Handbook of Local Area Networks, 1998 Edition:LAN Interconnectivity Basics
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LAN-based applications and services generate bursty traffic. These traffic bursts are random and can vary from a series of small packets (e.g., a simple remote procedure call) to a series of large file transfers (e.g., a remote load of a network based application). The new requirements for LAN-to-LAN connections can be summarized as follows:
• Provide high WAN transmission speeds, not to transmit large continuous data volumes but to reduce response time.
• Provide a mechanism to better use the WAN transmission facilities when large LAN-based traffic bursts are not occurring.
• Handle the many different protocols (e.g., Transmission Control Protocol/Internet Protocol (TCP/IP), Internetwork Packet Exchange (IPX), Apple Talk, DECnet, and SNA) that require simultaneous WAN transport.
Frame relay operates at the physical and the lower half of the link layers. The original frame relay specification is a derivative of the development work done for the Integrated Services Digital Network (ISDN) and is based on the 1988 International Telephone and Telegraph Consultative Committee (CCITT) I.122 Framework for Providing Additional Packet Mode Bearer Services documents. The original standards specify both permanent and switched virtual circuits (PVCs and SVCs) at speeds up to T3 and E3. However, there were some design constraints at the beginning and the pioneering vendors selected a subset of the specifications, consisting of PVCs at rates to T1, for initial product development.
Current frame relay is over PVCs on a frame switching fabric implemented in private networks and by the public digital carrier services. One digital link terminating at the customer premise can carry multiple PVCs. This packet-based multiplexing scheme has the advantage of providing bandwidth to multiple destinations through a single access line. The underlying frame relay specifications allow any of the PVCs to user any percentage of the available bandwidth on the digital access line, depending on the available bandwidth at any given time and the configuration of the PVC.
The frame relay carrier services are defined by several parameters. The data link circuit identifiers (DLCI) are assigned in pairs and uniquely define the PVC. The certified interface rate (CIR) is the guaranteed or minimum bandwidth allocated to any given PVC. Burst mode operation over a frame relay network allows a PVC to support data traffic bursts at rates up to access line speed provided no other PVCs require simultaneous access. Frame relay as a carrier-based service has the following tangible benefits for LAN-to-LAN connection:
• Higher interface speeds, eliminating the bandwidth constraints that exist with such older packet switching protocols as X.25.
• The statistical nature of the frame relay network provides a service with efficient bandwidth use.
• Frame relay networks rely on end-to-end connections, optimizing user response time by not performing error checking on interim hops.
BRIDGES
The fundamental difference between repeaters and bridges is the protocol layer at which they perform internetworking. Bridges operate at the link layer of the protocol stack, and therefore have a very different set of connection attributes. Functionally a bridge can be thought of as a selective repeater, that is, the connected LAN segments logically form one large segment but only traffic destined for a remote segment is forwarded across the bridge.
Bridges do not need to conform to the same configuration rules as repeaters. Bridges and repeaters can be intermixed to meet specific network requirements. A very important advantage the bridge gains over the repeater by operating in the link layer is that the specific CSMA/CD and ring passing functions are restricted to each LAN segment. This difference eliminates the propagation delay restriction incurred in the repeated environment. Also, by operating in the link layer bridges can set up filtering on specific link layer addresses or link layer attributes. Link layer bridging is actually subdivided into two separate sublayers: the logical link controls (LLC) and media access control (MAC).
The LLC functions define access points to higher level services, perform frame sequencing, and issue connection oriented acknowledgments. The MAC layer handles the well-oriented interface to the physical media (i.e., twisted pair or fiber) and provides structured link layer addressing. Before bridges are discussed further the concepts of Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) MAC layer protocols are introduced.
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