Computer Networks II

Transcription

1 Computer Networks II Asynchronous Transfer Mode Giorgio Ventre COMICS LAB Dipartimento di Informatica e Sistemistica Università di Napoli Federico II Nota di Copyright Quest insieme di trasparenze è stato ideato e realizzato dai ricercatori del Gruppo di Ricerca sull Informatica Distribuita del Dipartimento di Informatica e Sistemistica dell Università di Napoli e del Laboratorio Nazionale per la Informatica e la Telematica Multimediali. Esse possono essere impiegate liberamente per fini didattici esclusivamente senza fini di lucro, a meno di un esplicito consenso scritto degli Autori. Nell uso dovrà essere esplicitamente riportata la fonte e gli Autori. Gli Autori non sono responsabili per eventuali imprecisioni contenute in tali trasparenze né per eventuali problemi, danni o malfunzionamenti derivanti dal loro uso o applicazione. 1

2 ATM Networks TOPICS» The ATM header» The ATM protocol stack» The physical layer» ATM switch architectures» ATM adaptation layers» IP over ATM Asynchronous Transfer Mode (ATM) The word Asynchronous in ATM is in contrast to Synchronous Transfer Mode (STM) that was proposed earlier on, which was based on the SONET/SDH hierarchy. Transfer Mode refers to a telecommunication technique 2

3 Asynchronous Transfer Mode (ATM) ATM was standardized by ITU-T (CCITT) in 1988 as the transfer mode of B-ISDN It can carry a variety of different types of traffic, such as» Voice» Video» Data At speeds varying from fractional T1 to 2.4 Gbps Asynchronous Transfer Mode (ATM) These different types of traffic have different Quality-of-Service (QoS) requirements, such as:» Packet loss» End-to-end delay ATM, unlike IP networks, can provide each traffic connection a different type of quality of service. 3

4 ATM: The Grand Unifier? What is changing: Due to fiber, better copper technology, and improved hardware bandwidth is no longer the problem. How can we use it? CATV is moving to digital too. 500 D-CATV channels New services: video-on-demand, interactive TV. Data communication is changing as well. New applications: multimedia over LANs/WANs, distributed computing, telecontrolling, virtual reality. ATM Architecture Assumptions ATM is a cell-based, connection-oriented transfer methodology ATM can dynamically allocate bandwidth ATM can dynamically manage QoS specifications ATM works on fiber optic fabric and High Quality TP with extremely low error rates The devices to be connected to ATM networks might be very simple, like a telephone ATM is organized in a hierarchy, like today s phone network 4

5 ATM Architecture Packets vs Cells A Cell is a data entity of small, constant size Scheduling is more efficient and manageable ATM Architecture From Packets to Cells The transition from packets to cells can be inefficient Cell losses can have a critical impact 5

6 ATM Architecture Cell Information: why 48 bytes? 48 Bytes = 384 bits = 6 msec of PCM 64 Kbps With 48 bytes of payload, a very efficient and small addressing scheme must be used. Only 5 additional bytes are reserved for addressing, control and error check header fields. Cell Size = = 53 bytes ATM Architecture Switching is performed on a per-connection basis» Connection ID in the cell header» Cut-through switching» hardware-based switching Connections are identified by two elements» Virtual Channel Identifier (VCI) A concept to describe unidirectional transport of cells» Virtual Path Identifier (VPI) A concept to describe a set of virtual channels Both identifiers have only local significance 6

7 Global addressing vs. Local addressing Global addressing:» Unique World» Large addressing info» Global management» Global knowledge Local addressing» Local identifiers» Small addressing info» Local management» Translation needed in local-to-local transit ATM Architecture Public UNI Public NNI Private UNI Private NNI ATM ATM Router/Bridge ATM Switch LANE, NHRP, MPOA 7

8 Some features of ATM Connection-oriented packet-switched network Fixed cell (packet) size of 48+5 bytes Header Payload 5 bytes 48 bytes No error protection on a link-by-link No flow control on a link-by-link Delivers cells in the order in which they were transmitted The structure of the ATM cell UNI cell format NNI cell format GFC VPI VPI 2 VPI VCI 2 VPI VCI B y t e VCI VCI PTI HEC CLP B y t e VCI VCI PTI HEC CLP... Information payload... Information payload

