In this section I shall briefly review some of the main functions of the ATM layers but I shall not go into too much detail because at this stage we are interested in only the general points about protocols.
ATM physical layer
The ATM physical layer is divided into two sub-layers: the transmission convergence sub-layer and the physical medium sub-layer, (see Figure 22).
Functions of the transmission convergence sub-layer include generating and receiving cells, and generating and verifying the cyclic redundancy check in the header error control field. For correct interpretation of ATM cells it is important to identify the beginning of a cell. In theory, if ATM cells are transmitted as a continuous stream of bits, once a receiver has found the start of one cell, the start of the next cell starts 53 × 8 = 424 bits later. However, this still leaves the problem of identifying the start of the first ATM cell. The sending device inserts a cyclic redundancy check word in the header error control field in each ATM cell. A receiving device performs cyclic redundancy checks on 40 consecutive bits (five bytes) of the bit stream, so valid headers will pass this test. Of course, it is possible that any 40 bits may pass a cyclic redundancy check by chance. To avoid the possibility of misinterpretation, a receiver will check the header error control bytes of the next few cells before assuming that it has managed to synchronise with the arrival of ATM cells over a link. Unfortunately, because of mismatches in the timing of senders and receivers, it is possible to lose bits. Therefore, the receiver should still monitor the header error control field. If several consecutive cells fail the cyclic redundancy check, the receiver assumes that it has lost synchronisation over that link and starts again to look for ATM headers by examining 40 consecutive bits. This process is shown as a state diagram in Figure 27. The number of correct header error control checks that should occur before the receiver assumes synchronisation is expressed as and the number of incorrect checks that can occur before it assumes loss of synchronisation is expressed as .
Figure 27: ATM cell synchronisation
SAQ 15
Why is it not assumed that synchronisation is lost after a single header error control check failure, i.e. = 1?
Now read the answer
Answer
Header error control check failures are more likely to be caused by simple transmission errors of misinterpreting a 1 for a 0, or vice versa, rather than a timing error causing a loss or insertion of a bit. Therefore, it is sensible to discard the ATM cell that failed the check, but still assume for the time being that the link is synchronised.
The functions of the ATM physical medium sub-layer are associated with the transmission of bits over a specific physical medium. The main transmission medium of ATM is optical fibre, typically at data rates of approximately 155 Mbit/s or 622 Mbit/s.
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.
ATM adaptation layer
The basic function of the ATM adaptation layer is to convert the user data supplied by higher layers into 48-byte blocks of data. The ATM adaptation layer is divided into two sub-layers – the convergence sub-layer, and the segmentation and re-assembly sub-layer. The convergence sub-layer provides services to higher layers through a set of protocols, but I do not need to describe these here. The segmentation and re-assembly sub-layer separates the messages from the convergence sub-layer into ATM cells. Each of the two sub-layers adds some protocol information, which is transported in the payload of ATM cells as illustrated in Figure 28.
Figure 28: ATM adaptation layer functions (Source: based on Tanenbaum, 1996, Figure 6.38)
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