aal 3/4 aal-sap aal-sdu sscs sscs-pdu sscs-pdu payload sscs-pdu trailer sscs-pdu header cpcs-sdu...

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  • Slide 1
  • AAL 3/4 AAL-SAP AAL-SDU SSCS SSCS-PDU SSCS-PDU Payload SSCS-PDU Trailer SSCS-PDU Header CPCS-SDU CPCS-PDU Payload CPCS-PDU Trailer CPCS-PDU Header CPCS CPCS-PDU SAR-PDU Payload SAR-PDU Trailer SAR-PDU Header ATM-SAP SAR-PDU SAR
  • Slide 2
  • AAL 3/4 Non-Assured Mode (Unreliable) Assured Mode (ARQ Protocols) - Go_Back_N - Selective Repeat Request Message Mode Entire AAL-PDU needed Stream Mode Small AAL-PDUs allowed
  • Slide 3
  • a) MESSAGE MODE AAL-SDU is passed across the AAL interface in exactly one AAL-SDU. This service provides transport of fixed size of variable length AAL-SDUs. 1:1 mapping, i.e., one SSCS-PDU consists of one AAL-SDU. SSCS accepts a block of information from a user and creates a SSCS-PDU. This includes a Header & Trailer with protocol information and padding to make the PDU an integral multiple of 32 bits. SAR accepts the SSCS-PDU from SSCS and segments it into N 44-octet SAR-PDUs (this last segment may contain some unused portion).
  • Slide 4
  • AAL-SDU H H... AAL Interface SSCS-PDU SAR-PDUs H Data SSCS-PDU Header (4 octets) SSCS-PDU Trailer (4 octets) Padding octets ( 0-3 octets ) SAR-PDU Header SAR-PDU Trailer Unused Message Mode H
  • Slide 5
  • Message mode is used for framed data transfer, e.g., high level protocols and applications would fit into this category, e.g., LAPD or Frame Relay would be in message mode. Advantage: Detects errored SSCS-PDUs and discards them. Disadvantage: Requires large buffer capacity.
  • Slide 6
  • b)Streaming Mode The AAL service data unit is passed across the AAL interface in one or more AAL interface data units (AAL IDUs). The transfer of these AAL-IDUs across the AAL interface may occur separately in time and this service provides the transport of the variable length AAL-SDUs. It provides transport of variable length AAL-SDU. The AAL-SDU may be small as 1 octet and is always delivered as 1 unit because only this unit will be recognized by the application.
  • Slide 7
  • AAL SDUs AAL Interface SSCS-PDU SAR-PDUs HHH H Streaming mode Data SSCS-PDU Header (4 octets) Trailer (4 octets) Padding octets(0-3) SAR-PDU Header SAR-PDU Trailer Unused
  • Slide 8
  • Streaming mode is used for low speed continuous data with low delay requirements which may be as small as 1 octet. 1 block is transferred per cell. Data are presented to AAL in fixed size slots. Advantage: Transfer delay of a message is low. A single SDU is passed to the AAL layer and transmitted in multiple SSCS-PDUs (pipelined or streamed mode).
  • Slide 9
  • AAL 3/4 Details 0-65535 Bytes SAR-PDU Payload LICRC Cell Payload AAL-SAP Higher layer CPCS SAR ATM Layer CPIBtag BASize CPCS-PDU Payload 0-65535 Bytes PADAL ST Length Etag Length SN MID SAR-PDU Payload LICRC STSN MID Cell Header ATM-SAP H T 48 octets T H HT . 53 octets 44 CPI: Common Part Indicator (1 Octet) Btag: Beginning Tag (1 octet) BA Size: Buffer Size Allocation (2 octets) Length: Length of CPCS-PDU Payload (2 octets) AL: Alignment (1 octet) Etag: End Tag (1 octet) PAD: Padding (0-3 octets) ST: Segment Type (2 bits) SN: Sequence Number (4 bits) MID: Multiplexing Identification (10 bits) LI: Length Indicator (6 bits) CRC: Cyclic Redundancy Check Code (10 bits)
  • Slide 10
  • The SAR sublayer is depicted in the Figure. The SAR sublayer accepts variable length CS-PDUs from the convergence sublayer and generates SAR-PDUs with a payload of 44 octets, each containing a segment of the CS-PDU. ST (Segment Type) The ST identifies a SAR-PDU as containing a beginning of message (BOM), a continuation of message (COM), an end of message (EOM), or a single segment message (SSM). All BOMs and COMs contain exactly 44 octets where EOM and SSM may have variable lengths. STST Field BOM COM EOM SSM 10 00 01 11 Segment Type Value
  • Slide 11
  • AAL 3/4 Segmentation User Data CPCS-H CPCS-PDU Payload CPCS-T SAR-H SAR-PDU Payload SAR-T ATM-H ATM Cell Payload SAR-H SAR-PDU PayloadSAR-T BOM COM EOM ATM Cell CPCS PDU SAR PDU SAR PDU SAR PDU
  • Slide 12
  • SN (Sequence Number) The SN allows the sequence of SAR-PDUs to be numbered modulo 16. SN is incremented by 1 relative to the SN of the previous SAR-PDU belonging to the same AAL connection (numbering modulo 16). These two fields enable the segments of the CS-PDU to be reassembled in the correct sequence and minimize the effect of errors on the reassembly process (counts for lost or misinserted cells, buffer overflows, and underflows & bit errors).
