Data Communications Networking Devices - 4th Ed.pdf

692 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTcredit card data, their use for interconnecting geographically separated local areanetworks has been replaced to a large degree by the use of Frame Relay networks.Since the Frame Relay Access Device FRAD) is the key device used to obtainaccess to a Frame Relay network, Section 6.6 is focused upon the operation andutilization of that networking device.6.6 FRAME RELAY ACCESS DEVICEA Frame Relay Access Device FRAD) can be considered to represent acombination protocol conversion and encapsulation product, enabling a variety ofLAN and WAN protocols to be transported over a Frame Relay network. Since thebasic function of a FRAD is to provide access into a Frame Relay network, allFRADs transmit and receive data via the use of Frame Relay frames to and from aFrame Relay network. Depending upon the type of internetworking a FRAD ismanufactured to support, it may encapsulate or convert LAN and WAN protocolsin or into Frame Relay frames. In addition, based upon other functions included inthe design of a FRAD, it may perform a variety of additional functions. Oneexample of a relatively new feature added to FRADs is the ability to support voiceover Frame Relay. To do so, most FRADs include frame fragmentation andpriority queuing as well as modules that digitize voice at very low data rates.Hardware overviewFrom a hardware perspective, a FRAD consists of a series of software modulesoperating in memory that control the data ¯ow to at least one LAN via a networkadapter card, and to a Frame Relay networkvia a serial port. Figure 6.45 illustratesthe use of a pair of FRADs to interconnect two geographically separated EthernetLANs via a Frame Relay network.In examining Figure 6.45 note that some FRADs are modular devices, enablingthe addition of LAN and WAN port modules that can be used to serve additionaldevices. For example, adding Token-Ring or ATM port modules enables theFRAD to provide connectivity to Token-Ring and ATM networks. Similarly, theuse of serial port modules may permit a FRAD to support SDLC, bisynchronous,and asynchronous data streams.Figure 6.45 Using Frame Relay Access Devices to interconnect geographically separatedLANs via a Frame Relay network