The ATM AAL 3/4 protocol provides for reliable end-to-end transmission of SAR an
ID: 3646049 • Letter: T
Question
The ATM AAL 3/4 protocol provides for reliable end-to-end transmission of SAR and CPCS PDUs, as well as multiplexing of multiple data streams on the same virtual connection, in ATM networks.(a) Assume that AAL 3/4 is being used and that the receiver is in the idle state. Two blocks of user data are then transmitted as two separate sequences of SAR PDUs. Given that the EOM SAR PDU of the first sequence is lost, will this error be detected at the receiving end and, if so, how will it be detected? If this error can be detected in more than one way, be sure to enumerate each detection method.
(b) Assume that AAL 3/4 is being used and that the receiver is in the idle state. Two blocks of user data are then transmitted as two separate sequences of SAR PDUs. Given that the EOM SAR PDU of the first sequence and the BOM SAR PDU of the second sequence are both lost, will this error be detected at the receiving end and, if so, how will it be detected? If this error can be detected in more than one way, be sure to enumerate each detection method.
Explanation / Answer
The use of Asynchronous Transfer Mode (ATM) technology and services creates the need for an adaptation layer in order to support information transfer protocols, which are not based on ATM. This adaptation layer defines how to segment and reassemble higher-layer packets into ATM cells, and how to handle various transmission aspects in the ATM layer. Examples of services that need adaptations are Gigabit Ethernet, IP, Frame Relay, SONET/SDH, UMTS/Wireless, etc. The main services provided by AAL (ATM Adaptation Layer) are: Segmentation and reassembly Handling of transmission errors Handling of lost and misinserted cell conditions Timing and flow control AAL Type 3/4 supports VBR, data traffic, connection-oriented, asynchronous traffic (e.g. X.25 data) or connectionless packet data (e.g. SMDS traffic) with an additional 4-byte header in the information payload of the cell. Examples include Frame Relay and X.25. With a promise to support a full range of applications through a core broadband backbone, ATM has caught the imagination of a wide spectrum of users. To deliver on this promise, the ATM Forum and its participants have crafted a protocol layer flexible enough to provide the mechanisms needed to adapt the cell transmissions into the necessary higher level behaviors. ATM has been designed to support applications including broadband ISDN, carrying multiple synchronous information channels, as a transfer mechanism for frame-relay and Switched Multi-Megabit Data Service (SMDS) applications, and finally as a link-transport mechanism for Local Area Networks. Each of these types of interfaces requires special mapping services to operate over the ATM cell structure. The AAL provides these mappings. AAL protocols have been defined to support five classes of service: Class X: Control Sigalling - supports the in-band service characteristics. Class A: Constant Bit Rate - supports transfer of synchronous circuit services Class B: Variable Bit Rate - supports delivery of isochronous services such as variable rate video and voice Class C: Connection Oriented data Class D: Connectionless Data These classes of service are mapped to the AAL protocols 0 through 5, referred to as AAL0, AAL1, etc. Figure 1 shows the relationship of the AAL, its major components with the remainder of the ATM protocol stack. Residing above the ATM layer, the AAL protocol elements transfer their information over one or more ATM cells. Figure 1 - AAL Service Classes & AAL Types Figure 2 shows the internal components of the AAL. The services provided by each of these components is as follows: The Service Specific Functions (SSCS) sublayer provides additional functions and mechanisms that may be required for the a specific service. This layer is not required for all AAL services, and may be represented as a NULL layer. The Common Part Convergence Layer (CPCS) operates on the complete AAL frames, providing header and trailer record control as well as ensuring the integrity of delivered information. The Segmentation and Reassembly Layer (SAR) converts the CPCS frames to and from ATM cell payload information. As the information is transferred through each of these layers it takes on a different format. The Service Data Units (SDU) is the representation of data between two service layers; for example, the ATM SDU is presented to the SAR layer for AAL processing. The Interface Data Unit (IDU) is a subcomponent of the SDU; one or more IDUs can contribute to a single SDU. Finally, the familiar term Protocol Data Unit (PDU) is used to represent data between a sublayer and its supporting sublayer. With these options, the way information is named can become confusing AAL-3/4 End-To-End Data Transport Based on the SMDS standard, AAL-3/4 provides data transport