ATM Adaptation Layer (AAL)

1. ATM Adaptation Layer (AAL) • AAL is responsible for adaptation of information of higher layers to the ATM cells (in the transmission direction) or adaptation of ATM cells into the information of the higher layer (receiver direction). • AAL is subdivided into two sublayers: - SAR (Segmentation and Reassembly) - CS (Convergence Sublayer): Multiplexing, loss detection, timing recovery, message identification

2. ATM Adaptation Layer (AAL) • AAL provides a variety of services: Class 1: Circuit Emulation with Constant Bit Rates (CBR). Voice of 64 kbps Fixed Bit Rate (Voice,Video) Class 2: Connection-oriented service with Variable Bit Rates (VBR) and timing between source and destination. VBR Video & Audio Class 3: Connection-Oriented Service. Data Transfer and Signaling ABR Traffic with no timing Class 4: Connectionless Data Service SMDS, Ethernet, Internet, Data Traffic, No constraints.

3. A B C D Timing Between Source and Destination Yes No Bit Rate Constant Variable Connection Mode Connection Oriented Connectionless AAL 1 2 ¾ or 5 ¾ or 5 Example DS1, E1 N64 Kbps Emulation Packet Video, Audio (Real Time) Frame Relay IP, Ethernet Traffic Classes

4. Service Access Point AAL-SAP Convergence Sublayer (CS) CONVERGENCE SUBLAYER(CS) Primitives AAL Segmentatioin & Reassembly (SAR) Sublayer SAR SUBLAYER ATM-SAP General Structure of AAL • Service Data Unit (SDU) crosses the SAP • PDU is data unit between peer layers

5. AAL Interfaces AAL-SAP Convergence Sublayer(CS) AAL-SDU CS-PDU Header CS-PDU Payload CS-PDU Trailer CS-PDU No SAP is defined between CS and SAR AAL Segmentation of CS-PDU Segmentation And Reassembly (SAR) Sublayer SAR-PDU Header SAR-PDU Payload SAR-PDU Trailer SAR-PDU ATM-SAP ATM-SDU ATM Layer ATM Cell Cell Header Cell information Field (Cell Payload) PL-SAP Physical Layer General Data Unit Naming Convention

6. Structure of AAL with SSCS and CPCS AAL-SAP AAL-PDU Primitives Service Specific Convergence Sublayer (SSCS) SSCS SSCS-PDU CS Primitives AAL Common Part (CP) AAL Common Part Convergence Sublayer (CPCS) CPCS CPCS-PDU Primitives Segmentation And Reassembly (SAR) SAR SAR-PDU Primitives ATM-SAP

7. AAL Type 1 • AAL 1 provides the foll. services to the AAL users: • Transfer of service date unit with a constant source • bit-rate and their delivery with the same bit rate • - Voice traffic 64kbps: as in N-ISDN to be • transported over an ATM network. This • service is called circuit emulation. • In other words, how TDM type circuits can be • emulated over ATM. • CBR-Voice; CBR-Video (fixed (constant) bit rate • video)

8. AAL Type 1 • Transfer of timing information between source and • destination. • Transfer of structure information between source • and destination; some users may require to transfer • of structured data, e.g., 8 kHz structured data for • circuit mode device for 64 kbps (N-ISDN). • Indication of lost or errored information which is • not covered by AAL1, if needed.

9. AAL Type 1 (Cont.) • The functions listed below may be performed in the AAL in order to enhance the layer service provided by the ATM layer: • Segmentation and reassembly of user information • Handling of cell delay variation  to achieve constant rate delivery (playout buffer) • Handling of cell payload assembly delay • Handling of lost and misinserted cells (SN processing)  Discarded

10. AAL Type 1 (Cont.) • Source clock frequency recovery at the receiver • - 4 bit RTS is transferred by CSI • - handling of timing relation for Asynchronous • transfer (SRTS Synchronous Residual • Time Stamp) • Monitoring of AAL-PCI (Protocol Control • Information) for bit errors • Handling of AAL-PCI bit errors SAR-PDU Header CS-PDU Header CS-PDU Trailer PCI

11. AAL Type 1 (Cont.) • Monitoring of the user information field for bit • errors and possible corrective action • - FEC maybe performed for high quality • video or audio (124,128 Reed Solomon code)

12. AAL Type 1 (cont.) • Receiver’s Responsibilities are as follows. • Examine the CRC and parity bit for error detection. • Correct single bit errors in SN field. If multiple bit • errors in SN field, then declare invalid. • Reassemble the CS-PDU in correct sequence using • SN-numbers. • Discard misinserted CS-PDUs and generate dummy • information for missing CS-PDU.