  • Slide 13
  • MID (Multiplexing Identification) * The MID is used to identify a CPCS 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, variable- length CS-PDUs concurrently, over a single ATM layer connection between AAL entities. Different AAL connections on a single ATM layer connection where AAL connections must have identical QoS requirements. Multiplexiing/Demultiplexing is performed on an end-to-end basis. AAL 3/4 multiplex different streams of AAL/SDUs across a single Virtual Connection. For CO, each logical connection between AAL users is assigned a unique MID value. Thus, the cell traffic from up to 2 10 different AAL connections can be multiplexed and interleaved over a single ATM connection. For CL service, MID field can be used to communicate a unique identifier associated with each CL user and again traffic from multiple AAL users can be multiplexed.
  • Slide 14
  • From a single host to forward along the same VC and be separated at the destination. All sessions having the same QoS MID finds which cell belongs to which session. MID desirable Carriers charge for each connection set up and for each second for an open connection. If a pair of hosts have several sessions open simultaneously giving each one its own VC expensive. If 1 VC can handle the job (enough BW use) VC1 VP VC2 3 sessions Multiplexed onto VC2
  • Slide 15
  • AAL 3/4 Multiplexing Example A data communication terminal has 2 inputs with a 98-octet packets arriving simultaneously destined for a single ATM output port using the AAL 3/4 protocol. Two parallel instances of the CPCS sublayer encapsulate the packets the packets with a header and trailer. These are passed to 2 parallel SAR processes that request the CPCS-PDU or two different MIDs resulting in a BOM, COM, and EOM segment for each input packet. Since all these occurs in parallel, the ATM cells are interleaved on output.
  • Slide 16
  • Slide 17
  • LI (Length Indicator) The LI contains the number of octets (binary coded) from the CS-PDU which are included in the SAR-PDU payload. Maximum value is 44. It aids in the detection of reassembly errors such as loss or gain of cells.
  • Slide 18
  • CRC ( Cyclic Redundancy Check ) The CRC is a 10-bit sequence used to detect bit errors across the whole SAR-PDU. This includes the CS-PDU segment and hence the user data. Remainder of the division (modulo 2) by the generator polynomial of the product of x 10 and the content of the SAR-PDU, including the SAR-PDU header, SAR-PDU payload and LI field of SAR-PDU. The polynomial is G(x) = x 10 +x 9 +x 5 +x 4 +x+1. Result of CRC calculation is placed with the LSB right justified in the CRC-field (CRC-10 to detect errors).
  • Slide 19
  • CPI ( Common Part Indicator ) * The CPI is used to interpret subsequent fields for the CPCS functions in the Header/Trailer. * CPI of 0 indicates that the BAsize field contains an estimate of incoming CPCS-PDU and LI exact size.
  • Slide 20
  • BTag ( Beginning Tag ) Sender inserts same value in BTag and ETag for a given CPCS-PDU and changes the value for each successive CPCS-PDU. Receiver checks the values for each successive CPCS-PDU. It also checks the value of BTag in the CPCS-Header with the value of ETag in trailer. BTag and ETag are set to the same value to help error detection.
  • Slide 21
  • BASize ( Buffer Allocation Size ) The BAsize indicates the receiver the maximum buffering requirements to receive the CPCS-PDU. BAsize is binary encoded as number of counting units. Size of counting units is identified by the CPI field. BAsize field estimates the incoming CPCS-PDU size in bytes. Length field contains the exact size of CPCS-PDU in bytes. In Message Mode, BAsize value is encoded equal to the CPCS-PDU payload length. In Streaming Mode, BAsize value is encoded equal to or greater than the CPCS-PDU length.
  • Slide 22
  • PAD Between end of CPCS-PDU payload and 32-bit aligned CPCS-PDU trailer, there will be 0-3 unused octets for padding makes the CPCS-PDU an integral multiple of 32 bits to make end system processing more effificient. These are used as filler octets and do not convey any information. It may be set to zero and its value is ignored at the receiving end.
  • Slide 23
  • AL (Alignment ) The AL is used to achieve 32-bit alignment in the CPCS-PDU trailer. AL field complements the CPCS-PDU trailer to 32 bits. This unused octet is strictly used as a filler octet and does not convey any information, i.e., it simply makes the trailer a full of 32 bits to simplify the receiver design. AL field should be set to 0. ETag ( End Tag ) The ETag is used to associate the CPCS-PDU trailer with the CPCS-PDU header the transmitter will insert the same value into the BTag and ETag fields.
  • Slide 24