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While Figure 8.5 presents only a single VPI/VCI for both data directions, ATM considers each direction independently. In addition, each value has local significance from the port only—thus the VPI/VCI of 0/67 could be used for the entire definition. This usage is highly recommended since it facilitates troubleshooting.

In Figure 8.5, the terms “client” and “server” relate to Layer 7 functions, not ATM services.

The incorporation of a virtual path is illustrated in Figure 8.6. Virtual path switching considers only the path (VPI) for switching decisions; the VCI value is ignored. This permits the creation of a single PVC to transport multiple VP/VC transfers.

FIGURE 8.6  ATM virtual path switching

ATM Adaptation Layer 5

The most common ATM adaptation layer in use for data services is AAL 5 (ATM adaptation layer 5). This adaptation layer defines the methodology used by ATM equipment for the transmission of data cells. The use of a SNAP (Sub-Network Access Protocol) header in the encapsulation is also specified.

There are two different ATM cell formats in use for all adaptation layers, including AAL 5. Connections between end nodes and switches are carried via UNI, or User-to-Network Interface; UNI defines the way that ATM devices communicate with each other. There are three current versions of the UNI specification—3.0, 3.1, and 4.0. Version 3.1 is found in most implementations at present. The UNI header and cell format is illustrated in Figure 8.7.

FIGURE 8.7  The ATM cell format (AAL 5, UNI)

For switch-to-switch links, the ATM specification calls for the use of the Network-to-Network Interface (NNI). It omits the GFC (Generic Flow Control) field, as shown in Figure 8.8. The following sections describe each of the fields found in the UNI and NNI specifications, which should provide a better overview of how these protocols operate in the ATM environment.

FIGURE 8.8  The ATM cell format (AAL 5, NNI)

Generic Flow Control

The Generic Flow Control (GFC) bits are found only in the UNI specification; they have not been implemented in an open standard. As a result, most switches set them to all zeros and ignore them on receipt. Flow control has been incorporated into the payload type field, described below.

Payload Type

The three bits of the payload type (PT) are used to differentiate between user data and maintenance data, although the VPI/VCI effectively directs this traffic to the proper destination. In addition, the PT field may be used for flow control, and it is used for end of message markers in AAL 5.

Connection Associated Layer Management information is referred to as F5 flow. Congestion information is also incorporated into this section, depending on the PTI coding bit values. The PTI coding (most significant bit first) is interpreted as shown in Table 8.2.

TABLE 8.2 PTI Coding

PTI Coding Definition

000 User data cell with no experienced congestion. The SDU (Service Data Unit) type is 0.
001 User data cell with no experienced congestion. The SDU type is 1.
010 User data cell with congestion experienced. The SDU type is 0.
011 User data cell with congestion experienced. The SDU type is 1.
100 Segment OAM F5 flow-related cell.
101 End-to-end OAM F5 flow-related cell.
110 Reserved.
111 Reserved.

Segment OAM cells are limited to switch-to-switch connections; the endto-end OAM cells include the router interfaces or end station. F4 type cells are used for virtual paths and use a VCI of 3; F5 type cells are used for virtual circuits and use a VCI of 4.

OAM is a powerful tool for the designer, as it provides visibility to the entire PVC. Unlike LMI in Frame Relay, this tool allows the router (or other ATM device) to detect faults in the ATM cloud—an area that typically remains veiled from the administrator. As a result, OAM-managed PVCs can detect a failure within seconds and immediately trigger failover to an alternate circuit. Without OAM, the network may appear to be functioning properly while discarding all cells.

Cell Loss Priority

The Cell Loss Priority (CLP) bit identifies the cell as eligible to be discarded when the bit rate is not reserved. There are a number of bit rates, including:

  Unspecified bit rate
  Available bit rate
  Variable bit rate—real-time
  Variable bit rate—non-real-time
  Constant bit rate

These bit rate settings correspond to the type of data in the cell. For example, voice traffic is considered constant bit rate (CBR), while data typically uses unspecified, available, or variable bit rates—UBR, ABR, and VBR, respectively.

Header Error Control

The Header Error Control (HEC) is responsible for validating the ATM header of the cell only. It does not provide CRC for the payload data. The HEC can handle most single-bit errors without requiring additional data or retransmission. However, the medium used in ATM and the error-free nature of the medium significantly reduce the potential for an error.


The payload portion of an AAL 5 cell is 48 bytes. Therefore, a 64-byte frame in Ethernet would require two cells in ATM, and since each cell must equal 53 bytes, the ATM cell is padded. This leads to some concerns in the networking arena that there is too much overhead in ATM when linking frame-based networks.

Designers should note that ATM does not provide error checking on the payload section of the cell; it leaves that responsibility to the upper-layer protocols.

Network Design in the Real World: Other ATM Adaptation Layers

There are five adaptation layers in the ATM specifications, although layers 3 and 4 are generally regarded as a single layer. AAL 1 is typically used for voice traffic, while AAL 2 is rarely used at all.

Due to their similarities, AAL 3 and 4 are frequently listed as AAL 3/4. Unlike AAL 5, AAL 3/4 incorporates a message identifier, a sequence number, and a cyclical redundancy check in each cell. This reduces the payload portion of the cell to 44 bytes.

Because of this overhead, there are some advantages to AAL 3/4. Receivers can reassemble cells based on the message identifier and the sequence number, which permits reconciliation of out-of-sequence frames. While this overhead is beneficial for connectionless configurations, it also results in a significant performance penalty. In addition to the added cell tax, or the overhead per cell, the segmentation and reassembly process is substantially more involved, which can lead to further delay. As a result, AAL 3/4 is not as popular as AAL 5.

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