Further Reading Summary The Network Layer

72 Communication Networks Copyright © 2005 PragSoft Data Actual user data. The Protocol ID field uniquely identifies the IPDU from other network packets. Segmentation is facilitated by the Segment Length and Segment Offset fields. The former is set to the length of the segment for this IPDU; the latter denotes the relative position of this segment from the beginning of the original NSDU. The Total Length field denotes the total length of the original NSDU. Corrupt and expired IPDUs are discarded immediately. This causes an error report being generated by the respective station, which is then returned back to the originating station. ISO 8473 can be used in a variety of arrangements. It can be used on top of X.25, or even directly on top of LAP-B.

4.6. Further Reading

General descriptions of the network layer, its sublayers, protocols, and standards are provided by Freeman 1989, Tanenbaum 1989, Stamper 1991, and Stallings 1994. Detailed accounts of internetworking, gateways, and routers can be found in White 1992 and Perlman 1992. Dhas and Konangi 1986 and Schlar 1990 describe X.25 and related standards. Black 1992, Feit 1992, Comer 1993, and Piscitello and Chapin 1993 describe IP in detail. Lynch and Rose 1993 is a handy reference on the Internet. Stallings 1990 and 1993b are useful sources on networking standards.

4.7. Summary

• The generic method for establishing a path for point-to-point communication in a network is called switching. There are two general switching methods: circuit switching and packet switching. • In circuit switching two communicating stations are connected by a dedicated communication path. • In packet switching communication is discrete in form of packets. The packets are handled by the intermediate nodes in a store-and-forward fashion. Packet siwtching is either based on virtual circuits or on datagrams . • The task of selecting a path for the transport of packets across the network is called routing. The three classes of routing algorithms are: flooding, static routing , and dynamic routing. www.pragsoft.com Chapter 4: The Network Layer 73 • Congestion control measures ensure that a network’s performance is not eroded due to overload conditions. • Internetworking facilitates the communication between two stations situated on different networks subnets. This requires protocol conversions and is handled by Interworking Units IWUs. • A router is a protocol converter which interconnects two networks of the same architecture. A gateway is a protocol converter which interconnects two networks of different architectures. • The network layer is subdivided into three sublayers to facilitate internetworking: the subnet access sublayer deals with the services of a single subnet, the subnet dependence convergence sublayer deals with service mapping between the subnets, the subnet independence convergence sublayer deal with internetworking services. • X.25 is a widely-known network layer standard. It provides no routing or switching functions, but rather an interface between DTEs and network DCEs.

X.25 supports three types of packets: data packets, control packets, and interrupt