4.2.2 ATM layer
The primary functions of the ATM layer are associated with the routing and switching of ATM cells. Because ATM cells are packets, the switches are packet switches and the switching operation can be called forwarding, but by convention, because the ATM layer provides a connection-oriented service, the term ‘forwarding’ is generally not used.
The path cells take and the resources allocated to them depend on their service category. This is determined when a virtual connection is established. The following service categories are recognised by the ATM layer:
Constant bit rate – a constant data rate is allocated and is continuously available for the duration of a connection.
Real-time variable bit rate – there are some commitments about the data rate to be made available, and the delay and variation in delay are tightly controlled.
Non-real-time variable bit rate – there are some commitments about the data rate to be made available, but no delay limits are placed on the delivery of cells.
Unspecified bit rate – there are no commitments about the data rate to be made available.
Available bit rate – the data rate made available may be changed during the time a connection is maintained.
Guaranteed frame rate – there is a commitment about the minimum data rate of a connection.
There are two types of virtual circuit – switched virtual circuits and permanent virtual circuits. The two types are similar in that they must be established before user data can be transferred; the difference is how they are set up. Switched virtual circuits are set up in response to user requests to transfer data and are released once that exchange has been completed. Permanent virtual circuits are set up by management activities in response to contracts established between users and are expected to last much longer than switched virtual circuits. The word ‘permanent’ may be misleading because permanent virtual circuits do change in a network, but they change relatively infrequently, and from the point of view of users they are always available. Both types of virtual circuit are controlled by functions in the control plane of the ATM reference model and are very important for the routing and switching of ATM cells.
I do not intend to go into the details of particular control protocols adopted by ATM. For our purposes here it is sufficient to understand that virtual circuits are established in response to set-up messages which contain the address of the destination. These connections are virtual connections, similar to TCP connections, but they take place at a lower level of abstraction. ATM switches examine the destination address in a set-up message and decide the best path to take for the service category intended for that connection. Each link in the path is identified by a virtual path identifier and a virtual channel identifier. Once a virtual circuit has been established, ATM cells carrying user data are switched according to their virtual path and virtual channel identifiers. For the purposes of switching, permanent virtual circuits are treated identically to switched virtual circuits.
The switching information in each ATM switch takes the form of a forwarding table, like the example in Table 10. For connection-oriented networks, a forwarding table is sometimes called a connection table and the term ‘forwarding’ is restricted to connectionless networks.
Table 10: ATM forwarding table
|Interface number||Virtual path identifier||Virtual channel identifier||Interface number||Virtual path identifier||Virtual channel identifier|