Biblio5

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Figure 18.7 B-ISDN/ ATM functional layering.

The primitives identified at the border between the PM and TC sublayers are a continuous flow of logical bits or symbols with this associated timing information.

18.6.1.1 Transmission Convergence Sublayer Functions. Among the important functions of this sublayer is the generation and recovery of transmission frame. Another function is transmission frame adaptation, which includes the actions necessary to structure the cell flow according to the payload structure of the transmission frame (transmit direction), and to extract this cell flow out of the transmission frame (receive direction). The transmission frame may be a cell equivalent (i.e., no external envelope is added to

Source: Based on Refs. 3 and 8.

the cell flow), an SDH/ SONET envelope, an E1/ T1 envelope, and so on. In the transmit direction, the HEC sequence is calculated and inserted in the header. In the receive direction, we include cell header verification. Here cell headers are checked for errors and, if possible, header errors are corrected. Cells are discarded where it is determined that headers are errored and are not correctable.

Another transmission convergence function is cell rate decoupling. This involves the insertion and removal of idle cells in order to adapt the rate of valid ATM cells to the payload capacity of the transmission system. In other words, cells must be generated to exactly fill the payload of SDH/ SONET, as an example, whether the cells are idle or busy.

Section 18.12.5 gives several examples of transporting cells using the convergence sublayer.

18.6.2 ATM Layer

Table 18.4 shows the ATM layering functions supported at the UNI (U-plane). The ATM layer is completely independent of the physical medium. One important function of this layer is encapsulation. This includes cell header generation and extraction. In the transmit direction, the cell header generation function receives a cell information field from a higher layer and generates an appropriate ATM cell header except for the HEC sequence. This function can also include the translation from a service access point (SAP) identifier to a VPI and VCI.

In the receive direction, the cell header extraction function removes the ATM cell header and passes the cell information field to a higher layer. As in the transmit direction, this function can also include a translation of a VPI and VCI into an SAP identifier.

In the case of the NNI, the GFC is applied at the ATM layer. The flow control information is carried in assigned and unassigned cells. Cells carrying this information are generated in the ATM layer.

In a switch the ATM layer determines to where the incoming cells should be forwarded, resets the corresponding connection identifiers for the next link, and forwards the cell. The ATM layer also handles traffic-management functions between ATM nodes on both sides of the UNI (i.e., single VP link segment) while the virtual channel identified by a VCI value c 4 can be used for VP level end-to-end (user ↔ user) management functions.

What are flows such as “F4 flows”? OAM (operations, administration, and management) flows deal with cells dedicated to fault and performance management of the total system. Consider ATM as a hierarchy of levels, particularly in SDH/ SONET, which are

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the principal bearer formats for ATM. The lowest level where we have F1 flows is the regenerator section (called the section level in SONET). This is followed by F2 flows at the digital section level (called the line level in SONET). There are the F3 flows for the transmission path (called the path level in SONET). ATM adds F4 flows for virtual paths (VPs) and F5 flows for virtual channels (VCs), where multiple VCs are completely contained within a single VP. We discuss VPs and VCs in Section 18.8.

18.6.3 ATM Adaptation Layer (AAL)

The basic purpose of the AAL is to isolate the higher layers from the specific characteristics of the ATM layer by mapping the higher-layer protocols data units (PDUs) into the payload of the ATM cell and vice versa.

18.6.3.1 Sublayering of the AAL. To support services above the AAL, some independent functions are required for the AAL. These functions are organized in two logical sublayers: (1) the convergence sublayer (CS), and (2) the segmentation and reassembly (SAR) sublayer. The primary functions of these layers are:

•SAR—The segmentation of higher-layer information into a size suitable for the information field of an ATM cell. Reassembly of the contents of ATM cell information fields into higher-layer information;

•CS—Here the prime function is to provide the AAL service at the AAL-SAP (service access point). This sublayer is service dependent.

18.6.3.2 Service Classification of the AAL. Service classification is based on the following parameters:

•Timing relation between source and destination (this refers to urgency of traffic): required or not required;

•Bit rate: constant or variable; and

•Connection mode: connection-oriented or connectionless.

When we combine these parameters, four service classes emerge, as shown in Figure 18.8. Examples of the services in the classes shown in Figure 18.8 are as follows:

•Class A: constant bit rate such as uncompressed voice or video;

•Class B: variable bit rate video and audio, connection-oriented synchronous traffic;

•Class C: connection-oriented data transfer, variable bit rate, asynchronous traffic; and

•Class D: connectionless data transfer, asynchronous traffic such as SMDS.

Note that SMDS stands for switched multimegabit data service. It is espoused by Bellcore and is designed primarily for LAN interconnect.

