Circuit-Switched Networks

Circuit-Switched Networks

There are two main types of circuit-switched WANs: those based on leased lines and customer premises equipment (CPE) to manage the leased lines, and those based on ISDN, including both Basic Rate Interface (BRI) and Primary Rate Interface (PRI).

Leased Lines

Leased lines can be configured in one of two ways. The first approach uses point-to-point leased lines, as shown in Figure 7.2. In this approach, a communications link joins two nodes and only two nodes. The good thing about this setup, of course, is that there is no contention. The two devices always have an available communications path. The disadvantage becomes pronounced when the network starts to grow, either in the number of devices or in the distance between the locations. Because leased lines are calculated on a mileage-sensitive basis, the cost increases as the network scope increases.

Figure 7.2. Point-to-point leased lines

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To increase the cost-efficiency of a leased-line network, you can use a multipoint leased-line network, in which you have a shared communications facility where multiple nodes vie for access to the communications link. The advantage of this configuration is that it combines mileage, so the overall monthly cost associated with the leased line is reduced. The disadvantage is that you must introduce some type of intelligent scheme that allows you to determine which device gets to use the communications pathway at what time. Figure 7.3 shows a multipoint leased-line network.

Figure 7.3. Multipoint leased-line network

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Leased lines, then, are configured on a point-to-point or on a multipoint basis. This can be accomplished by using a number of approaches, including these:

         DDSs, which essentially use 56Kbps or 64Kbps leased lines

         T-, E-, or J-carrier backbone

         SDH/SONET backbone

         Dark fiber backbone

The following sections describe these approaches.

DDSs

A DDS uses leased lines that operate at either 56Kbps or 64Kbps, depending on whether you are being served over a T-, E-, or J-carrier infrastructure. Figure 7.4 illustrates the equipment associated with a DDS network. Two pieces of equipment are located at the customer site: the DTE (for example, host computers, routers) and DCE (for example, the data service unit [DSU]), which are connected to one another via a physical interface (for example, an RS-232, a V.35). The data access line (that is, the 56Kbps or 64Kbps facility) runs from the DCE to a specialized DDS hub that is a digital circuit switch. (Remember that when this service was introduced, local exchanges were analog, so in order to introduce DDS into a metropolitan area, the telcos had to put into the exchange environment a specific digital circuit-switched hub for those services.) The DDS hubs connect into the digital transport network; destination cities attach to the network the same way, using the same equipment configurations.

Figure 7.4. DDS equipment

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The DSU is a device that connects various DTE together via RS-232 or V.35 interfaces with the digital services that offer 56Kbps or 64Kbps access. DSUs can be used to accommodate DDS, Frame Relay, and ATM facilities. When the DSU is combined with a channel service unit (CSU), it interfaces with services at the 64Kbps level, as well as n 64Kbps, up to either T-1, E-1, or J-1 capacities. (These levels are described in detail in Chapter 5, "The PSTN.") The DSU converts the binary data pulse it receives from the DTE to the bipolar format required by the network. Within the computer, the one bits are positive voltages, and the zero bits are no voltages or low-level voltages. The ones density rule says that if you transmit more than 15 zeros in a row, the network may lose synchronization, which means transmission errors could occur. Therefore, the DSU performs a bipolar variation it alternates the one bits as positive and as negative voltages. The DSU also supplies the transmit and receive logic, as well as the timing. The CSU provides a means to perform diagnostics.

DDS facilities can be used for LAN interconnection, access to the Internet, and remote PC access to local hosts. One thing to bear in mind with a traditional DDS approach is that it is a leased-line service. If the leased line goes down, the network is out of service, and recovering the service can be a lengthy process, depending on how many network providers are associated with the link end-to-end. The mere process of troubleshooting and identifying within whose network the problem lies can often lead to resolution times of 24 to 48 hours or even more, if network managers do not cooperate with one another. If you rely on the DDS network for critical applications, you need a backup in the event that the leased line fails at some point, and the best backup option is generally switched digital access, which is a dialup option in which facilities are allocated based on demand rather than being associated with a specific customer all the time. Switched digital access supports transmission rates of 56Kbps, 64Kbps, 384Kbps, and 1,536Kbps. Another potential backup is ISDN, which is described later in this chapter.

Videoconferencing Data Rates

Switched 384Kbps data rate can support full-motion videoconferencing, which requires a frame rate of 30 frames per second. Below a 384Kbps data rate, the frame rate begins to drop to only 10 to 15 frames per second, and jerky video results. Therefore, enterprises that require applications such as full-motion videoconferencing are often interested in services that offer 384Kbps.

