Data Communications Networking Devices - 4th Ed.pdf

5.4 LAN SWITCHES _______________________________________________________________ 615signal. As the transmitting device delays further transmission, the switch's destinationport has the opportunity to empty the contents of its buffer, precluding theoccurrence of data loss.Although backpressure is an effective ¯ow control mechanism, its use requires asecond wire pair. This makes it mutually exclusive with full-duplex transmissionFDX) since workstations and servers directly cabled to switch ports neverencounter a collision and can operate in a FDX mode by using the collision wirepair for transmission in the opposite direction. If you require full-duplex transmissionand want to preclude the loss of frames via ¯ow control, you must turn tothe use of a server software module.Server software moduleSeveral switch vendors developed software that operates on Windows NT andNetWare servers that regulate the ¯ow of data between switch ports and thoseservers. To accomplish ¯ow control the switch transmits a prede®ned signal to themodule operating on the server, while a second signal is used to inform the moduleto resume transmission. The major difference between the use of backpressure andserver software modules is that the latter can provide support for full-duplextransmission.ATM considerationsAlthough ATM equipment has been gaining considerable momentum for use forwide area networks, its use in a LAN environment is primarily as a backbone forinterconnecting what many persons refer to as legacy LANs, such as Token-Ringand Ethernet, instead of as a replacement of those networks. The reason for the lackof ATM to the desktop is more than likely due to its cost, lack of multimediadesktop applications, and the fact that both Ethernet and Token-Ring LANs cancontinue to provide a reasonable level of performance by segmentation and connectionof segments via a high-speed backbone.The use of ATM switches as a backbone for connecting legacy LAN traf®cresults in a series of incompatibilities that must be overcome. The major incompatibilitiesbetween ATM and legacy LANs include their connection method,address, and transport mechanism. An ATM network uses a connection-orientedswitching mechanism to route traf®c in the form of 53-byte cells from source todestination using Virtual Path Identi®ers VPIs) and Virtual Channel Identi®ersVCIs). In comparison, legacy LANs are connectionless, transmitting variablelengthframes containing 48-bit MAC addresses to all stations on a network, withthe addressed station then reading the frame. To overcome these incompatibilities,the ATM Forum developed a speci®cation referred to as LAN Emulation LANE).LAN Emulation represents a method whereby ATM's connection-oriented infrastructureprovides a service which enables stations on legacy LANs to connect toother legacy stations as well as devices connected to ATM switches by mimickingor emulating the connectionless operation of legacy networks.