ELEMENTOS DE PRIORIZACIÓN

The ATM layer is concerned with the end-to-end transfer of information, i.e., from the transmitting end-device to the receiving end-device. Below, we summarize its main features.

Connection-oriented packet switching

The ATM layer is a connection-oriented packet-switched network. Unlike the IP network, an ATM end-device cannot transmit cells to a destination ATM end-device over an ATM network, without first establishing a virtual channel connection. Cells are delivered to the destination in the order in which they were transmitted.

A connection is identified by a series of VPI/VCI labels, as explained in section 4.2, and it may be point-to-point or point-to-multipoint. Point-to-point connections are bi- directional, whereas point-to-multipoint connections are unidirectional. Connections may

be either permanent virtual circuits (PVC) or switched virtual circuits (SVC). PVCs are set-up using network management procedures, whereas SVCs are set-up on demand using ATM signalling protocol procedures.

Fixed size cells:

In the ATM layer, packets are fixed-size cells of 53 bytes long, with a 48-byte payload and 5-byte header. The structure of the header was described in detail in section 4.2.

Cell switching

Switching of cells in an ATM network is done at the ATM layer. An example of the ATM stacks used when two end-devices communicate with each other is given in figure 4.6. Both end-devices run the complete ATM stack, that is, the physical layer, the ATM layer, the ATM adaptation layer (AAL), and the application layer. The ATM switches only need the physical layer and the ATM layer in order to switch cells. Different types of ATM switch architectures are described in Chapter 6.

No error and flow control

We recall that in the OSI model the data link layer provides error and flow control on each hop using the ARQ mechanism. In ATM networks, there is neither error control nor flow control between two adjacent ATM switches which are connected with a point-to- point link. A cell simply gets lost if it arrives at an ATM switch at a time when the switch experiences congestion. Also, it is possible that the ATM network may deliver a cell to a destination end-device with an erroneous payload.

The probability that a cell is delivered to the destination end-device with an erroneous payload is extremely small because of the high reliability of fiber-based transmission links. Typically, the probability that a bit will be received wrongly is over 10-8.

. Now, if we

End-device

Application Application

ATM switch End-device

ATM switch

Figure 4.6: Cell switching in an ATM network

assume that bit errors occur independently of each other, then the probability that the payload of an ATM cell, which consists of 48 bytes or 384 bits, will not contain errors is (1-10-8

)384

. Therefore, the probability that it will contain one or more erroneous bits is 1- (1-10-8

)384

,which is very low.

The cell loss rate is a quality-of-service parameter that can be negotiated between the end-device and the ATM network at set-up time. Different applications tolerate different cell loss rates. For instance, video and voice are less sensitive to cell loss than a file transfer. Cell loss rates typically vary from 10-3

to 10-6

. The ATM network guarantees the negotiated cell loss rate for each connection.

In the ATM standards there is a mechanism for recovering lost cells or cells delivered with erroneous payload, but this mechanism is only used to support the ATM signalling protocols, see SSCOP in Chapter 10. The recovery of the data carried by lost or corrupted cells is expected to be carried out by a higher-level protocol, such as TCP. We note that depending upon the application that created the data, it may not be necessary or there may not be enough time to recover such cells. For instance, it may be deemed unnecessary to recover lost cells when transmitting video over ATM.

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 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 the next Chapter, the ATM adaptation layer will break the IP PDU to 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.

Now, let us 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.

Addressing

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 10.5.

We note that ATM addresses are different to IP addresses. In view of this, when running IP over ATM, it is necessary to translate IP addresses to ATM addresses and vice versa. Address resolution protocols are discussed in Chapter 8.

Quality of service

Each ATM connection is associated with a quality-of-service category. Six different categories are provided by the ATM layer, namely, 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 do not have real-time constraints. This category can be used by frame relay, when it is carried over an ATM network. 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 which 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 may require a minimum guaranteed rate.

Each quality-of-service category is associated with a set of traffic parameters and a set of quality-of-service parameters. The traffic parameters are used to characterize the traffic transmitted over a connection, and the quality-of-service 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 quality-of-service for each connection.

The topic of quality of service in ATM networks is discussed in Chapter 7.

Congestion control

In ATM networks, congestion control permits the network operator to carry as much traffic as possible without affecting the quality of service requested by the users. Congestion control may be either preventive or reactive. In preventive congestion control, one prevents the occurrence of congestion in the network using a call admission algorithm (CAC) to decide whether to accept a new connection, and subsequently policing the amount of data transmitted on that connection. In reactive congestion control, one controls the level of congestion in the network by regulating how much the end-devices transmit through feedback messages. These two scheme are described in detail in Chapter 7.