services for both connection and connectionless data. These services correspond to Class C and Class D data. Unlike AAL-0 and AAL-1, AAL-3/4 includes a range of service options. Information is transferred in either message or streaming mode. Optional delivery assurance techniques include discarding faulty SDUs, end-to-end data recovery, and delivery of all SDUs regardless of their integrity. Assured transmission through retransmission is discussed in the standards, but is not fully specified. Non-assured transmission is supported in the standard, with options to deliver or discard faulty SDUs. Typically, it is most useful to discard faulty SDUs. Delivery of unreliable information is typically something to avoid. Several logical AAL connections can be multiplexed over a single ATM Virtual Circuit (VC). Finally, AAL-3/4 provides a capability to support multipoint delivery of information from a single source. To support this set of services, the AAL-3/4 architecture is considerably more involved than the other AAL protocols. Active processing is performed by both the CPCS and SAR layers to provide additional functions. Processing steps include receipt of the User data frame to AAL, CPCS formatting of the AAL-PDU for transmission to the destination, followed by forwarding of the CPCS-PDU to the SAR layer for segmentation into a set of 44-byte PDUs for transmission (with 3 octets of SAR control information) over the ATM layer. On receipt of the ATM PDU, the opposite process ensues, with the SAR reassembling the stream of SAR-PDUs into a single CPCS-PDU, and delivering the received CPCS-PDU to the CPCS for final processing and delivery of information to the user. A summary of these processes follows. The AAL-3/4 CPCS is responsible for managing the integrity of AAL-3/4 information. It additionally pads the information data-block size to an even multiple of 4 octets, simplifying protocol processing by CPUs such as the Motorola 680x0 series that are popular in telecommunications systems. Figure 4 shows the format of the CPCS-PDU. The fields of the PDU and their use are: CPI - Common Part Indicator: Acts as a protocol identifier that defines how the remaining fields of the PDU are to be processed. At this point, it is largely a placeholder to support future changes to the protocol, the CPI is specified to be always zero by ITU-T I.363. Btag - Beginning Tag. This value is used to determine whether or not the CPCS-PDU has been properly delivered. When the CPCS-PDU is constructed, the same value is entered into both the Btag and Etag fields. If a reassembled CPCS-PDU is received with these values being different, the PDU is assumed to have been corrupted. The tag value is incremented (with a wrap at all ones) as each PDU is constructed. BASize - Buffer Allocation Size Indication. Tells the receiver the amount of buffer space to allocate to process the PDU. This value is equal-to or greater than the PDU size. Pad - Ensures that the PDU is a multiple of 4 octets in length. This simplifies processing of the PDU in 32-bit chunks. AL - Alignment Field. This is unused pad space with the sole purpose of padding the trailer to 4-octets Etag - End Tag. Matches the beginning tag. Used for data integrity verification Length - Field Length. Specifies theactual length of the CPCS-PDU (in octets when CPI=0). As the SAR is responsible for framing the data, this is provided to support additional error checking. Figure 4 - AAL-3/4 CPCS PDU Format The Segmentation and Reassembly Sublayer manages the fragmentation and reassembly of the AAL-3/4 CPCS-PDUs into payload information that can be carried by the ATM cells. The format of the SAR-PDU is shown in Figure 5. The SAR-PDU fields and there application are as follows: ST - Segment Type (2-bits) defines where the segment belongs in the SAR-SDU (essentially the CPCS-PDU). Possible values include: (10) beginning of the message, (00) continuation of the message, (01) end of the message, and (11) single segment message. SN - Sequence Number (4-bits). Used to ensure the in-order delivery of segments. MID - Multiplexing Identification Field. (10-bits) Provides compatibility with SMDS, this field carries multiplex channel identification, allowing for the transmission of multiple channels of information over a single ATM VC. Payload - (44-octets) the segment of the CPCS-PDU information to be transferred within this cell. LI - length Indication. (6 bits) Identifies the amount of information in the payload portion of the cell. Only the last cell in a transmission should have less than 44 octets of information. Therefore, this value will be 44 for all segments with the exception of the end-of-message segment, in which case it is an even multiple of 4 between 4 and 44 (remember the CPCS padding). CRC - cyclic redundancy check (10-bits). Calculated over the complete SAR-PDU with the exception of the CRC field.
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