing that both SSPs are local to each other. Only call setup and teardown procedures would be sent over this linkset.

 

		Figure 2.16 Extended links connect an SSP with an STP not considered its home STP. This is done for diversity and provides an alternate route around its home STP pair. This configuration can also be used when there is a high volume of traffic to a particular destination, to prevent the home STP pair from becoming congested.

		Physical Link Interfaces

		The signaling data links are connected to network equipment using electrical interfaces. These interfaces are industry-standard interfaces defined by standards bodies such as ITU-TS and EIA. The interface type will depend on the type of equipment used with the links. For example, if a data service unit (DSU) is used, a V.35 interface will be needed to connect from the DSU to the signaling point.

		Interfaces operate at level one of the SS7 protocol stack, and provide the electrical/optical medium for transmission of data packets within the SS7 network. Following are descriptions of the most commonly used interfaces in the SS7 network.

 

		Figure 2.17 Fully associated links connect two SSPs directly, allowing signaling traffic to follow the same path (in parallel) as the voice circuits. This is commonly found when there is a large volume of signaling traffic (ISUP) between two exchanges. This configuration can also be used when there is no direct access to the SS7 network.

		V.35

		This interface is commonly used from a data service unit to the SS7 signaling point. The V.35 interface can also be used from a digital x-connect (DSX) panel. V.35 provides data rates up to 56 or 64 kbps. Slower data rates are supported as well.

		The V.35 interface was originally intended for use with high-speed modems. Interfacing analog modems to a digital line at 48 kbps was the first implementation of this interface. Later, the ITU-TS adopted this interface for use in all digital lines, with data rates of 48, 56, 64, and 72 kbps.

		The ITU-TS Blue Book considers this interface obsolete and no longer recommends its usage. Instead, the ITU-TS Blue Book recommends use of V.36 or other similar standards. The V.35 interface is still very common, however, and is found in many equipment types (see Figure 2.18).

		When using a V.35 interface, a clock source must be provided. This clock source is typically provided by the switch itself, but can be provided from an external source. One side of the V.35 connection must be defined as master (clock source) and the other as slave (uses master clock). In the event the master side should fail, a mechanism should be provided to allow the slave to become master and provide its own clocking.

Pin 1 - Protective Ground

Pin 2 - Transmitted Data

Pin 3 - Received Data

Pin 4 - Request to Send

Pin 5 - Ready for Sending

Pin 6 - Data Set Ready

Pin 8 - Receive Line Signal Detect

Pin 15 - Tx Signal Element Timing

Pin 17 - Rx Signal Element Timing

		Figure 2.18 The ITU V.35 interface may use a 37-pin or a 15-pin connector.

		DS0A (Digital Signal 0)

		This is a 56/64-kbps channel located in a DS1 (or higher) facility. The link can be carried with an existing DS1/DS3 circuit between offices, as long as one of the DS0As is dedicated to SS7 signaling. A DSU/CSU is required to terminate the DS1/DS3 and separate the various DS0As from the circuit.

		The DSU/CSU is usually located close to the entrance point of the digital facility into the telephone company building. From there, the various DS0As are cross-connected through a digital cross-connect to their final destinations.

		This is the most commonly used interface in U.S. SS7 networks. Signaling points in the network will usually have the ability to terminate the DS0A circuit without interface adapters. Maximum cable length for a DS0A is 1500 ft. This limitation is not a transmission limitation, but a consideration due to propagation delay. The Telcordia requirements also specify the nominal impedance as 135 ohms with a balanced transmission path in each direction.

		In the future, DS0A with clear channel capability may be used. At the time of this publishing, this had not yet been written. However, work is being accomplished in the area of 64 kbps DS0s. The designation for a 64 kbps DS0 is DS0C.

		Clear channel means that the data source can transmit a full 64 kbps without any restrictions on ones density or all zeroes. Currently, the DS1 level, which carries the DS0 signal, enforces the ones-density rules.

