Frame Relay uses a common error checking mechanism known as the Cyclic Redundancy Check (CRC). The CRC compares two calculated values to determine whether errors occurred during the transmission from source to destination. Frame Relay reduces network overhead by implementing error checking rather than error correction. Because Frame Relay is typically implemented on reliable network media, data integrity is not sacrificed because error correction can be left to higher-layer protocols, such as OSPF, which runs on top of Frame Relay.
The Local Management Interface (LMI) is a set of enhancements to the basic Frame Relay specification. The LMI offers a number of features (called extensions) for managing complex internetworks. Some of the key Frame Relay LMI extensions include global addressing and virtual circuit status messages (see Figure 1-13).
Switched Virtual Circuits (SVCs)
SVC technology is the newest kid on the block, and MCI was the first carrier to offer SVCs to customers via their Hyperstream Frame Relay network. SVCs, unlike PVCs, are set up and torn down on-the-fly as needed. Through this capability, SVCs are able to save organizations thousands of dollars a month in service charges when compared to PVCs. When used as a true bandwidth on demand, service router capacity and management is conserved. This is done by putting one entry for each router in its routing table, which allows the SVC to do the rest. For additional information refer to http://www.mci.com/.
Point-to-Point Protocol (PPP)
PPP is an encapsulation protocol for transporting IP traffic over point-to-point links. It provides a method for transmitting packets from serial interface to serial interface. PPP also established a series of standards dealing with IP address management, link management, and error checking techniques. PPP supports these many functions through the use of Link Control Protocol (LCP) and Network Control Protocols (NCP) to negotiate optional configuration parameters.
Asynchronous Transfer Mode (ATM)
ATM was originally developed to support video, voice, and data over WANs. ATM was developed by the International Telecommunications Union Telecommunication Standardization Sector (ITU-T). ATM has also been referred to as Broadband ISDN or B-ISDN.
ATM is a cell-switching and multiplexing technology that provides flexibility and efficiency for intermittent traffic, along with constant transmission delay and guaranteed capacity.
An ATM network consists of an ATM switch and endpoints that support the LAN Emulation (LANE) technology. LANE uses an ATM device to emulate a LAN topology by encapsulating the packet in an Ethernet or Token Ring frame when going from media to media. Essentially, LANE enables an ATM device to behave as if it were in a standard LAN environment. LANE supports all versions of Token Ring and Ethernet but currently is not compatible with FDDI. The support for these technologies is possible because these protocols use the same packet format regardless of link speed.
ATM can be configured to support either PVCs or SVCs. PVCs provide for a point-to point-dedicated circuit between end devices. PVCs do not require a call set up or guarantee the link will be available but are more manual in nature and require static addressing than SVCs. SVCs, however, are dynamically allocated and released. They remain in use only as long as data is being transferred. SVCs require a call set up for each instance of the circuit s connection. The switched circuits provide more flexibility and efficiency; however, they are burdened by the overhead associated with the call set up, in terms of the extra time and configuration. Figure 1-14 illustrates a typical ATM network.
Integrated Systems Digital Network (ISDN)
ISDN is defined by ITU-T Standards Q.921 and Q.931. The Q.921 specification requires the user to designate a network interface that is needed for digital connectivity. The Q.931 determines call setup and configuration. ISDN components include the following:
It is important to point out that there is specialized ISDN equipment known as terminal equipment type 1 (TE1). All other equipment that does not conform to ISDN Standards is known as terminal equipment type 2 (TE2). TE1s connect to the ISDN network through specialized cables. TE2s connect to the ISDN network through a terminal adapter.
Another ISDN device is the network connection type network termination type 1 or 2 devices. These termination devices connect the specialized ISDN cables to normal two wire local wiring.
ISDN reference points define logical interfaces. Four reference points are defined:
Figure 1-15 illustrates the various devices and reference points found in ISDN implementations, as well as their relationship to the ISDN networks they support.
The ISDN Basic Rate Interface (BRI) service provides two B channels and one D channel. The BRI B-channel service operates at 64Kbps and carries data, while the BRI D-channel service operates at 16Kbps and usually carries control and signaling information.
The ISDN Primary Rate Interface (PRI) service delivers 23 B channels and one 64Kbps D channel in North America and Japan for a total bit rate of up to 1.544Mbps. PRI in Europe and Australia carry 30 B channels and 1 D channel for a total bit rate of up to 2.048Mbps.
The ISDN network layer operation involves a series of call stages that are characterized by specific message exchanges. In general, an ISDN call involves call establishment, call termination, information, and miscellaneous messages.