ATM Cells

ATM Cells

The asynchronous transfer mode makes use of fixed-size cells, which consist of a 5-

octet header and a 48-octet information field. There are several advantages to the

use of small, fixed-size cells. First, their use may reduce queuing delay for a highpriority

cell, as it waits less if it arrives slightly behind a lower-priority cell that has

gained access to a resource (e.g., the transmitter). Secondly, it appears that fixedsize

cells can be switched more efficiently, which is important for the very high data

rates of ATM. With fixed-size cells, it is easier to implement the switching mechanism

in hardware.

Header Format

Figure 11.4a shows the header format at the user-network interface. Figure 11.4b

shows the cell header format internal to the network, where the generic flow control

field, which performs local functions, is not retained. Instead, the virtual path

identifier field is expanded from 8 to 12 bits; this allows support for an expanded

number of VPCs internal to the network, to include those supporting subscribers

and those required for network management.

The generic flow control ,field does not appear in the cell header internal to the

network, but only at the user-network interface. Hence, it can be used for control

of cell flow only at the local user-network interface. The details of its application are

for further study. The field could be used to assist the customer in controlling the

flow of traffic for different qualities of service. One candidate for the use of this

field is a multiple-priority level indicator to control the flow of information in a service-

dependent manner. In any case, the GFC mechanism is used to alleviate shortterm

overload conditions in the network.

The virtual path identifier (VPI) constitutes a routing field for the network. It

is 8 bits at the user-network interface and 12 bits at the network-network interface,

allowing for more virtual paths to be supported within the network. The virtual

indicates user information-that is, information from the next higher layer. In this

case, the second bit indicates whether congestion has been experienced; the third

bit, known as the ATM-user-to-ATM-user (AAU) indication bit is a one-bit field

that can be used to convey information between end users. A value of 1 in the first

bit indicates that this cell carries network management or maintenance information.

This indication allows the insertion of network-management cells into a user's VCC

without impacting the user's data, thereby providing in-band control information.

The cell-loss priority (CLP) is used to provide guidance to the network in the

event of congestion. A value of 0 indicates a cell of relatively higher priority, which

should be discarded only when no other alternative is available. A value of 1 indicates

that this cell is subject to discard within the network. The user might employ

this field so that extra information may be inserted into the network, with a CLP of

1, and delivered to the destination if the network is not congested. The network may

set this field to 1 for any data cell that is in violation of an agreement concerning

traffic parameters between the user and the network. In this case, the switch that

does the setting realizes that the cell exceeds the agreed traffic parameters but that

the switch is capable of handling the cell. At a later point in the network, if congestion

is encountered, this cell has been marked for discard in preference to cells that

fall within agreed traffic limits.

Header Error Control

Each ATM cell includes an 8-bit header error control field (HEC) that is calculated

based on the remaining 32 bits of the header. The polynomial used to generate the

code is X^8 + x^2+ X + 1. In most existing protocols that include an error control

field, such as HDLC and LAPF, the data that serve as input to the error code calculation

are, in general, much longer than the size of the resulting error code; this

allows for error detection. In the case of ATM, the input to the calculation is only

32 bits, compared to 8 bits for the code. The fact that the input is relatively short

allows the code to be used not only for error detection but, in some cases, for actual

error correction; this is because there is sufficient redundancy in the code to recover

from certain error patterns.

Figure 11.5 depicts the operation of the HEC algorithm at the receiver. At initialization,

the receiver's error-correction algorithm is in the default mode for single-

bit error correction. As each cell is received, the HEC calculation and comparison

is performed. As long as no errors are detected, the receiver remains in error-correction

mode. When an error is detected, the receiver will correct the error if it is a

single-bit error or it will detect that a multi-bit error has occurred. In either case, the

receiver now moves to detection mode. In this mode, no attempt is made to correct

errors. The reason for this change is a recognition that a noise burst or other event

might cause a sequence of errors, a condition for which the HEC is insufficient for

error correction. The receiver remains in detection mode as long as errored cells are

received. When a header is examined and found not to be in error, the receiver

switches back to correction mode. The flowchart of Figure 11.6 shows the consequence

of errors in the cell header.

The error-protection function provides both recovery from single-bit header

errors, and a low probability of the delivery of cells with errored headers under

bursty error conditions. The error characteristics of fiber-based transmission sys-

tems appear to be a mix of single-bit errors and relatively large burst errors. For

some transmission systems, the error-correction capability, which is more timeconsuming,

might not be invoked.

Figure 11.7, based on one in ITU-T 1.432, indicates how random bit errors

impact the probability of occurrence of discarded cells and valid cells with errored

headers, when HEC is employed.

 

Transmission of ATM Cells

The ITU-T Recommendations for broadband ISDN provide some detail on the

data rate and synchronization techniques for ATM cell transmission across the

user-network interface. The approach taken for broadband ISDN is also used in

many other ATM networks.

The BISDN specifies that ATM cells are to be transmitted at a rate of 155.52

Mbps or 622.08 Mbps. As with ISDN, we need to specify the transmission structure

that will be used to carry this payload. For 622.08 Mbps, the matter has been left for

further study. For the 155.52-Mbps interface, two approaches are defined in 1.413:

a cell-based physical layer and an SDH-based physical layer. We examine each of

these approaches in turn.

Cell-Based Physical Layer

For the cell-based physical layer, no framing is imposed. The interface structure

consists of a continuous stream of 53-octet cells. Because there is no external frame

imposed on the cell-based approach, some form of synchronization is needed. Synchronization

is achieved on the basis of the header error control (HEC) field in the

cell header. The procedure is as follows (Figure 11.8):

1. In the HUNT state, a cell delineation algorithm is performed bit by bit to

determine if the HEC coding law is observed (i.e., match between received

HEC and calculated HEC). Once a match is achieved, it is assumed that one

header has been found, and the method enters the PRESYNC state.

2. In the PRESYNC state, a cell structure is now assumed. The cell delineation

algorithm is performed cell by cell until the encoding law has been confirmed

consecutively 6 times.

3. In the SYNC state, the HEC is used for error detection and correction (see

Figure 11.5). Cell delineation is assumed to be lost if the HEC coding law is

recognized as incorrect a times consecutively.

The values of a and 6 are design parameters. Greater values of 6 result in

longer delays in establishing synchronization but in greater robustness against false

delineation. Greater values of a result in longer delays in recognizing a misalignment

but in greater robustness against false misalignment. Figures 11.9 and 11.10

show the impact of random bit errors on cell delineation performance for various

values of a and 6. The first figure shows the average amount of time that the

receiver will maintain synchronization in the face of errors, with a as a parameter.

The second figure shows the average amount of time to acquire synchronization as

a function of error rate, with 6 as a parameter.

The advantage of using a cell-based transmission scheme is the simplified

interface that results when both transmission- and transfer-mode functions are

based on a common structure.