9 Fields in the ATM cell header GFC: (Generic Flow Control) Connection identifier: VPI/VCI, Payload type indicator: (PTI) Cell loss priority (CLP) Head error control (HEC) ATM connections Identified by the combined fields» virtual path identification (VPI), and» virtual channel identification (VCI) VPI field:» 256 virtual paths at the UNI interface, and» 4096 virtual paths at the NNI interface. VCI field:» a maximum of 65,536 VCIs. 9

10 ATM connections VPI/VCI values have local significance. That is, they are only valid for a single hop. A connection over many hops, is associated with a different VPI/VCI value on each hop. Each switch maintains a switching table. For each connection, it keeps the incoming and outgoing VPI/VCI values and the input and output ports. ATM Switching Transmission Path VCI 1 VCI 2 VPI 4 VPI 4 VCI 1 VCI 2 VCI 1 VCI 2 VPI 5 VPI 5 VCI 1 VCI 2 Virtual Path Link Virtual Path Connection (VPC) ATM Switch ATM Switch ATM Switch Virtual Channel Connection (VCC Virtual Channel Computer Networks II a.a. Link

11 ATM Switching VC1 VC2 VC3 VC1 VC2 VC3 VP1 VC1 VC2 VC3 VC1 VC2 VC3 VP1 VC1 VC2 VC3 VC1 VC2 VC3 VC4 VC4 VC4 VC4 VC4 VC4 VC5 VC5 VC5 VC5 VC5 VC5 VC6 VC7 VC6 VC7 VP2 VC6 VC7 VC6 VC7 VP2 VC6 VC7 VC6 VC7 VC8 VC8 Virtual Path Switching VC8 ATM Switch VP Switch ATM Switch End-to-end Virtual Channel Connection (VCC) set up No matter of routing across the network- VCs associated with VP are globally switched without processing the individual VC or changing VCI number Virtual Circuit can be permanent or switched VC8 VC8 VC8 ATM Switching VC 1 VC 2 VC Switch VC 3 VC 4 VPI 2 VCI 1 VCI 2 VPI 1 VPI 1 VPI 3 VPI 2 VPI 3 VCI 4 VCI 3 VCI 1 VCI 2 VPI 4 VP Switch VPI 5 VCI 1 VCI 2 Virtual Circuit Switching In this case both VPI and VCI are transformed by the switch In theory, mere VPI switching can also be performed 11

12 Label swapping A C VPI=40 VCI=62 2 VPI=30 VCI= ATM switch VPI=10 VCI=89 VPI=30 VCI=53 4 ATM switch 2 VPI=100 VCI= D ATM switch 3 6 VPI=50 VCI=77 B PVCs and SVCs Depending on how a connection is setup, it may be» Permanent virtual circuit (PVC)» Switched Virtual circuit (SVC) PVCs are set-up administratively. They remain up for a long time. SVCs are set-up in real-time using ATM signalling. Their duration is arbitrary. 12

13 Call & Connection Control ATM Connections on Demand Poin-to-Point & Point-to-Multipoint Symmetric & Asymmetric bndwdt allocation Single Connection calls Procedures for» Call setup» Request» Answer» Clearing» Out-of-band signaling Call & Connection Control ATM Connections on Demand (cont.) Non-negotiation of QoS among users Support for A, C, and X traffic classes Specification of VPI/VCI ranges Guidelines for Addressing Formats Designation of OOB Signaling Channel Error Recovery Client Address Registration Procedures Non-support of Multicasting Operations 13

14 Call & Connection Control Connection Control Messages Call establishment» SETUP, CALL PROCEEDING, CONNECT, CONNECT ACK Call Clearing» RELEASE, RELEASE COMPLETE Point-to-multipoint» ADD PARTY, ADD PARTY ACK, ADD PARTY REJ, DROP PARTY, DROP PARTY ACK Misc» RESTART (VC), STATUS ENQUIRY, STATUS Call & Connection Control UNI UNI Call Ref. Addresses QoS Traffic Char. USER SETUP CALL PROC Call Ref. CONNECT CONN ACK ATM Network Allocate Res. Path Disc. Build VC Complete VC Creation USER SETUP CALL PROC CONNECT CONN ACK Call Ref. Call Ref. Addresses QoS Traffic Char. VPI/VCI Call Ref. Call Ref. Call Ref. 14

