Asynchronous Transfer Mode (ATM)

ATM is a connection-oriented transfer mode. Before sending cells that carry user data, a virtual connection between source and destination has to be established. All packets of a connection follow the same path within the network. During the connection setup each switch generates an entry in the Virtual Path Identifier (VPI) / Virtual Channel Identifier (VCI) translation table. This enables the switch to move an incoming packet from its VP / VC to corresponding outgoing VP / VC. As an advantage, this kind of routing requires a smaller header. Just a locally valid address (i.e. VPI / VCI) has to be carried in the packet.

ATM transmits, switches and multiplexes information in fixed-length cells. The length of a cell is 53 bytes, consisting of a 5-byte cell header and 48 bytes of data (fig. 1).

Fig. 1 ATM Cell

Header Format

The ATM header contains information about destination, type and priority of the cell.

The Generic Flow Control (GFC) field allows a multiplexer to control the rate of an ATM terminal. The GFC field is only available at the User-to-Network Interface (UNI). At the Network-to-Network Interface (NNI) these bits belong to the Virtual Path Identifier (VPI).

The Virtual Path Identifier (VPI) and the Virtual Channel Identifier (VCI) hold the locally valid relative address of the destination. These fields may be changed within an ATM switch.

The Payload Type (PT) marks whether the cell carries user data, signaling data or maintenance information.

The Cell Loss Priority (CLP) bit indicates which cells should be discarded first in the case of congestion.

Finally, the Header Error Control (HEC) field is to perform a CRC check on the header data. Only the header is error checked in the ATM layer. Error check for the user data is left to higher layer protocols and is performed on an end-to-end base.

Table 1 ATM cell header format (UNI)

Table 2 ATM cell header abbreviations

ATM Networks are thought to transmit data with varying characteristics. Different applications need various Qualities of Service (QoS). Some applications like telephony may be very sensitive to delay, but rather insensitive to loss, whereas others like compressed video are quite sensitive to loss.

The ATM Forum specified several Quality of Service (QoS) categories:

• CBR (Constant Bit Rate)
• rt-VBR (real-time Variable Bit Rate)
• nrt-VBR (non-real-time Variable Bit Rate)
• ABR (Available Bit Rate)
• UBR (Unspecified Bit Rate)

Table 4 QoS abbreviations

During a connection setup CBR reserves a constant amount of bandwidth. This service is conceived to support applications such as voice, video and circuit emulation, which require small delay variations (jitter). The source is allowed to send at the negotiated rate any time and for any duration. It may temporarily send at a lower rate as well.

VBR negotiates the Peak Cell Rate (PCR), the Sustainable Cell Rate (SCR) and the Maximum Burst Size (MBS). VBR sources are bursty. Typical VBR sources are compressed voice and video. These applications require small delay variations (jitter). The VBR service is further divided in real-time VBR (rt-VBR) and non-real-time VBR (nrt-VBR). They are distinguished by the need for an upper bound delay (Max CTD).

MaxCTD is provided by rt-VBR, whereas for nrt-VBR no delay bounds are applicable.

ABR and UBR services should efficiently use the remaining bandwidth, which is dynamically changing in time because of VBR service. Both are supposed to transfer data without tight constraints on end-to-end delay and delay variation. Typical applications are computer communications, such as file transfers and e-mail.

UBR service provides no feedback mechanism. If the network is congested, UBR cells may be lost.