T-, E-, and J-Carrier Backbone

In the 1980s, as networks, traffic, and applications were evolving and growing, customers saw a rise in the amount of data traffic they were carrying, and they began to institute various data services to address those, but this resulted in a hodgepodge of single-purpose networks based on leased lines that included unique universes of equipment or specific applications.

For example, Figure 7.5 shows that you have a variety of networks under operation: Both point-to-point and multipoint leased lines are being used to provide connectivity between LANs in various cities around the world. The PBXs for the most part are relying on the PSTN, but there is a leased line between San Francisco and London because of the volume of traffic and security requirements. The videoconferencing systems between San Francisco and London make use of a specially provisioned satellite link. In essence, there are four separate networks, you're paying for four different infrastructures, and you don't have complete connectivity between all the locations, users, and resources. You therefore want to build an enterprise backbone network that ties together everybody with one common infrastructure and that provides for more fluid connectivity between the various locations and applications. This requires intelligent equipment at each location, to manage the transmission resource and to properly allocate the capacity to voice, data, image, video, fax, or other forms of transmissions.

Figure 7.5. Single-purpose leased-line networks

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Fractional Services

Some locations may be too small to justify a full T-1 or E-1 facility. In those cases, fractional services can be used; they allow you to subscribe to capacity in small bundles of 56Kbps or 64Kbps channels, which are generally provided in increments of four channels. These are referred to as fractional T-1 (FT-1) and fractional E-1 (FE-1) services.

The facilities used to transport the information on this revised network can be combinations of privately owned and leased facilities, but the equipment to manage those transmission facilities is owned and managed by the customer. You might be leasing a T-1 or an E-1 from a network operator, or you might be leasing dark fiber from a railroad company. Between some locations, you might deploy a privately owned digital microwave system. You can integrate what used to be four separate networks into one cohesive backbone (see Figure 7.6). In terms of the capacities and components that would then be used within that backbone, the majority of customers today would rely on the dimensions that are deliverable from the T-, E-, and J-carrier infrastructure, although early adopters, and those with high levels of visual information processing, are already migrating to the optical carrier levels of the SDH/SONET hierarchy.

Figure 7.6. An enterprise backbone network

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The customer needs the following equipment to manage the transmission facilities (refer to Figure 5.5 in Chapter 5):

         Transmission media The customer might be using copper twisted-pairs with T-1 or E-1 and might be using higher-bandwidth media such as coax, microwave, or fiber with T-3 or E-3 capacities. These are four-wire circuits operating in full-duplex, which means that you can communicate in both directions simultaneously.

         CSU The CSU terminates each end of the T-1/E-1 or J-1 carrier facility. The CSU equalizes the received signal, filters the transmitted and received wave forms, and interacts with the customer's, as well as the carrier's, test facility so that diagnostics can be performed. Essentially, the CSU is used to set up the T-1/E-1/J-1 line with a customer-owned PBX, channel banks as stand-alone devices, intelligent multiplexers (for example, T-, E-, or J-carrier multiplexers), and any other DS-x/CEPT-x compliant DTE, such as digital cross-connects.

         Time-Division Multiplexers One type of mux, channel banks, consolidate the individual channels: 24 channels are associated with T-1, and 32 channels are associated with E-1. These voice and data channels are 64Kbps each, and they can be aggregated onto a higher-speed transmission line. Channel banks were designed to accept analog input, so if there is an analog switch either at the PBX or at the local exchange, that analog signal can be digitized, using Pulse Code Modulation (PCM), as described in Chapter 5. The channel banks provide a first level of aggregation. Beyond that, the customer might want to migrate to higher bandwidth, which would involve using T-1/T-3, E-1/E-3 multiplexers. (Refer to Chapter 5 for a description of the rates associated with the various digital signal levels.)

The most important piece of equipment in building out the enterprise backbone network is the intelligent multiplexers because they act dynamically to manage transmission resources. They allow you to make on-the-fly decisions about who is allocated the capacity, how much capacity needs to be allocated to each user, and whether individual users have rights to access the resource they want to access. As shown in Figure 7.7, the intelligent muxes basically form a smart computer. An intelligent mux has a port side to which you interface the universe of information resources, which could be the videoconferencing systems, or the voice systems, or the variety of data universe that you have. On the trunk side, you terminate the T-1s/E-1s or T-3s/E-3s.

Figure 7.7. T-1/T-3 and E-1/E-3 muxes

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Inside the muxes are databases that allow you to make decisions in real-time. One of the greatest benefits of managing your own bandwidth between locations is that you can use that bandwidth as you need it. There are two ways you can use T-, E-, or J-carrier facilities. First, you can use them as an access pipe. For example, you can use a T-1 to access the local exchange and to replace combined voice and data trunks. When you are using it to access the PSTN, you have to work within the subscribed standards. For example, with T-1 you get 24 channels at 64Kbps per channel, so if you have only a little bit of data to send, you can't reallocate your extra bandwidth to another application. You are stuck with static bandwidth allocation (see Figure 7.8), either twenty-four 64Kbps channels or one 1,536Kbps channel.