15 Payload Type Indicator PTI Meaning 000 User data cell, congestion not experienced, SDU type=0 001 User data cell, congestion not experienced, SDU type=1 010 User data cell, congestion experienced, SDU type=0 011 User data cell, congestion experienced, SDU type=1 100 Segment OAM flow-related cell 101 End-to-end OAM flow-related cell 110 RM cell (resource management) 111 Reserved The ATM protocol stack voice Video Data ATM adaptation layer ATM layer Physical layer 15

16 Protocol Architecture Control Plane Higher Layer Protocols Plane Management Layer Management User Plane Higher Layer Protocols ATM Adaptation Layer ATM Layer Physical Layer I.121 Protocol Reference Model U-Plane The User Plane provides for the transfer of user application information. C-Plane The Control Plane protocols deals with call and connection control for switching. M-Plane The Management Plane provides management function and exchange information between U-Plane and C-Plane Headers and Trailers 16

17 ATM Networks Higher Layer Protocol Higher Layer Protocol Convergence Segmentation & Reassembly CS SAR AAL Convergence Segmentation & Reassembly CS SAR AAL Generic Flow Control Cell Header Generation & Extraction Cell VPI/VCI Routing/Translation Cell Multiplex/Demultiplex ATM HEC Generation & Verification TC Cell Delineation PHY Frame Multiplexing PMD Transmission Frame Gener. & Recov. UNI ATM PHY NNI ATM PHY Generic Flow Control Cell Header Generation & Extraction Cell VPI/VCI Routing/Translation Cell Multiplex/Demultiplex ATM HEC Generation & Verification TC Cell Delineation PHY Frame Multiplexing PMD Transmission Frame Gener. & Recov. UNI User Network Interface (UNI) Protocol adapted to define connection between ATM users (end-station) and ATM network switch Network Network Interface (NNI) Subnet of UNI defines interface between network nodes (switches) ATM Networks Higher Layer Protocol Convergence Segmentation & Reassembly Generic Flow Control Cell Header Generation & Extraction Cell VPI/VCI Routing/Translation Cell Multiplex/Demultiplex CS SAR AAL ATM HEC Generation & Verification TC Cell Delineation PHY Frame Multiplexing PMD Transmission Frame Gener. & Recov. UNI HLP AAL ATM PHY NNI HLP AAL ATM PHY Higher Layer Protocol Convergence Segmentation & Reassembly Generic Flow Control Cell Header Generation & Extraction Cell VPI/VCI Routing/Translation Cell Multiplex/Demultiplex CS SAR AAL ATM HEC Generation & Verification TC Cell Delineation PHY Frame Multiplexing PMD Transmission Frame Gener. & Recov. UNI For OAM operations we need to invoke AAL Therefore we must have AAL capabilities also in the switches for dealing with C-plane and M-plane commands For some OAM messages SAR is not required This is the case of messages contained in just one ATM cell 17

18 ATM and the OSI model ATM does not map directly with the OSI layers ATM layer performs operations typically found in OSI layers 2 and 3 AAL combines features of layers 4, 5, and 7 of the OSI model Consequentely, it is difficult to separate functions so that internetworking can be performed efficiently E.g.: TCP/IP on top of ATM Layering and delay PD PD PD PD ATM ATM ATM C/S QD & SD QD & SD QD & SD C/S End to end delay is made of different contributions C/S: Convergence /SAR 6000 s variable PD: Propagation Delay 2000 s e2e 500 km SD: Switching Delay 24 s /swicth variable QD: Queueing Delay 225 s/swicth variable Another important factor: link speed However it contributes only to constant delay 18

19 ATM Layers and Sublayers - Physical Layer Physical Medium Dependent (PMD) sublayer Defines actual speed at which ATM traffic can be transmitted across a given physical medium SONET/SDH» Synchronous Transfer Signal. STS-n: basic unit for SONET n= n= Mbps» Synchronous Transfer Module. Basic unit for SDH n=1-2-4 n= Mbps Other Interfaces : E1(2.048 Mbps), T1(1.544 Mbps), T3 (DS Mbps), etc. ATM Layers and Sublayers - Physical Layer Transmission Convergence (TC) Sublayer Deadline protocol for preparing cells for transmission across the physical medium defined by PMD Functions differ according to physical medium Transmission Convergence Sublayer Physical Medium Dependent Sublayer HEC Generation/Verification Cell scrambling/descrambling Cell delineation (H4, HEC) Path Signal Identification (C2) Frequency justification/pointer processing Multiplexing Scrambling/Descrambling Transmission frame generation/recovery Bit timing, Line coding Physical medium B-ISDN specific function SONET or SDH 19

