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U. Glaeser

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for establishing and terminating connections between hosts across the network and for providing hostto-host<br />

flow control.<br />

The session layer is responsible for establishing and managing connections between two processes.<br />

Session establishment typically requires authentication, billing authorization, and agreement on a set of<br />

parameters that will be in effect for the session. The session layer is also responsible for recovery from a<br />

transport failure and for providing virtual circuit service if the transport layer does not do so.<br />

The presentation layer performs services that are commonly requested by users, such as text compression,<br />

code conversion, file formatting, and encryption.<br />

The application layer contains routines specific to a particular application.<br />

Networks<br />

LAN<br />

Local area network (LAN) is a privately owned network of up to a few kilometers in size. LANs are used<br />

to connect computers, workstations, and file servers, and attach printers and other devices. The restricted<br />

size of LANs allows for prediction of transmission time, and simplifies network management. Traditional<br />

LANs run at speeds of 10–100 Mbps and have low delay of tens of miliseconds.<br />

Broadcast topologies include bus and ring. In a bus network, at any instant one machine is the master<br />

and is allowed to transmit. At the same time, all other machines are required to refrain from sending.<br />

IEEE 802.3, which is the Ethernet, is a bus-based broadcast network with decentralized control operating<br />

at 10 or 100 Mbps. Computers on an Ethernet can transmit at any time. If two or more packets collide,<br />

each computer waits for a random time and tries again later.<br />

In a ring network, each bit propagates around on its own, not waiting for the rest of the packet to which<br />

it belongs. IEEE 802.5, which is the IBM token ring, is a ring-based LAN operating at 4 and 16 Mbps.<br />

WAN<br />

Wide area network (WAN) covers a large area, a country or a continent. The hosts in WAN are used to<br />

run application programs and are connected by a communication subnet, which consists of transmission<br />

lines and switching elements. A switching element, also called router, is used to forward packets to their<br />

destinations.<br />

Cellular Network<br />

The most widely employed wireless network topology is the cellular network. This network architecture<br />

is used in cellular telephone networks, personal communication networks, mobile data networks, and<br />

wireless local area networks (WLAN). In this network configuration, a service area, usually over a wide<br />

geographic area, is partitioned into smaller areas called cells. Each cell, in effect, is a centralized network,<br />

with a base station (BS) controlling all the communications to and from each mobile user in the cell.<br />

Each cell is assigned a group of discrete channels from the available frequency spectrum (usually radio<br />

frequency). These channels are in turn assigned to each mobile user, when needed.<br />

Typically, BSs are connected to their switching networks using landlines through switches. The BS is<br />

the termination point of the user-to-network interface of a wireless cellular network. In addition, the BS<br />

also provides call setups, cell handoffs and various network management tasks, depending on the type of<br />

network.<br />

TCP/IP Protocol<br />

Transmission control protocol/Internet protocol (TCP/IP) suite is used in the network and transport layers.<br />

TCP/IP is a set of protocols allowing computers to share resources across the network. Although the protocol<br />

family is referred to as TCP/IP, user datagram protocol (UDP) is also a member of this protocol suite.<br />

TCP/IP protocol suite used as network and transport layers has the following advantages:<br />

1. It is not vendor-specific.<br />

2. It has been implemented on most systems from personal computers to the largest supercomputers.<br />

3. It is used for both LANs and WANs.<br />

© 2002 by CRC Press LLC

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