Atm Introduction

" The Wise Way to The World of Communications"

ATM POSITIONING

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	INTRODUCTION

Before ATM (Asynchronous Transfer Mode) appeared, there were two different multiplexing techniques to transport several channels on the same physical links:

TDM: Time Division Multiplexing whereby, within the multiplex frame, well-defined time slots are assigned to each channel (see figure1). Time positioning locates the different user channels, which have to be synchronous with the multiplex aggregate channel. Accuracy and reliability of clock generation, recovery and distribution are critical to proper operation of TDM.

PM: Packet Multiplexing whereby user information is sent in packets marked with a unique channel identifier (represented by a color on figure1). The packets are of variable length and are delineated by flags. When no information is present no packet is sent; the link remains idle and can be used by another channel.

The two schemes are compared in the following table.

	TDM	PM (X25, FR, IP)	ATM template like
Error rate	+ Low to the high availability of the digital links.	-- Uncontrolled in case of IP & FR . Low error rate with error recovery (X25)	TDM
Latency	+ Low	-- High if network is loaded	TDM
Transit delay variation	+ Extremely low	-- Uncontrolled, unpredictable	TDM
Throughput	+ Large available range: 9.6Kbps to 2.4Gbps	--Restricted in maximum speeds: 2Mbs (X25), 45Mbps (IP and FR)	TDM
Throughput granularity	-- Low . 64 Kbps on T1/E1 links . 2Mbps on SONET/SDH . No granularity on T3/E3	+ High , thanks to Asynchronous transmission: as low as 2.4 Kbps	PM
Multiplexing Efficiency	-- Low	+ High	PM
flexibility	-- Static connection .No communication rerouting .Point to Point Channel	+ Dynamic with rerouting .Multicast channels .Point and Multipoint channels	PM
Transparency	+ Full transparency	-- Content dependent - optimized for data applications	TDM
Services integration	+ voice, data and video services (point to point)	-- Suitable for data mainly (although voice over FR and IP exists)	TDM
Security	+ Channel dedicated to a single user	-- Shared between several users raising security problems	TDM
Ubiquity	+ Worldwide Coverage via PTT networks	-- Much less coverage even with Internet and X25	TDM

In the previous table we have indicated the wish list for ATM requirements. ATM should take advantage of:

TDM, with its capacity to transport integrated services and media as required by the new information era where communications are based on multiple combined media (data, text, graphics, voice and video)
PM, with its multiplexing efficiency and communication flexibility.

To achieve this, ATM shall combine the "packet and virtual channel" advantages of PM, with the real time slots of TDM.
The solution is the cell concept , with fixed size packets transmitted regularly (with full or empty cells), and a cell header that identifies not only the destination but also the user's traffic characteristics (real time, throughput, transit delay, error rate, "best effort").
ATM multiplexing is compared with TDM and PM on the next figure.

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