		Because DS0A is a digital facility, clocking is critical. For this reason, whenever a DS0A is used in the SS7 network, the DS0A must be synchronized according to the Digital Synchronization Network Plan (TA-NPL-000436, Digital Synchronization Network Plan, Issue 1: November 1986). Network synchronization is explained in the first chapter.

		Synchronization is accomplished at two different levels: bit synchronization and byte synchronization. Bit synchronization is what ensures that the transmitter and receiver are operating at the same data rate. Byte synchronization is what ensures the receiver can properly define alignment of frames. This is critical in defining the beginning and the end of a received frame.

		The DS0A used within the central office uses bipolar encoding. The nominal pulse width of this signal is 15.6 microseconds, with rise and fall times of 0.5 s. Clocking is provided by the Building Integrated Timing Supply (BITS).

		The DS0A interface provides reliable digital transfer of data. There are literally no configurable options to worry about in DS0A circuits, except for encoding schemes and data rates. The encoding method used will vary from network to network, depending on the type of multiplexer used.

		There has been some discussion regarding a full DS1 interface to replace the DS0A. This would provide 1.544 Mbps on one link. This is not necessary today, but as the traffic mix changes and becomes more complex, this may become a requirement.

		The major obstacle in DS0A links is timing. When a DS3 is used between two exchanges, four multiplexers must be used (end to end) before the DS0A signal can get to the signaling point. Whenever there are this many multiplexers, timing synchronization can become a problem.

		When the timing on a link is not synchronized between any two multiplexers, the links cannot carry data properly, because the signaling point will not be able to read the data. Remember that timing is used to delineate between bits. If the synchronization between any two devices is not correct, it causes the receiving device to see bits that do not exist.

		When using DS0A links, the most common problem encountered is related to losing the clock synchronization, which causes the links to be taken out of service. The fastest correction is to reset all the multiplexers and allow them to resynchronize with one another.

 

High-speed Links

		There are three interfaces defined to support ATM in the SS7 networks: SONET, DS1, and DS3. This provides a migratory path for telephone companies looking to deploy ATM in their signaling networks. The transmission rates vary.

		DS1 supports transmission rates of 1.544 Mbps, while DS3 supports transmission rates of 44.736 Mbps. SONET supports much higher transmission rates. For SS7 signaling links, transmission rates of 51.840 Mbps, 155.520 Mbps, 622.080 Mbps. and 2.48832 Gbps have been defined using the ATM_SAAL protocol in place of the MTP levels 1 and 2. A subset of MTP level 3 is used to deliver BISUP messages over these links. Refer to Telcordia document GR-1417-CORE for more details on the ATM interface.

		There are two ways in which telephone companies can deploy ATM in their SS7 networks. Most companies will likely choose to use DS1 interfaces initially, because they already have these facilities available. However, as they migrate to SONET, the DS1 and even DS3 facilities may be replaced. SONET facilities will then be used to carry all traffic through the network. Currently, only the ATM interface has been implemented because SS7 does not require the amount of bandwidth provided by the other options.

		SS7 messages will be included in this migration. The concept of ATM is to use one facility for all traffic. The SS7 traffic will then be carried with the voice and data traffic. Due to the inherent features of ATM routing, signaling traffic will be routed the same way voice and data are routed in ATM networks, using the Signaling ATM Adaptation Layer (SAAL) instead of MTP. Only a portion of MTP level 3 will be used for routing SS7 messages on ATM links.

		TCP/IP Links

		Next generation networks are based on TCP/IP packet networks. Voice, data, video, and audio are all carried on TCP/IP rather than channelized facilities. This type of network offers considerable cost savings, which is why every carrier is examining the use of TCP/IP.

		Using TCP/IP for SS7 does present some problems. The TCP/IP protocols do not support real-time applications such as voice. SS7 requires a higher Quality of Service (QoS) as well, which is mandating changes to the TCP/IP protocols. MTP levels 2 and 3 provide many services not currently supported in TCP/IP. The IETF as well as the ITU are actively defining new protocols to replace MTP level 2 and 3.

