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When TCP/IP runs over ATM, the loss or corruption of the payload of a single cell results in the retransmission of an entire TCP PDU. In order to clarify this point, let us
End device ATM switch ATM switch End device
Figure 3.7 Cell switching in an ATM network.
THE ATM LAYER
assume that we want to send a single TCP PDU over an ATM network. This PDU will be encapsulated by IP and it will be passed on to the ATM network. (For simplicity, we assume no fragmentation of the IP PDU.) As will be seen in Section 3.7, the ATM adaptation layer will break the IP PDU into small segments, and each segment will be placed in the payload of an ATM cell. Let us assume that the IP PDU will be carried in n ATM cells. When these n cells arrive at the destination, their payloads will be extracted and the original IP PDU will be reconstructed, from which the TCP PDU will be extracted.
Assume that one of these n cells is either lost or its payload is corrupted. If this causes the IP header to get corrupted, then IP will drop the PDU. TCP will eventually detect that the PDU is missing and it will request its retransmission. On the other hand, if the cell in question causes the TCP PDU to get corrupted, then TCP will again detect it and it will request its retransmission. In either case, the loss of a cell or the corruption of the payload of a cell will cause the entire PDU to be retransmitted. Since this is not expected to happen very often, it should not affect the performance of the network.
Each ATM end device and ATM switch has a unique ATM address. Private and public networks use different ATM addresses; public networks use E.164 addresses and private networks use the OSI NSAP format. Details on ATM addresses are given in Section 5.5.
ATM addresses are different from IP addresses. Therefore, when running IP over ATM, IP addresses must be translated into ATM addresses, and vice versa (see Section 3.8 below).
Quality of service (QoS)
Each ATM connection is associated with a QoS category. Six different categories are provided by the ATM layer: constant bit rate (CBR), real-time variable bit rate (RT-VBR), non-real-time variable bit rate (NRT-VBR), available bit rate (ABR), unspecified bit rate (UBR), and guaranteed frame rate (GFR). The CBR category is intended for real-time applications that transmit at a constant rate, such as circuit emulation. The RT-VBR category is intended for real-time applications that transmit at a variable rate, such as encoded video and voice. The NRT-VBR category is for delay-sensitive applications that transmit at a variable rate but that do not have real-time constraints. For example, when frame relay is carried over an ATM network, it can use this category. The UBR category is intended for delay-tolerant applications, such as those running on top of TCP/IP. The ABR category is intended for applications that can vary their transmission rate according to how much slack capacity there is in the network. Finally, the GFR category is intended to support non-real-time applications that might require a minimum guaranteed rate.
Each QoS category is associated with a set of traffic parameters and a set of QoS parameters. The traffic parameters are used to characterize the traffic transmitted over a connection, and the QoS parameters are used to specify the cell loss rate and the end-to-end delay required by a connection. The ATM network guarantees the negotiated QoS for each connection. QoS for ATM networks is discussed more in Chapter 4.
In ATM networks, congestion control permits the network operator to carry as much traffic as possible without affecting the QoS requested by the users. Congestion control can be
either preventive or reactive. In preventive congestion control, network congestion can be prevented by using a call admission control (CAC) algorithm. CAC decides whether or not to accept a new connection; if accepted, then CAC polices the amount of data that is transmitted on that connection. In reactive congestion control, network congestion is managed by regulating how much the end devices transmit through feedback messages. These two schemes are described in detail in Chapter 4.
Various applications (such as voice, circuit emulation, and video) can run on top of AAL. Connection-oriented protocols (such as frame relay) and connectionless protocols (such as IP, signaling protocols, and network management protocols) also run on top of AAL.
3.6 THE ATM SWITCH ARCHITECTURE
The main function of an ATM switch is to transfer cells from its incoming links to its outgoing links. This is known as the switching function. A switch also performs several other functions, such as signaling and network management. A generic model of an ATM switch consisting of N input ports and N output ports is shown in Figure 3.8. Each input port can have a finite capacity buffer, where cells wait until they are transferred to their destination output ports. The input ports are connected to the output ports via the switch fabric. Each output port can also be associated with a finite capacity buffer, where cells can wait until they are transmitted out. Depending upon the structure of the switch fabric, there might be additional buffering inside the fabric. An ATM switch whose input ports are equipped with buffers is referred to as an input buffering switch (this is irrespective of whether or not its output ports have buffers). If an ATM switch only has buffers for its output ports, then it is referred to as an output buffering switch. Depending on the switch architecture, cell loss might occur at the input ports, within the switch fabric, or at the output ports.