13. AAL Type 1 (Cont.) • Buffer the received CS-PDUs to compensate for cell • delay variation (jitter) to achieve constant rate • delivery. (PLAYOUT Buffer) • Clock frequency recovery (Handling of timing • relationship for asynchronous circuit transport) • Monitoring and handling AAL-PCI (Protocol • Control Information) SAR-PDU Header, SAR-PDU • Trailer, CS-PDU Trailer are collectively called • AAL-PCI.

14. Adaptation Layer Type 1 (AAL1) STACK Convergence Sublayer - accepts 124-byte blocks from user - appends 4-byte FEC - writes to matrix “row” - forwards CS-PDU to SAR when 47 blocks (rows) have been written • Forward Error Correction • No Retransmission * (124,128) Reed-Solomon Code * Polynomial undefined * Corrects 2 errored bytes per row * Corrects 4 “erasure” bytes (knows position) * Uses interleaving Segmentation/Re-assembly Sublayer - reads matrix “columns”(47bytes) - effect: interleaving ATM

15. FEC in AAL1 R-S Code with 4 byte FEC Reading Cell 1 Byte 1 Cell 2 Byte 1 Cell 124 Byte 1 Cell 1 Byte 2 Cell 2 Byte 2 Cell 124 Byte 2 Cell 1 Byte 47 Cell 124 Byte 47 Cell 2 Byte 47 Reed-Solomon Code recovers up to 4 lost cells in a block of 128.

16. SAR-PDU Payload SAR-PDU Payload SAR-PDU Payload AAL 1 Higher Layers User Data Bit Stream AAL-SAP CS CPCS-PDU Payload AAL SAR H … H H 1B 47B 48 Bytes ATM-SAP ATM Layer … H Cell Payload H Cell Payload H Cell Payload 5B 53 Bytes

17. SAR-PDU of AAL 1 1 Octet 47 Octets Cell Header SN SNP SAR-PDU Payload 4 bits 4 bits SAR-PDU Header SAR-PDU (48 Octets) SN (Sequence Number) for numbering of the SAR-PDUs SNP (Sequence Number Protection) to protect the SN field To detect lost or misinserted cells (Error Detection & Correction)

18. SAR-PDU Header of AAL 1 1 bit 3 bits 3 bits 1 bit Sequence Count Even Parity CSI CRC SN Field SNP Field

19. CSI Field • Sequence Count 0, .., 7 • CSI bit used to transfer timing info. or info. about data structure. • CSI values in cells 1,3,5,7 are inserted as a 4-bit timing value. • In even numbered cells 0,2,4,6, CSI used to support blocking • of info. from a higher layer. • If CSI bit is set to 1 in a cell 0,2,4,6, then the first octet of • SAR-PDU payload is a pointer that indicates the start of the next • structured block within the payload of this cell and the next cell, • i.e., 2 cells (0-1, 2-3, 4-5, 6-7) are created as containing a 1-octet pointer • and a 93-octet payload and pointer indicates where in that 93 octet • payload is the first octet of the next block of data.

20. 1 Octet 47 Octets SN SNP SAR-PDU Payload (User Data) Non P-format SAR-PDU Header SAR-PDU (48 Octets) 1 Octet 1 Octet 46 Octets SN SNP SAR-PDU Payload P-format Reserved for Pointer SAR-PDU Header (CSI = 1) P & Non-P Formats AAL-1 CS uses a pointer to delineate the structure boundaries. Supported by 2 types of CS_PDUs called non-P & P Can be used only in SAR PDUs with even SN values (because SRT scheme uses the CSI bits in SAR PDUs with odd SN values)

21. P-Format Sequence Counter 0,2,4,6 Structure Pointer Field SAR-PDU Header User Data Reserved Bit Offset Field 7 Bits 7 Bits are the offset measured in Bytes between the end of the pointer field & start of the structured block in 93 bytes consisting of remaining 46 bytes in this CS-PDU & 47 Bytes of the next CS-PDU. This offset may range from 0-92. SN even uses 1 Octet Pointer field to indicate the offset into the current payload of the first octet of a n*DSO payload. Value of n may be as large as 92 in the P-format since pointer is repeated every other cell when supporting AAL 1.