		Packet networks are far more efficient than channelized facilities because they allow more efficient use of the available bandwidth. In channelized facilities, a 56 kbps link is used for one purpose, whether or not the full bandwidth is being used. In packet networks, everything is sent over the same facilities, allowing the full bandwidth to be utilized.

		TCP/IP links use an Ethernet connection at level 1, supporting bandwidth of 10 or 100 Mbps. Eventually, all networks will be based on TCP/IP, eliminating legacy channelized equipment. This migration has already begun and is widespread throughout the industry both in North America and internationally.

		Miscellaneous Interfaces

		The V.35 and DS0A are the most commonly used interfaces for connecting links to network nodes today, with TCP/IP gaining popularity. There are other interfaces used for interconnecting adjunct equipment such as terminals and communications equipment. They are mentioned here so that the reader might understand their usage.

		RS-232/V.24

		The The RS-232 interface is a serial interface designed originally for connecting modems to computer equipment. Over the years, the RS-232 has found many other uses as a serial interface. Printers, terminals, and any other device requiring a serial interface can use the RS-232. The RS-232 provides a separate transmit and receive path, as well as flow control. However, there are limitations. The maximum cable distance for the RS-232 is 50 ft. In many cases, this is not a problem. But in the central office, this may be unacceptable, since terminal equipment and switches are often placed in different areas. In addition to the distance limitation, RS-232 has a maximum data rate of 19.2 kbps.

		The signals provided by the RS-232 interface are divided into four categories: data signals, control signals, timing signals, and grounds. Data signals consist of the transmit and receive paths for the user data.

		There are eight control signals provided by the RS-232. Not all of these control signals are needed, as some were developed for use with modems specifically. Request to send, clear to send, and data set ready are typically used with printers and terminals for flow control. Data terminal ready, ring indicator, data carrier detector, data modulation detector, and speed selector are all optional control signals used specifically for modems.

		The mechanical requirements of the RS-232C call for a 25-pin connector (typically the DB-25 type). Most any connector fitting the application and the equipment can be used. The DB-25 is the most common connector used; however, many smaller connectors are being sought since equipment is getting smaller.

		The electrical requirements of the RS-232C are specified as:

		Binary 1 = voltage more negative than -3 V

		Binary 0 = voltage more positive than +3 V

		Signal rate = <20 kbps

		Distance rate = <15 m

		The functional requirements call for an unbalanced transmission path. One ground is provided as a return for both data leads. The other ground is a protective isolation ground. Synchronous transmission is accomplished by sending timing signals over the leads designated as receiver signal element timing.

		An unbalanced line means that one lead is used to transmit the data, while the common ground is used as the return path. Interference can cause signals to be altered and, because the signal path is only over one lead, the voltage difference can be damaging to the data. In a balanced circuit, the data is sent over one lead with another lead used as the return for the same circuit. This means that current is carried in one direction (data flow) and returned on another lead, creating a complete circuit. Interference may occur, but it will not affect both leads. Thus, balanced circuits are better for data transmission over long distances. RS-232 is limited to short distances because of its use of unbalanced transmission circuits.

		When a device is ready to send data, the request to send (RTS) lead goes high. The receiving device acknowledges RTS and raises the clear to send (CTS) lead high. These are the minimal signaling requirements of the RS-232C. There are many other signaling and control leads which can be incorporated, but most manufacturers find that RTS and CTS are all that are needed. Modems require most of the leads indicated for reliable transmission.

		In a modem configuration, connections are established in a different fashion. Modems are most often used by maintenance personnel wishing to connect to a remote SS7 signaling point from their computer workstation. They can then perform maintenance and administration tasks from their location. To understand the sequences that take place when a modem is concerned, let's look in more detail at the steps involved.

		When the craftsperson is ready to connect to the modem, he or she will choose a communications software application on the workstation. The application will perform the steps necessary to connect to the modem. The interface will go through various stages before transmission actually begins. When the computer is ready to transmit, the data terminal ready (DTR) lead from the workstation will go high.