20 The transmission convergence (TC) sublayer HEC cell generation and verification» Implements the HEC state machine Decoupling of cell rate» Maintains a continuous bit stream by inserting idle cells Transmission frame generation and recovery» Such as SONET frames Cell delineation Head Error Control (HEC) Multiple bit error detected (cell discarded) No Error detected (no action) Correction mode No Error detected (no action) Detection mode Error detected (cell discarded) Single bit error detected (correction) ATM HEC: Header Error Correction & Detection» Based on a 8 bit field in the header» Allows Detection of Single bit and Multiple bit errors» Single Error allows Correction -> Cell recovered» Multiple Error -> Cell discarded» Based on an Hamming coding: Bose - Chaduri - Hocquengem (BHC)» 40 bit header needs 6 bits for 1 bit error recovery» With 8 bits we reach 84% of multiple bits error detection 20

21 The ATM layer The ATM layer is concerned with the end-to-end transfer of information, i.e., from the transmitting end-device to the receiving end-device. The ATM layer is a connection-oriented point-to point packet-switched network with fixed-size packets (known as cells). Cell switching is performed at the ATM layer. Cells are delivered to the destination in the order in which they were transmitted. No error and flow control on each hop Low probability of a cell getting lost or delivered to the destination end-device in error. The recovery of the data carried by lost or corrupted cells is expected to be carried out by a higher-level protocol, such as TCP. When TCP/IP runs over ATM, the loss or corruption of the payload of a single cell results in the retransmission of an entire TCP PDU. 21

22 Addressing Each ATM end-device and ATM switch has a unique ATM address. Private and public networks use different ATM addresses. Public networks use E.164 addresses and private networks use the OSI NSAP format. ATM addresses are different to IP addresses. Quality of service in ATM Each ATM connection is associated with a qualityof-service category. Each quality-of-service category is associated with a set of traffic parameters and a set of quality-ofservice parameters. The ATM network guarantees the negotiated quality-of-service for each connection. 22

23 ATM Traffic Classes Different classes of applications require different adaptation layer protocols: Constant Bit Rate Applications: They require that delay from source to destination be bounded, so that isochrony is respected. Variable Bit Rate Applications: They require that delay be bounded but for variable data transmission. Connection-oriented Data Applications: ATM should support traditional data communication traffic. E.g. X.25. Connectionless Data Applications: Most of data community currently uses datagram networking protocols. E.g. TCP/IP. Service Classes and Resource Allocation ABR ABR UBR? VBVBR R CBR 23

24 Service Classes and QoS Guarantees ATM Forum Service Categories (in prior. order)» CBR: assured steady supply of bndwdt at PCR values» VBR: assured supply of bndwdt at ACR with rt and nrt reqs» ABR: bndwdt to keep the application running» UBR: bndwdt as available with no assurance Service Bndwdt Delay Var. Through. Cong. fbck CBR yes yes yes no rt-vbr yes yes yes no nrt-vbr yes no yes no ABR yes no min. Yes UBR no no no no Original Service Classes Service Classes Class A Class B Class X Class C Class D Applications Circuit Emulation Compresse Media Cell Relay Bursty data Datagram service Constant Bit Rate Variable Bit Rate Parameters Timing required Timing not required Connection Oriented Connectionless AAL3/4 AALs AAL1 AAL2 AAL0 AAL5 ITU-T Traffic Classes» Class A: CBR - Connection oriented - Timing relationship» Class B: VBR - Connection oriented - Timing relationship» Class C: VBR - Connection oriented - No timing req.» Class D: VBR - Connectionless - No timing req. ATM Forum Traffic Classes» Class X: ABR & UBR 24

25 Quality of Service in ATM Parameter Cell Loss Ratio Cell Misinsertion Rate Cell Error Ratio Severely Errored Cell Block Ratio Cell Transfer Delay Mean Cell Transfer Delay Cell Delay Variation Definition Ratio of lost cells to transmitted cells Number of misinserted cells per connection per second Errored cells to delivered cells Number of errored cells blocks to total number of cell blocks Transfer delay for a single cell Delay averaged over multiple cells Difference between average delay and a single observation Cell loss rate» This is a very popular QoS parameter and it was the first one to be used extensively in ATM networks.» It is easy to quantify, as opposed to other QoS parameters such as jitter and cell transfer delay. It has been used extensively as a guidance to dimensioning ATM switches, and in call admission control algorithms. 25