22. AAL1 STD Mode (Structured Data Transfer) Unstructured Data Transfer n x DSO (64kbps) Service (supports an octet structured n – DSO Service) DS1/E1 (1.544Mbps) DS3/E3 (45Mbps) including timing SRTS Method CSI bit (in even SN values) for SDT to convey information about internal byte alignment structure of the user data bit stream. (4-bit RTS included in CSI Bit !!) One sent in (1,3,5,7)

23. Structured Data Transfer • Kind of fractional DS1/E1 service where the user only requires • an n*64kbps (DS0) connection where n can be small as 1 • and as high as 24 for DS1 (T1) and 30 for E1. • An n*64 kbps service generates blocks of n bytes which are • carried in P and non-P format CS-PDUs. • The beginning of a block is pointed to by the pointer in the • 1-byte header of the CS-PDU-- > P format.

24. EXAMPLE: STRUCTURED DATA TRANSFER 192 1 1 192 1 192 1 192 1 192 1 192 1 192 1 192 DS1 Signal 193 175 18 193 165 28 193 147 46 193 137 46=368 47=376 46=368 47=376 p CS-PDUs SN=0 CSI=1 P-Format SN=1 CSI=0 Non-P-Format SN=2 CSI=1 P-Format SN=3 CSI=0 Non-P-Format 0-1 = 93 Octets 2-3=93 Octets Pointer indicates where in that 93 octet payload is the first octet of the next block of data. No structured boundary, then use dummy offset value of 127.

25. Unstructured Data Transfer • * The entire DS-1/E1 signal is carried over an ATM network. • The DS-1 signal is received from user A which is packed • bit-by-bit into the 47-byte non-P format CS-PDU which then • becomes the payload of a SAR-PDU.

26. DS1 CIRCUIT EMULATION USING AAL 1 EXAMPLE: UNSTRUCTURED DATA TRANSFER octets octets octets Header Header Header 5 5 5 Header Header Header 1 1 1 SAR- PDU SAR- PDU SAR- PDU SAR- PDU SAR- PDU SAR- PDU 48 48 48 47 47 47 ATM Cells DS1 Signal SRTS CS SAR-PDUs octets bits RTS 1 192 1 1 192 192 1 192 1 192 1 Time 192 Transmitter uses AAL 1 operating in SRTS mode to emulate a DS 1 digital bit stream created by a video codec. DS1 frame has 193 bit frames that repeat 8000 times per second (192 user data bit + 1 framing bit). CS computes the RTS every 8 cell times and provides this to the SAR sublayer for insertion in the SAR header. 193 bit frames are packed into 47 octet SAR-PDUs by SAR layer. SAR then adds the SN, inserts the data from CS, computes CRC and parity over SAR header and passes 48-octet SAR-PDU to ATM layer.

27. Handling of Lost and Misinserted Cellsin AAL1 • At the transmitter, CS provides SAR with a Sequence Count Value and a CSI associated with each SAR-PDU payload. Sequence Count Value starts with 0, and incremented sequentially and is numbered modulo 8. • At the receiver, CS receives Sequence Count, CS indication from SAR, and check status of Sequence Count and CS indication. CS identifies SAR-PDU payload sequence SAR-PDU loss, and SAR-PDU misinsertion. • CSI is used to transfer timing information and default value of CSI is “0”. 4 bit RTS is sent in odd-sequence-numbered PDUs (1,3,5,7) in SRTS approach.

28. Remark: • For each SAR-PDU, SAR receives a sequence number (SN) value from CS. • At the receiver, SAR passes the SN to CS. The CS may use these SNs to detect lost or misinserted SAR-PDU payloads. • SAR protects the SN value and CSI against bit errors. It informs the CS when SN value and the CSI are in error and cannot be corrected. • Transmitter computes the CRC value across the 4 bits of SAR-PDU header and inserts into CRC field. CRC contains the remainder of the division (mod 2) by polynomial of the product multiplied by the contents of SN field. • After completing the above operations, transmitter inserts the even parity bit.  7 bit code word protected. Handling of Lost and Misinserted Cells in AAL1(Cont.)

