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LAN Internetworking is the networking of two or more LANs, which may be similar or dissimilar in nature. The LANs may be in close proximity, or they may be geographically distant. Equipment used for LAN Internetworking includes that discussed above, plus a few other options. Specifically, the range includes bridges, routers, gateways, PADs (X.25), and TAs (ISDN). In a Frame Relay environment, FRADs (Frame Relay Access Devices) perform as the functional equivalent of PADs in the X.25 world.
WAN option for internetworking geographically distant LANs, include dialup options such as X.25 Packet Switching, ISDN, and Switched 56/64 Kbps. Dedicated leased-line options include DDS, T-carrier, and Fractional T1. Recent and developing fast packet network services include Frame Relay, SMDS (Switched Multimegabit Data Service), and ATM (Asynchronous Transfer Mode). These services are available from a wide variety of vendors, including LECs, IXCs, and specialized data carriers.
These provide the means by which the LANs can interconnect, at some level, including resolving issues of data formatting, addressing, and sequencing, as well as flow control and error control. The internetworking protocols which will be addressed include TCP/IP, XNS, SPX/IPX, and APPN.
TCP/IP (Transmission Control Protocol/Internet Protocol) integrates dissimilar systems and networks, operating at OSI Layers 3 and 4. TCP/IP initially was developed for the U.S. Department of Defense for use in ARPANET, which became standardized as X.25. Because early ARPANET protocols were subject to frequent network crashes, a new set of core protocols were proposed in 1974 by Vinton G. Cerf and Robert E. Kahn; that set of design protocols became the foundation for the development of the TCP/IP protocol suite. The U.S. Department of Defense stimulated development by funding Bolt, Beranek, and Newman (BBN) to implement TCP/IP for UNIX operating systems and the University of California at Berkeley to incorporate the BBN code into the Berkeley UNIX variation [9-20].
The TCP/IP protocol suite to bring together disparate networks, independent of host hardware, operating systems, transmission media and data link technologies. The resulting internet is transparent to the user, appearing as a single network. Additionally, TCP/IP is capable of supporting internetworking in the face of high error rates, as well as node or link failures [9-20].
TCP/IP has become widely popular as an internetworking protocol, as it is effective, inexpensive, and well-understood. TCP/IP remains a foundation of the Internet, and enjoys wide usage in all variety of networks and internetworking applications. The fact that the TCP/IP code is in the public domain encourages its use and further development, which continues to this day. The TCP/IP protocol suite is divided into two distinct protocol layers, TCP and IP.
Transmission Control Protocol (TCP) is a virtual circuit protocol, managing the flow of user application data and providing translations of application names to lower-level addresses. TCP supports reliable, peer-to-peer communications on a connection-oriented basis. TCP accepts the data stream, segments it, and passes it to IP for routing. On the receiving end of the data transfer, TCP accepts the segmented data from IP, resolves error conditions, resequences the data segments, and passes them to the target device [9-20] and [9-21].
Internet Protocol (IP) is the protocol layer that defines the nature of the format of the data (packets, or datagrams) and the routing of the data by IP address. The IP element of the TCP/IP protocol suite accepts the segmented data from TCP, routes it, resolves error conditions, and presents the data to TCP for resequencing. IP is a best effort protocol, meaning it provides no assurance of deliveryit is the responsibility of TCP to detect errored or lost packets and to effect resolution of such conditions [9-20] and [9-21].
XNS (Xerox Networking System) originally was developed for integrating Xerox office systems. Each packet is routed like a datagram in an X.25 network, with each packet routed independently and over the best available link.
SPX/IPX (Sequenced Packet Exchange/Internet Packet Exchange), Novells derivative of XNS, using echo protocol and error protocol. SPX/IPX is the defined interface for Novell NetWare.
APPN (Advanced Peer-to-Peer Networking) is an IBM protocol that allows routing of LAN traffic, independent of a Front End Processor (FEP). Supporting X.25, Token Ring and other network protocols, APPN is an integral part of SNA and SAA, and provides a transition toward routers, rather than IBMs traditional approach.
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