Title - An Introduction to the ATM Protocol

Connection Management

As in the phone system, ATM needs to have a way to establish a connection between two points and, after completing the session, a way to terminate the connection. The mechanism that the originating computer uses to inform the network that a connection needs to be established is called signaling.


When an ATM device wants to establish a connection with another ATM device, it sends a signaling-request packet to its directly connected ATM switch. This request contains the ATM address of the desired ATM endpoint, as well as any QoS parameters required for the connection.
ATM signaling protocols vary by the type of ATM link can be either User-Network Interface (UNI) signals or Network Node Interface (NNI) signals. UNI is used between an ATM end-system and ATM switch across ATM UNI, while NNI is used across NNI links.


The ATM Forum UNI 3.1 specification is the current standard for ATM UNI signaling. The UNI 3.1 specification is based on the Q.2931 public network signaling protocol developed by the ITU-T. UNI signaling requests are carried in a well-known default connection: VPI = 0, VPI = 5.
Standards currently exist only for ATM UNI signaling, but standardization work is continuing on NNI signaling.


ATM Connection-Establishment Process

ATM signaling uses a mechanism to do connection setup that is similar to the telephone network. An ATM connection setup proceeds in the following manner. First, the source end system sends a connection request which is propagated through the network and connections are set up through the network. The connection request reaches the final destination, which either accepts or rejects the connection request.


ATM Connection-Management Messages

A number of connection management message types, including Setup, Call Proceeding, Connect, and Release, are used to establish and tear down an ATM connection. The source end-system sends a Setup message (including the destination end-system address and any traffic QoS parameters) when it wants to set up a connection. The ingress switch sends a Call Proceeding message back to the source in response to the Setup message. The destination end system next sends a Connect message if the connection is accepted. The destination end system sends a Release message back to the source end system if the connection is rejected, thereby clearing the connection.

Connection-management messages are used to establish an ATM connection in the following manner. First, a source end system sends a Setup message, which is forwarded to the first ATM switch (ingress switch) in the network. This switch sends a Call Proceeding message and invokes an ATM routing protocol. The signaling request is propagated across the network. The exit switch (called the egress switch) that is attached to the destination end system receives the Setup message. The egress switch forwards the Setup message to the end system across its UNI, and the ATM end system sends a Connect message if the connection is accepted. The Connect message traverses back through the network along the same path to the source end system, which sends a Connect Acknowledge message back to the destination to acknowledge the connection. Data transfer then can begin.

Figure 1. Connection Establishment

  1. Source PC uses Q.2931 procedures (VPI=0/VCI=5) to setup an ATM connection. The local ATM switch receives the signaling message and passes it to the Connection Manager. The Connection Manager the level of services for that machine calculates the bandwidth required to setup this connection for the forward and reverse paths. If the bandwidth on either the reverse channel or the forward channel is not available, alternate paths are examined.

  2. The setup message is then sent back to the Network Switch to allocate the necessary links.

  3. The Network switch forwards the connection setup request to the destination machine.

  4. The Network switch receives the connection-accept message from the destination machine.

  5. Once the Network switch receives the connection-accept message from the called PC, it passes the information to the Connection Manager.

  6. The Connection Manager allocates the reverse channel capacity for the called party.

  7. Connection Manager informs the Local switch that the connection has been accepted.

  8. Connection Manager informs the Local switch that the connection has been accepted.

  9. Connection Manager confirms the connection to the destination PC.

  10. Connection Manager informs the source PC and data transmission can begin.

Types of Connections

Unlike a phone connection, data connections usually have several different types of connections that area established. Voice traffic is continuous, and in the network, samples of speech are transmitted every 125 micro-seconds (8,000 times a second).

ATM supports Quality of Service (QoS) which allows voice and video to be transmitted smoothly. Constant Bit Rate (CBR) guarantees bandwidth for real-time voice and video. Variable Bit Rate (VBR) allows time delivery guarantees but the amount of data at each interval may vary. Available Bit Rate (ABR) adjusts bandwidth according to congestion levels for LAN traffic. Unspecified Bit Rate (UBR) provides a best effort for remote users.

In order to support QoS delivery of cells through a network, network devices must implement algorithms that control when cells are transmitted. These mechanisms are comprised of traffic contract, traffic shaping, and traffic policing.


A traffic contract specifies an envelope that describes the intended data flow. This envelope specifies values for peak bandwidth, average sustained bandwidth, and burst size, among others. When an ATM end-system connects to an ATM network, it enters a "contract" with the network based on QoS parameters.