Figure 7.8. Static versus dynamic bandwidth allocation

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A second way in which you can use T-, E-, or J-carrier facilities is to build a private network. For example, if you use a T-1/E-1 to tie together two of your own locations, then basically you can do as you wish with that pipe. You're in control of it, and you have the intelligent equipment at either end that can manage it on your behalf. For example, you can dynamically assign bandwidth; you can allocate only as much capacity as is necessary for an application, which more efficiently uses the capacity available on the digital facility. When you use the T-1/E-1 to build a private network, you can also perform dynamic alternative routing. Figure 7.9 shows an example of dynamic alternative routing. The primary route between Los Angeles and New York City is the direct diagonal line between them. But say there is a problem and that link fails. The multiplexer in Los Angeles will be smart enough to know to reroute its highest-priority traffic through Denver to get it to New York. And in Denver, it may take the second-priority traffic and reduce it to priority three in order to make room for the incoming high-priority traffic from Los Angeles. When the primary link is recovered, the network will revert to the original routing mechanisms. Dynamic alternative routing is useful in the face of congestion, failure, and when a customer needs to reconfigure capacities based on activities and personnel at given locations at random times.

Figure 7.9. Dynamic alternative routing

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Another element that a customer can use in creating a comprehensive enterprise backbone is the digital cross-connect. This device was introduced in 1985, with the purpose of automating the process of provisioning circuits in essence, replacing the use of manual patch panels. The key feature of the digital cross-connect system (DCS) is its capability to "drop an insert," which means the cross-connect can exchange channels from one facility to another (refer to Figure 5.7 in Chapter 5). It is used to implement appropriate routing of traffic, to reroute around congestion or failure, or to allow customers to dynamically reconfigure their networks. With digital cross-connects, network configurations are defined entirely in software, and this gives the customer great control and allows reconfigurations to be implemented in a matter of minutes rather than hours or days. The levels at which switching can be performed are at the DS-3/CEPT-3 level, DS-1/CEPT-1 level, and DS-0/CEPT-0 level. Sub-DS-O and sub-CEPT-0 levels are also possible. This capability is also offered by some of the intelligent multiplexers the T-1/E-1 and T-3/E-3 multiplexers so the functionality can be bundled.

The main applications for the digital access cross-connects are disaster recovery, bypassing systems during scheduled maintenance, addressing peak traffic demand, and implementing temporary applications. For customers that need to support even more advanced applications such as computer-aided design, three-dimensional modeling and simulation, visualization, and multimedia the capacities of the T-, E-, and J-carrier infrastructure may not suffice, so the next step is to migrate to the SDH/SONET signal hierarchy.

SDH/SONET Backbone

As discussed in Chapter 5, SDH/SONET is the second generation of digital infrastructure, based on the use of fiber optics. With SDH/SONET, an enterprise might today subscribe to OC-1 and OC-3 in order to build an enterprise backbone. Remember from Chapter 5 that the OC-1 level provides roughly 51Mbps, 50Mbps of which are available for payload. OC-3 provides a total data rate of 155Mbps, with a little over 150Mbps for the customer payload. In today's environment, given the high cost of such leased lines, the customers that use the SDH/SONET levels are still considered early adopters. They include airports, aerospace companies, universities that have medical campuses or significant art schools, large government agencies such as the U.S. Internal Revenue Service, and the military. These early adopters typically have a rather minor presence in the marketplace at this time, but as we move toward greater use of visual and sensory applications, more users will require this type of bandwidth.

To build out a private SDH/SONET network, a customer would need two types of multiplexers: terminal muxes and add/drop muxes (ADMs). A customer would also need two types of cross-connects to build out a private SDH/SONET network: wideband cross-connects and broadband cross-connects. (Chapter 5 describes these muxes and cross-connects.)

Dark Fiber

With dark fiber, the customer leases the fiber itself and buys the necessary equipment to actually activate the fiber. The customer pays for the physical media, not for bandwidth, and as the customer adds equipment that can either pulse more bits per second or extract more wavelengths out of the underlying fiber, the bandwidth essentially becomes cheaper and cheaper.