18.6.3.3 AAL Categories or Types. There are five different AAL categories. The simplest is AAL-0. It just transmits cells down a pipe. That pipe is commonly a fiberoptic link. Ideally it would be attractive that the bit rate here be some multiple of 53 × 8 bits or 424 bits. For example, 424 Mbps could handle 100 million cells per second.

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Figure 18.10 SAR-PDU format for AAL-3/ 4. (From ITU-T Rec. I.363, Figure 6/ I.363, p. 13 [Ref. 10].)

stamp extracted from CSI from cells with odd sequence numbers. The residual time stamp is transmitted over eight cells. It supports DS1, DS3, and E1 digital streams. Another mode of operation is structured data transfer (SDT). SDT supports an octetstructured n × DS0 service.

18.6.3.3.2 AAL-2. AAL-2 handles the variable bit rate (VBR) scenario such as MPEG (Motion Picture Experts Group) video. It is still in the ITU-T organization definitive stages.

18.6.3.3.3 AAL-3/ 4. Initially, in ITU-T Rec. I.363 (Ref. 10) there were two separate AALs, one for connection-oriented variable bit rate data services (AAL-3) and one for connectionless service. As the specifications evolved, the same procedures turned out to be necessary for both of these services, and the specifications were merged, to become the AAL-3/ 4 standard. AAL-3/ 4 is used for ATM transport of SMDS, CBDS (connectionless broadband data services, an ETSI initiative), IP (Internet protocol), and frame relay. AAL-3/ 4 has been designed to take variable-length frames/ packets and segment them into cells. The segmentation is done in a way that protects the transmitted data from corruption if cells are lost or missequenced. Figure 18.10 shows the cell format of an AAL-3/ 4 cell. These types of cells have only a 44-octet payload, and additional overhead fields are added to the header and trailer.9 These carry, for example, the BOM, COM, and EOM indicators [carried in segment type (ST)] as well as an MID (multiplexing identifier) so that the original message, as set up in the convergence sublayer PDU (CS PDU), can be delineated. The header also includes a sequence number for protection against misordered delivery. There is the MID (multiplexing identification) subfield, which is used to identify the CPCS (common part convergence sublayer) connection on a single ATM layer connection. This allows for more than one CPCS connection for a single ATM-layer connection.

The SAR sublayer, therefore, provides the means for the transfer of multiple, vari-

9A trailer consists of overhead fields added to the end of a data frame or cell. A typical trailer is the CRC parity field appended at the end of a frame.

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a given reference point, that share the same VPC. A specific value of VPI is assigned each time a VP is switched in the network. A VP link is a unidirectional capability for the transport of ATM cells between two consecutive ATM entities where the VPI value is translated. A VP link is originated or terminated by the assignment or removal of the VPI value.

Routing functions for VPs are performed at a VP switch/ cross-connect. This routing involves translation of the VPI values of the incoming VP links into the VPI values of the outgoing VP links. VP links are concatenated to form a VPC. A VPC extends two VPC endpoints or, in the case of point-to-multipoint arrangements, more than two VPC endpoints. A VPC endpoint is the point where the VCIs are originated, translated, or terminated. At the VP level, VPCs are provided for the purpose of user–user, user–network, and network–network information transfer.

When VPCs are switched, the VPC supporting the incoming VC links are terminated first and a new outgoing VPC is then created. Cell sequence integrity is preserved by the ATM layer for cells belonging to the same VPC. Thus cell sequence integrity is preserved for each VC link within a VPC.

Figure 18.14 is a representation of a VP and VC switching hierarchy, where the physical layer is the lowest layer composed of, from bottom up, a regenerator section level, a digital section level, and a transmission path level. The ATM layer resides just above the physical layer and is composed of the VP level; just above that is the VC level.

18.9 SIGNALING REQUIREMENTS

18.9.1 Setup and Release of VCCs

The setup and release of VCCs at the user–network interface (UNI) can be performed in various ways:

•Without using signaling procedures. Circuits are set up at subscription with permanent or semipermanent connections;

•By meta-signaling procedures, where a special VCC is used to establish or release a VCC used for signaling. Meta-signaling is a simple protocol used to establish and remove signaling channels. All information interchanges in meta-signaling are carried out via single cell messages;

•User-to-network signaling procedures, such as a signaling VCC to establish or release a VCC used for end-to-end connectivity; and

•User-to-user signaling procedures, such as a signaling VCC to establish or release a VCC within a preestablished VPC between two UNIs.

18.9.2 Signaling Virtual Channels

18.9.2.1 Requirements for Signaling Virtual Channels. For a point-to-point signaling configuration, the requirements for signaling virtual channels are as follows:

•One virtual channel connection in each direction is allocated to each signaling entity. The same VPI/ VCI value is used in both directions. A standardized VCI value is used for point-to-point signaling virtual channel (SVC).