Introduction

Telephone companies all over the world are faced with a very fundamental problem: MULTIPLE NETWORKS. Each of the networks like Telex, SMDS, DQDB, SSN, etc. use different kinds of switching mechanisms. Maintaining all these separate networks is a major headache. The solution is to invent a single new network to replace the entire telephone system and all the specialized networks with a single integrated service network for all kinds of information transfer. This new wide area service is called B-ISDN (Broadband Integrated Service Digital Network). The underlying technology that makes B-ISDN possible is called ATM (Asynchronous Transfer Mode) because it is not synchronous (tied to a master clock), as most telephone lines are. The basic idea behind ATM is to transmit all information in small, fixed-size packets called CELLS. Each cell is 53 byte long, of which 5 bytes are header and 48 bytes are payload, as shown in Figure 1.

  

Figure 1: An ATM Cell

The Cell Switching technology used in ATM is highly advantageous and is a gigantic break with the 100-year old tradition of circuit switching within the telephone system. Cell switching is highly flexible and can handle both constant rate traffic (video, audio) and variable rate traffic (data) easily. Cell switching can provide broadcasting needed for television transmission while circuit switching cannot provide this facility.

ATM networks are connection oriented. Making a call requires first sending a message to set up the connection. After that, subsequent cells follow the same path to the destination. Cell delivery is not guaranteed, but their order is. If cells 1 and 2 are sent in that order, then if both arrive, they will arrive in that order, never first 2 then 1.

ATM networks are organised like traditional WANs, with lines and switches (routers). The intended speeds for ATM networks are 155 Mbps and 622 Mbps. The 155.52 Mbps speed was chosen for making it compatible with AT&T's SONET transmission system. The 622 Mbps speed was chosen so that 4 155 Mbps channels could be sent over it.

The ATM Forum, (http://www.atmforum.com/) is an organisation which is standardizing the ATM architecture.