ISDN

Another circuit-switched WAN option is ISDN. The International Telecommunication Union Telecommunications Standardization sector (ITU-T) formalized the ISDN standard in 1983. According to the ITU-T (www.itu.org), ISDN is "a network evolved from the telephony integrated digital network that provides end-to-end connectivity to support a wide range of services, including voice and nonvoice services, to which users have access by a limited set or standard multipurpose customer interfaces." One of the ideas behind narrowband ISDN (N-ISDN), as the first generation of ISDN was called, was to give customers one access into the network, from which they could then engage in circuit-switched, leased-line, or packet-switched options. Although all these options were available before ISDN, each one generally required its own special access line and device, which meant extra costs and administrative responsibilities because of the large number of options. The goal of ISDN was to provide one plug into the network, from which you could then go out over multiple alternatives.

ISDN Networks

A couple of key elements are required to form an ISDN network (see Figure 7.10). First, you must have a digital local exchange, and that digital exchange must be loaded with ISDN software. This is not an inexpensive proposition. The ISDN software alone costs around US$1 million, and each exchange costs in the neighborhood of US$3 million to US$5 million. Not all exchanges today are digital; a fairly significant number of exchanges some 30% to 40% in the United States, for example are analog, so not every locale can get ISDN services. But if you do have an exchange and the proper ISDN software, you also need a Signaling System 7 (SS7) network and a CPE that is compatible with the ISDN network. (SS7 is discussed in detail in Chapter 5.)

Figure 7.10. ISDN components

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As discussed in Chapter 3, "Transmission Media: Characteristics and Applications," N-ISDN has two interfaces (see Figure 7.11): BRI and PRI.

Figure 7.11. N-ISDN network interfaces

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BRI is primarily used for residential service, small businesses, and centrex environments. BRI is offered only by local telcos, and how they offer it varies greatly. It can be configured as either 1B, 2B, 1B+D, or the full 2B+D; 128Kbps transmission requires that two B-channels be used. One application for this is Internet access, so you might have a BRI device, such as an ISDN modem, that would bond together two B-channels to provide the 128Kbps access to the ISP.

PRI is primarily used for business applications, and both local and interexchange carriers offer it. In support of the voice environment, PRI applications would include access to PBX and call center networks, replacement of existing analog trunk groups, and configuration of PBX tie-lines. Q-Sig, a standard that's an enhanced version of the D-channel signaling protocol, supports feature transparency between different vendors' PBXs. A key data application of PRI is LAN/WAN integration.

ISDN Applications

The following are the main applications for N-ISDN:

         Internet access You would use N-ISDN to increase the speeds otherwise supported by your analog voice-grade line, when you do not have available other broadband access options, such as DSL, cable modems, or broadband wireless, available.

         Remote access You would use N-ISDN to give teleworkers or telecommuters access to corporate resources.

         LAN/WAN connections As mentioned earlier, N-ISDN is a technique for LAN interconnection, so bridging multiple LANs across a WAN could be done over ISDN connections.

         High-capacity access You could use N-ISDN if you needed to increase your capacity for things such as graphics, file transfer, video, and multimedia networking. Keep in mind that N-ISDN will not provide motion as good as you would get with the higher-capacity services, but it will be much better than what you get with an analog facility.

         Private line backup You could use N-ISDN as a backup to the private line services discussed earlier (for example, DDS).

         Dial-up Frame Relay access Frame Relay is a very popular data networking option, particularly for LAN-to-LAN interconnection. You can use ISDN to provide measured-use dialup access to Frame Relay services in which the user dials in to a remote access port on the carrier's Frame Relay switch at either 64Kbps or 128Kbps connections. This can be used at smaller sites and for remote access (for example, for telecommuters).

         BRI 0B+D for packet data One 16Kbps D-channel can be shared by up to eight devices. BRI 0B+D makes use of 9.6Kbps of D-channel capacity to support low-speed data terminals. This requires a terminal adapter that encapsulates the user data in D-channel frames. Applications for BRI 0B+D include credit card terminals and automatic teller machines.

         ISDN DSL (IDSL) IDSL delivers full-duplex, dedicated data services; it does not support voice services. It is provided either on a 1B or a 2B configuration (that is, 64Kbps or 128Kbps). IDSL is compatible with existing digital loop carrier systems, which, as mentioned previously, serve 30% to 40% of the U.S. population. Digital loop carriers are especially common in remote rural areas. Some of the new DSL services (which are discussed in Chapter 13, "Broadband Access Solutions") are incompatible with these older-generation digital loop carriers. IDSL, however, is compatible with them, so it can be used to deliver digital private line service at speeds up to 128Kbps. In today's environment, 128Kbps is not what we strive for, so even though IDSL can facilitate some speedier communications, it is not as fast as some other broadband access options, making its future rather short.

 



Telecommunications Essentials
Telecommunications Essentials: The Complete Global Source for Communications Fundamentals, Data Networking and the Internet, and Next-Generation Networks
ISBN: 0201760320
EAN: 2147483647
Year: 2005
Pages: 84

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