322 DIGITAL BUILDING BLOCKS AND COMPUTER SYSTEMS
the packeting/framing methods, and the virtual circuit/connection services; responsible
for the transfer of data across the link; provides for the detection and correction of data-
transmission errors.- Network—Standardizes the way in which addressing is accomplished between linked
networks; performs networking functions and internetworking. - Transport—Handles the task of reliable message delivery and flow control between
applications on different stations; provides source-to-destination data integrity. - Session—Establishes two-way communication between applications running on different
stations on the network; provides the user interface into the transport layer. - Presentation—Translates data formats so that computers with different “languages” can
communicate; provides the syntax (rules) of representation of data between devices. - Application—Interfaces directly with the application programs running on the stations;
provides services such as file access and transfer, peer-to-peer communication among
applications, and resource sharing; provides support to process end users’ applications
such as electronic mail, database management, and file management.
Note that the interconnection channel is not a part of the OSI specification.
Network Topology
This deals with the geometrical arrangement ofnodes(endpoints consisting of physical devices
such as terminals, printers, PCs, and mainframes) interconnected bylinks(transmission channels).
Network topologies may be classified as bus topology (multidrop topology), star topology, ring
topology, tree topology, and distributed (mesh or hybrid) topology, as illustrated in Figure 6.4.2.
Bus topology is used predominantly by LANs, whereas star topology is commonly used by
private-branch exchange (PBX) systems. Ring topology may have centralized control (with one
node as the controller) or decentralized control (with all nodes having equal status). Tree topology
is used in most of the remote-access networks, whereas distributed topology is common in public
and modern communications networks. A fully distributed network allows every set of nodes to
communicate directly with every other set through a single link and provides an alternative route
between nodes.
The Internet is physically a collection of packet switching networks interconnected by
gateways along with protocols that allow them to function logically as a single, large, virtual
network. Gateways (often called IP routers) route packets to other gateways until they can
be delivered to the final destination directly across one physical network. Figure 6.4.3 shows
the structure of physical networks and gateways that provide interconnection. Gateways do not
provide direct connections among all pairs of networks. The TCP/IP is designated to provide a
universal interconnection among machines, independent of the particular network to which they
are attached. Besides gateways that interconnect physical networks, as shown in Figure 6.4.3,
Internet access software is needed on each host (any end-user computer system that connects to
a network) to allow application programs to use the Internet as if it were a single, real physical
network. Hosts may range in size from personal computers to supercomputers.Transmission Media
These, also known asphysical channels, can be eitherboundedorunbounded. Bounded media,
in which signals representing data are confined to the physical media, are twisted pairs of wires,