Download (direct link):
THE ATM LAYER
Transmission frame generation and recovery
In frame-oriented transmission systems (such as SONET), TC generates frames at the senderís side by placing frame-related information and ATM cells into a well-defined frame structure. At the receiverís side, it recovers the frames and subsequently the ATM cells from the bit stream.
3.4.2 The Physical Medium-Dependent (PMD) Sublayer
The following are the main functions performed by this sublayer:
This is used to synchronize the transmitting and receiving PMD sublayers. It generates timing for transmitted signals; it also derives correct timing for received signals.
PMD can operate either on a bit-by-bit basis or with a group of bits (as in the 4B/5B and 8B/10B schemes). In the 4B/5B encoding scheme, each group of four bits is coded by a 5-bit code. In the 8B/10B scheme, each group of eight bits is coded by a 10-bit code. Coding groups of bits is known as block coding. Block coding requires more bandwidth than it effectively provides. For instance, FDDI uses 4B/5B block coding and runs at 125 Mbps, which gives an effective bandwidth of 100 Mbps. There are several benefits with block coding, such as bit boundary detection and exchange of control information. In addition, enhanced speed in the execution of the protocol is achieved, since it operates in chunks of bits.
3.5 THE ATM LAYER
The ATM layer is concerned with the end-to-end transfer of information. That is, it oversees functionality 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-switching network. Unlike the IP network, an ATM end device cannot transmit cells to a destination ATM end device over an ATM network unless a virtual channel connection is established first. 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 3.2). A connection can be either point-to-point or point-to-multipoint. Point-to-point connections are bidirectional, whereas point-to-multipoint connections are unidirectional. Connections can 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 signaling 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 3.2.
Switching of cells in an ATM network is done at the ATM layer. For an example of the ATM stacks that are used when two end devices communicate with each other, see Figure 3.7. Both end devices run the complete ATM stack: the physical layer, the ATM layer, the AAL, and the application layer. The ATM switches only need the physical layer and the ATM layer in order to switch cells.
No error and flow control
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 that are connected with a point-to-point link. If a cell arrives at an ATM switch when the switch is experiencing congestion, then the cell is simply lost, or perhaps is delivered to a destination end device with an erroneous payload.
Because of the high reliability of fiber-based transmission links, the probability that a cell is delivered to the destination end device with an erroneous payload is extremely small. Typically, the probability that a bit will be received wrongly is less than 10ó8. So, if we 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 ó 10-8)384. Therefore, the probability that it will contain one or more erroneous bits is 1 - (1 ó 10ó8)384, which is very low.
The cell loss rate is a QoS parameter that can be negotiated between the end device and the ATM network at setup time. Different applications tolerate different cell loss rates. For instance, video and voice are less sensitive to cell loss than is a file transfer. Cell loss rates typically vary from 10ó3 to 10ó6. 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. However, it is only used to support the ATM signaling protocols (see SSCOP in Chapter 5). 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. Depending upon the application that created the data, there might not be enough time to recover lost cells. Also, it might be deemed unnecessary to recover lost cells, such as when transmitting video over ATM.