Asynchronous Transfer Mode (ATM) – Lecture 1 Notes
Asynchronous Transfer Mode (ATM) is a standard developed by the ATM Forum for transfer of
data, voice and video over SONET (Synchronous Optical Network).
In its early years, ATM was a brainchild of IEEE (Institute of Electrical and Electronics Engineers) and
was predominantly only used within the IEEE, but it was from the 1985 – 1990 was when ATM was
re-created for commercial use.
The inspiration for ATM came from the fact that Time Division Multiplexing (TDM) wasted
bandwidth when there was no data to transfer in a particular time slot. This is explained with the
diagram below. From the figure, we observe that if there is no data available to transmit (on data
lines), then that time slot is not filled by the next data. Instead that time slot is left free leading to
Figure 1 : Time Division Multiplexing
ATM is an improvement to Time Division Multiplexing in the sense that if there is no data to be
transmitted in a time slot, the next available data is transmitted instead. ATM ARCHITECTURE :
ATM transmits data in the form of small size cells. The payload is 48 bytes long and the overhead is
5 bytes thus making the cell length to 53 bytes. This small size of cells helps to provide better QoS
(Quality of Service).
In an ATM architecture, the fibre links between devices within networks and between networks too
are called virtual circuits. A group of fibres or virtual circuits are called Virtual Paths. Virtual paths
In ATM, Virtual Paths (VP) & Virtual Circuits (VC) transfer data. All VPs and VCs are numbered by
every switch and this information is stored in a local database register on the switch. This
information is however valid only locally and the numbering keeps changing with each switch.
The 5 Bytes ATM cell header format is as shown in Figure 2. Here, the Virtual Path Identifier (VPI)
occupies 1 Bytes which is 8bits of the header. 2 = 256 unique IDs can be generated for 256 virtual
paths. The Generic Flow Control (GFC) uses 4bits and is used only when data is transmitted from an
ATM network to user terminals (outside ATM network). Since switches within networks are more
robust than user terminals, GFC is not required for data transmission within ATM networks. But
GFC is required for flow control during transmission between ATM networks and user terminals.
GFC makes sure that the user terminals do not overload with data coming from ATM networks.
When the GFC field is not required, VPI occupies the 4 bits instead. This then makes the VPI 12 bits
i.e. 2 = 4096 unique IDs for virtual paths.
Figure 2 : ATM Cell Header Format
In figure 2, Virtual Channel Identifier occupies 2 bytes = 16bits i.e. 2 = 65536 unique IDs for virtual
circuits. Considering that communication takes place only within an ATM network, this architecture can
support 2 × 2 = 268435456 unique connections available for data transmission!
Two types of connections are used by an ATM network : Switched Virtual Circuits & Permanent
Switched Virtual circuits are used when the demand arises for data transfer. These circuits are
switched off and are not available when not required. Permanent Virtual Circuits are made
available all the time. Examples are Emergency Lines or Direct Lines from the President’s Office.
Distribution of ci