Intermediate Devices in a Network

In a packet switched network, and in store and forward networks we have special purposes boxes, variously called:

Bridges - these connect LANs, of possibly different Media Access and/or physical technology.
Switches - these connect multiple communications links, and have a virtual or logical circuit model of what is connected to what.
Gateways- these are devices that translate one protocol stack or layer to another.
Router - an intelligent device like a switch, but that maintains no state between one packet and the next except that needed to carry out the forwarding function.
Relays - this is the ISO term for all of the devices above.

These will normally run real time operating systems, and perform many of the same functions as general purpose end hosts, except for the upper levels which are not required in intermediate nodes, except in so far as they may be needed for example by network management applications. There is often special hardware support to facilitate communications. e.g.:

Clever Interfaces
Clever Buses (e.g. binary tree buses or even no buses).
Specialized use of MMUs.

Depending on the architecture of the network, and the choice of end host protocols, these intermediate nodes support different complexities of protocols, usually ranging from connectionless to connection oriented. By way of example, the DoD Internet Datagram protocol is a commonly used format for a packet, used on LANs (Ethernets) and WANs (the Internet). It is shown in figure #fnip#1376>.

#figure1377#
Figure: Example Internet Datagram

There are many common internet protocols, of which IP, illustrated above, and Novell IPX are most wideley used. THe key fields in the packet header are the source and destination addresses which allow the packet to be forwarded along the way (just as addresses on envelopes permit postal sorting offices to forward letters), and the protocol field which indicates what upper layer service is being carried by the packet (i.e. what is in the data part of the packet!). The other fields are largely concerned with internal book-keeping. The order of bits and bytes must be unambiguously specified too. The order of transmission of the header and data described in this book is resolved to the octet level. Whenever a diagram shows a group of octets, the order of transmission of those octets is the normal order in which they are read in English. For example, in the following diagram (#fnbandb#1380>) the octets are transmitted in the order they are numbered.

#figure1381#
Figure: Transmission Order of Bytes

Whenever an octet represents a numeric quantity the left most bit in the diagram is the high order or most significant bit. That is, the bit labeled 0 is the most significant bit. For example, the following diagram represents the value 170 (decimal).

#figure1384#
Figure: Significance of Bits

Similarly, whenever a multi-octet field represents a numeric quantity the left most bit of the whole field is the most significant bit. When a multi-octet quantity is transmitted the most significant octet is transmitted first. This is the simplest possible form of presentation syntax.