29. TIMING (CLOCK) RECOVERY TECHNIQUES IN AAL 1 • Adaptive Clocking in AAL 1 • (No Network clock is available). • Synchronous Residual Time Stamp Approach (SRTS) • (Global Network Clock is available)

30. Network FIFO Data Data Terminal Filling Level . Local Clock Jitter Filter PLL Common network reference clock is not available!!! Adaptive Clocking in AAL 1 1. Adaptive Clocking (Receiver) Used for Transfer Delay Variable Cells PLAYOUT BUFFER Receiver writes received info field in this buffer. Receiver reads info. with a local clock. CONTROL is performed bycontinuously measuring the fill level around its median position & by using this measure to drive the PLL providing the local clock. (Content) Filling level of the buffer is used to control the frequency of the local clock. PLL (Phase Lock Loop) Provides local clock. The content level of the buffer may be maintained within an upper limit and lower limit to present buffer overflow and underflow. Underflow => PLL slowed down Overflow=> PLL speeded up

31. Synchronous Residual Time Stamp (SRTS) Approach BASIC IDEA: Convey a measure of the frequency difference between the reference clock and source clock. Network reference clock is available, source clock is not syncronized! NETWORK Sender Receiver Common Network Clock Local Clock Local Clock Odd # of segments CSI field Sequence # field TIMESTAMP Difference between the local and network clocks. Transport this info. in odd numbered Cells (CSI Field) to destination Difference between 2 clocks

32. (Assumed) • CommonNetwork clock is available • Source (local) clock is not synchronized with it. Source Transmitter • SRTS method conveys a measure of the frequency difference between the derived network reference clock and the source (local) clock. • The derived network reference clock is determined from the frequency of the network clock divided by some integer. • Within a time interval of N “source clock cycles” suppose there are M cycles of the derived “network reference clock”. • There is a nominal value Mnom (fixed and known for the service) and the actual value of M may vary anywhere within a certain range (Mmin & Mmax) around this nominal value Mnom. • The actual value of M will be the sum of Mnom and a residual part. • By transmitting the residual part, the receiver has enough info to construct the source clock.

33. Tolerance Source clock N cycles T seconds Source Frequency (fs) t M nom M M M min nom max Derived Network Frequency (fnx) t y y Residual value M 4 2

34. Sample & Hold 1 C fs t N 4 Bit SRTS encoded in CSI bit for SAR-PDUs with Sequence Numbers 1,3,5,7 fnx 1 4 Bit Counter fn X Network Reference clock frequency fn is divided by x such that 1 < fnx/fs< 2

35. Source clock fs is divided by N to sample the 4-bit counter Ct driven by • the network clock fnx once every N = 3008 = 47 x 8 x 8 bits generated • by the source. • This sampled counter output 4 bits (residual part) is transmitted as • the SRTS in SAR-PDU. • It is sent in the CSI bits of SAR-PDUs which have odd SN values. • The even SAR-PDUs have CSI=0. • The CSI bits in SAR-PDUs with even SN values are used in SDT. • The method can accept a frequency tolerance for source frequency of • 200 parts per million (ppm). • Ct, x, Mnom, N, fn are available at the destination and the clock • value can be recovered accordingly!!!! IFA’2004

36. AAL2 • For low bit rate communications, e.g., for compressed voice traffic. • Main Idea: multiplex many users within a single ATM VCC, where each • user’s information (SDT) is carried in variable length packets with a • header (3 octets) identifying the user channel with control information. • (kind of variable ATM cell) • In the minicell header, the field for user identification has 8 • bits limiting the number of AAL 2 users sharing a VCC to 256. • Short and variable length payload. • User packet multiplexing Minicell Header 3 octets Payload (1-64) octets SDU IFA’2004

37. WHY AAL 2? AAL 1 needs not be filled with full 47 bytes. e.g., to transmit digitized voice at a rate of 1 byte every 125 sec, filling a cell with 47 bytes means collecting samples for 5.875 msec. If this delay before transmission is unacceptable, we send partially filled cells  waste of bandwidth!!!

