Switch Modifications

The switch must make a forwarding decision for each packet that arrives and must forward the packet to the corresponding output port. Better look up algorithms can speed up the forwarding decision. New switch architectures speed up the forwarding of packets. Different applications have vastly different delay, throughput, and loss requirements. Switches are designed that can handle packets according to the requirements.

Circuit Switching

Circuit Switching is easy to implement relative to other schemes, but because a stream is assigned a fixed rate circuit, capacity utilization may be low if the data stream is bursty. It is therefore used in voice networks but not in networks designed for data transfer. The data stream originating at the source is divided into packets of fixed or variable size. The time interval between consecutive packets may vary , depending on the business of the stream. The switch routes the packet over one of its outgoing links. The packet remains queued in its buffer until the outgoing link becomes idle.

Network Elements

A communication network is a collection of network elements inter connected and managed to support the transfer of information from a user at one network location or node. The principal network elements are links and switches. A link transfers a stream of bits from one end to the other at certain rate with a given bit error rate and a fixed propagation time. Links are unidirectional. The most important links are optic fiber, copper coaxial cable, and micro wave or radio wireless links. Several incoming and outgoing links terminate at a switch, which is a device that transfers bits from its incoming links to its outgoing links.

Messages

Many user applications on a network are implemented by processes that exchange messages. When browsing the web, a user sends a server requests for web pages, audio or video clips or documents. The server replies by sending the requested records to the user. The message traffic generated by various user applications can have a wide range of characteristics. Some applications, such as email, generate isolated messages. Other applications such as a distributed computation generate long streams of messages. The rate of messages can vary greatly across applications and devices. The network must transfer the message with an acceptable delay and it can corrupt only a small fraction of the messages.

Variable Bit Rate

Some signal compression techniques convert a signal into a bit stream that has variable bit rate; MPEG2 is a family of standards for such variable bit rate compression of video signals. The bit rate is larger when the scenes of the compressed movie are fast moving than when they are slow moving. DBS uses MPEG2 with an average rate of 4 Mbps. The acceptable delay and BER of applications are similar to those of CBR applications.

Constant bit rate

To transmit a voice signal, the telephone network equipment first convert s it into a stream of bits with a constant bit rate of 64 Kbps. Some video compression standards convert a video signal into a bit stream with a constant bit rate. The rate of the compressed bit stream depends on the parameters selected for the compression algorithm such as the size of the video window, the number of frames per second, and the number of quantization levels. The end to end delay should be less than 200 ms for real time video and video conversations.