Traffic shaping is the use of queues to constrain data bursts, limit peak data rate, and smooth jitters so that traffic will fit within the promised envelope. ATM devices are responsible for adhering to the contract by means of traffic shaping. ATM switches can use traffic policing to enforce the contract. The switch can measure the actual traffic flow and compare it against the agreed-upon traffic envelope. If the switch finds that traffic is outside of the agreed-upon parameters, it can set the cell-loss priority (CLP) bit of the offending cells. Setting the CLP bit makes the cell "discard eligible," which means that any switch handling the cell is allowed to drop the cell during periods of congestion.

CBR (Constant Bit Rate)

The CBR service class is intended for real-time applications, i.e. those requiring tightly constrained delay and delay variation, as would be appropriate for voice and video applications. The consistent availability of a fixed quantity of bandwidth is considered appropriate for CBR service. Cells that are delayed beyond a value specified are assumed to be significantly less value to the application.

Figure 2. CBR Connection


VBR (Variable Bit Rate)

Real time VBR

The real time VBR service class is intended for real-time applications, i.e., those requiring tightly constrained delay and delay variation, as would be appropriate for voice and video applications. Sources are expected to transmit at a rate which varies with time. Equivalently the source can be described "bursty". Cells which are delayed beyond a value are assumed to be of significantly less value to the application. Real-time VBR service may support statistical multiplexing of real-time sources, or may provide a consistently guaranteed QoS.

Non-real time VBR

The non-real time VBR service class is intended for non-real time applications which have 'bursty' traffic characteristics and which can be characterized in terms of a GCRA. For those cells which are transferred, it expects a bound on the cell transfer delay. Non-real time VBR service supports statistical multiplexing of connections.

Figure 3. VBR Connection

ABR (Available Bit Rate)

Many applications have the ability to reduce their information transfer rate if the network requires them to do so. Likewise, they may wish to increase their information transfer rate if there is extra bandwidth available within the network. There may not be deterministic parameters because the users are willing to live with unreserved bandwidth. To support traffic from such sources in an ATM network will require facilities different from those for Peak Cell Rate of Sustainable Cell Rate traffic. The ABR service is designed to fill this need.


Figure 4. ABR Connection

UBR (Unspecified Bit Rate)

The UBR service class is intended for delay-tolerant or non-real-time applications, i.e., those which do not require tightly constrained delay and delay variation, such as traditional computer communications applications. Sources are expected to transmit non-continuous bursts of cells. UBR service supports a high degree of statistical multiplexing among sources.


UBR service includes no notion of a per-VC allocated bandwidth resource. Transport of cells in UBR service is not necessarily guaranteed by mechanisms operating at the cell level. However it is expected that resources will be for UBR service in such a way as to make it usable for some set of applications. UBR service may be considered as interpretation of the common term "best effort service".

Virtual Circuits

A circuit is a path through the network. For phone service, a circuit was an actual wire connection set up between two phones. Virtual Circuits embody the same idea of connecting two machines together having many attributes of having the security of a private line. It becomes a virtual circuit because there are many of these individual circuits sharing a common wire.


Figure 5. Virtual Circuits

Two main forms of these Virtual Circuits (VCs) are Switched Virtual Circuits (SVCs) and Permanent Virtual Circuits (PVCs).


PVCs are the simpler of the two types of Virtual Circuits because the path the network is established at some point in time and then the stations "turn on" the link or "turn off" the link. The PVC is defined only between two specific end points.


SVCs act more like a phone connection. When you enter the phone number (i.e. the address), the best path for that type of connection is computed and the requested bandwidth, delay, and jitter are guaranteed. You may enter any address (i.e. phone number) and each time you establish a connection, a new route is computed and the connection is not limited to specific end stations. While SVCs are more flexible, they require a lot of network information. For example, public and private networks need to share information about sources, destinations, capabilities and maps of network switches.

More Information

The ATMForum has specifications available at: www.atmforum.com.


Additional ATM seminars:

An Introduction to ATM - An overview of ATM, the types of connections, and a description of the various cell formats.

The ATM Adaptation Layers (AAL) - Large data packets are required to be segmented into the smaller ATM cells and later the ATM cells are reassembled back into packets. There are several different AALs that can be used based on the type of data being transported.


In Summary:

  • Connection Management requires the involvement of all elements along the network path.

  • There are four primary classes of Quality of Service (QoS): CBR, VBR, ABR, UBR.

  • There are two types of Virtual Circuits: PVCs and SVCs.


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