38. STRUCTURE OF AAL TYPE 2 AAL SAP Service Specific Convergence Sublayer (SSCS) AAL-SDU SSCS-PDU Header User Packet SSCS-PDU Trailer A A L SSCS-PDU CPS-SDU Common Part Sublayer (CPS) CS-Packet Payload CPS-Packet Header CPS Packet Start Field CPS-PDU Header CPS- Packet CPS-Packet PAD CPS-PDU (48 octets) ATM SAP A T M ATM Layer ATM Header ATM Cell Payload ATM Cell PHY SAP • Transfer of Service Data Unit with a Variable Bit Rate • Transfer of timing information between source and destination • Indication of lost or errored information which is not covered by AAL 2 IFA’2004

39. CPS-PACKET FORMAT CID| PPT| LI UUI HEC CPS-INFO CPS-Packet Payload (Variable length) CPS-Packet Header (3 octets) CPS-Packet (45 octets default 64 octets optimal) CID: Channel Identifier (8 bits): • 0: Not used • 1: Reserved for Layer Management (AAL2 ANP packets) • 2-7: Reserved • 8-255: ID of SSCS entity (valid CID values to identify channels)

40. CPS-PACKET FORMAT (Contd) • CID helps to multiplex multiple AAL2 users/streams • (channels) onto a single VCC (ATM connection). • Each channel is identified by the CID. • A channel is bidirectional and has the same CID value. • * CID field supports up to 248 individual users per VCC.

41. A’ B’ C’ D’ A B C D AAL2 AAL2 ATM ATM ATM Network PHY PHY AAL2 can multiplex several data streams

42. AAL-SAP SSCS SSCS CID=Z CID=Y SSCS CID=X CSP ATM-SAP Functional model of AAL2 (sender side)

43. CPS-PACKET FORMAT (Contd) • Packet Payload Type (2 bits): serves 2 functions: • * When PPT =/ 3, the CPS packet is serving a specific • application, such as carrying voice data, or carrying an • ANP packet. • * When PPT=3, the CPS packet is serving • an AAL network management function associated with • the management of the channel identified in the • CID field.

44. CPS-PACKET FORMAT (Contd) * LI: Length Indicator (6 bits) * LI specifies the number of octets (minus 1) in the variable length user payload. * LI Coding: One less than CPS-Packet payload length CPS-Packet payload length = LP => LI = LP -1 * CPS-INFO: Information (variable size: (min. 1- max. 45 or 64 octets))  45 means that exactly one CPS packet fits inside the 48 octet ATM cell payload.

45. CPS-PACKET FORMAT (Contd) • * UUI: User-to-User Information (5 bits): Allows the functions • of an SSCS to be specific according to a purpose. • UUI serves two purposes: • To convey specific info transparently between CPS users, • SSCS entities or layer management. • To distinguish between SSCS entities and layer management • users. • Codepoints: 0-27 SSCS entities • 28-29 Future use • 30-34 Layer management

46. CPS-PACKET FORMAT (Ctd) HEC: Header Error Control (5 bits) 5 bit CRC : Generator Polynomial x5+x2+1 (excluding CPS packet payload and error correction). Detectable 1 and 2 bit errors.

47. CPS-PDU FORMAT Start Field (STF) indicates the position of the first packet OSF SN P CPS-Packet CPS-Packet PAD CPS-PDU Payload (47 octets) CPS-PDU Header CPS-PDU (48 octets) OSF: Offset Field (6 bits) 6 bit pointer => Position Indication of first CPS-packet (starting point of the next CPS packet header within the cell) Values: 0-40: First CPS packet boundary (0=Next to OSF) 47-63: No CPS packet boundary SN: Sequence Number (1 bit): mod 2 (value 1 or 0) P: Parity (1 bit) : Odd parity for STF PAD: Padding (0-47 octets)

48. Packets are streamed into successive payloads CPS Packet Cell Period Pointer in OSF points to find start of a CPS packet in cell ATM Header First Packet Padding: All 0’s ATM Cell • OSF identifies the starting point of the next CPS packet header • within the cell. • If more than one CPS packet is present in a cell, then AAL2 uses • the LI in the CPS packet header to compute the boundary of the • next packet.

49. EXAMPLE 48 bytes 48 bytes Rt VBR Sources User 1 User 2 User 3 User 4 User 5 … 16 16 16 16 16 … CPS Packets 3 16 3 16 3 16 3 16 3 16 CPS PDUs SAR … 1 19 19 9 1 10 19 18 STF STF ATM Layer … ATM Header ATM Header PURPOSE: Accommodation of low bit rate (below 64 kbps) and delay sensitive applications into ATM networks, e.g., cellular systems. Requirements: Short Cell Assembly Time and High Efficiency.

50. PACKET CAN STRADDLE CELLS!! ATM Cell 1 ATM Cell 2 H H Packet 1 Packet 2 Pac- ket 3 Packet 4 Packe-