		The modem has now been alerted that the workstation wants to place a call. The workstation will need to send the telephone number of the signaling point to the modem. This can be accomplished over the transmitted data (TD) pins of the interface. The modem then dials the number over the analog telephone line. Most modems will have a speaker incorporated into the modem, so the user can actually hear the dialtone as the modem goes off-hook and begins dialing the number. This can be extremely helpful when trouble is encountered, because you can hear if the call actually went through or not.

		When the line begins ringing, the distant modem should detect ring generator on the line. When it detects ring generator, the distant modem will raise the ring indicator lead to high to alert the signaling point that there is an incoming call.

		The signaling point should then raise the DTR lead high to indicate it is ready to receive data. This indicates to the modem that it should answer (go off-hook) and establish a connection with the distant modem.

		The distant modem then answers the line and places a carrier signal to the calling modem. At the same time, the called modem raises the Data Set Ready (DSR) lead high to indicate to the signaling point that it has answered the incoming call and it is ready to communicate.

		The calling modem then sets DSR high to indicate to the workstation that a connection has been established and it is ready to transmit data. The calling modem will return a carrier signal to the called modem so that full-duplex transmission can be established. The called modem will set the carrier detect (CD) lead high to indicate receipt of a carrier signal.

		The workstation then sets Request to Send (RTS) high, indicating it is ready to transmit data. The modem responds and sets Clear to Send (CTS) high. This is an acknowledgment to the workstation. The workstation then begins transmitting the data in serial fashion (1 bit at a time) over the TD lead, using the carrier signal to send the data. The carrier signal is modulated (using any means of modulation) to represent the bit stream in an audible tone.

		The called modem receives the modulated data, demodulates it, and sends it to the signaling point over the received data (RD) lead. When the transmission is complete, either modem can drop the connection. The RTS lead is set low (off), which causes the called modem to set CTS low. The carrier is dropped and the connection is released.

 

This entire procedure can be monitored at any end of the circuit by using a breakout box or RS-232 monitoring device (a simple device with a series of LEDs for each circuit). Whenever a modem is connected to a signaling point or any other communications equipment in the network, it is strongly suggested that such a tool be kept in the toolkit for troubleshooting the connection. Most modem troubles can be detected and isolated through the use of such a tool.

		V.24 is the ITU version, after which the RS-232 was designed. The V.24 interface provides the same functions as the RS-232 interface, plus additional signals for automatic calling. In addition to the RS-232 signals, the V.24 provides many more signals for timing, control, and data transmission.

		RS-449

		The RS-449 interface was intended to replace the RS-232, providing increased distance and higher data rates. However, manufacturers have been using the RS-232 for so long, and there is already a large number of these interfaces in use today, that the RS-449 has not shared widespread acceptance. In the PC market, there really is no incentive for the RS-449, because distance is not a problem. Devices using a serial interface are typically found right next to the computer. But where distance and speed are issues, the RS-449 is a better interface.

		RS-449 supports distances up to 200 ft with a data rate of 2 Mbps. In addition to the enhanced performance, the RS-449 interface provides 37 basic circuits and 10 additional circuits to support loopback testing and other maintenance functions.

		The mechanical requirements specified for the RS-449 call for a 37-pin connector for the basic interface, and a separate 9-pin connector if the secondary channel is used. The electrical requirements show a significant improvement over RS-232 interfaces, which are limited to an unbalanced line.

		The RS-423-A standard specifies an unbalanced mode for the RS-449 interface, while the RS-422-A standard specifies the balanced mode. In a balanced mode, the electrical characteristics are:

		100 kbps at 1200 m

		10 Mbps at 12 m

		In an unbalanced mode, the performance is not as good, but is still an improvement over the RS-232 standard. In unbalanced mode, the performance rating is:

		3 kbps at 1000 m

		300 kbps at 10 m