26 Jitter An important QoS parameter for voice and video. It refers to the variability of the inter-arrival times at the destination Sender Receiver cell i-1 cell i cell i+1 ATM cloud cell i-1 cell i cell i+1 t i-1 t i s i-1 s i Inter-departure gaps Inter-arrival gaps Cell transfer delay (CTD) The time it takes to transfer a cell end-to-end, that is, from the transmitting end-device to the receiving end-device. It comprises of» Fixed cell transfer delay Propagation delay, fixed delays induced by transmission systems, and fixed switch processing times» Variable cell transfer delay, known as the peak-to-peak cell delay variation Queueing delays in switches 26

27 Maximum Cell transfer delay (max CTD) This is a statistical upper bound on the end-to-end cell transfer delay. Cell Delay variation pdf 1% of the total area Fixed CTD max CTD cells delivered late Cell error ratio (CER) and Cell misinsertion rate (CMR) The CER of a connection is the ratio of the number of errored cells to the total number of cells transmitted by the source. An errored cell is a cell delivered with erroneous payload. CMR is the rate of cells delivered to a wrong destination, calculated over a fixed period of time. 27

28 Traffic Descriptor Peak Cell Rate (PCR) Sustained Cell Rate (SCR) Minimum Cell Rate (MCR) Cell Delay Variation Tolerance (CDVT) Maximum Burst Size (MBS) Flow Specification 28

29 Attributes for: CBR, RT-VBR, NRT-VBR, UBR CBR» Class attributes: PCR, CDVT» QoS attributes: peak-to-peak CDV, MaxCTD, CLR rt-vbr» Class attributes: PCR, CDVT, SCR, MBS, CDVT» QoS attributes: peak-to-peak CDV, MaxCTD, CLR nrt-vbr» Class attributes: PCR, CDVT, SCR, MBS, CDVT» QoS attributes: CLR UBR» PCR is specified, but it may not be subject to CAC and policing» No QoS parameters are signaled Attributes for ABR and GFR ABR» Class attributes: PCR, CDVT, MCR» QoS attributes: CLR (possible, depends on network)» Other attributes: feedback messages GFR» Class attributes: PCR, CDVT, MCR, MBS, MFS, CDVT» QoS attributes: CLR (possible, depends on network) 29

30 Quality of Service in ATM QoS can be guaranteed only if traffic control mechanisms are enforced» Virtual Path Traffic Management» Connection Admission Control» Flow Control Usage Parameter Control (UPC) Network Parameter Control (NPC)» Priority Control» Traffic Shaping» Fast Resource Management» Congestion Control Quality of Service in ATM Connection Admission Control (CAC)» The mechanisms in the call set-up phase to decide if a VC/VP connection can be accepted» Users specify the traffic characteristics peak cell rate average cell rate burstiness peak duration» and the requested QoS» The network evaluates its capability to offer that QoS given the current network occupancy 30

31 Quality of Service in ATM Flow Control» Can be performed at both UNI and NNI levels» Checks on the validity of VPI/VCI values» Traffic volume monitoring Three basic mechanisms to punish misbehaviors» Connection release» Cell discarding» Cell tagging Effective if combined with Traffic Shaping The ATM adaptation layer The purpose of AAL is to isolate higher layers from the specific characteristics of the ATM layer. AAL consists of the» convergence sublayer, and the» segmentation-and-reassembly sublayer. 31

32 ATM layers and sublayers Headers and Trailers introduced by AAL 32

33 ATM Adaptation Layer 1- AAL 1 This AAL can be used for applications such as:» Circuit emulation services It emulates a point-to-point TDM circuit over ATM» Constant-bit rate audio Used to provide an interconnection between two PBXs over a private or public ATM network ATM Adaptation Layer 1- AAL 1» Constant Bit Rate (CBR); traffic sensitive to cell loss and delay -Digital voice and video, circuit emulation (transport for E1 link)» Include mechanisms for recovering the source timing» Requires an additional byte of header for sequence numbering (47 bytes of payload) CS SAR User data from higher layer protocol CS-PDU CS-PDU CS-PDU 47-Bytes 47-Bytes CSI SN SNP 47-Bytes CSI 1-bit SN sequen.num.3 bit SNP protection 4 bit 33

34 The SAR encapsulation for AAL 1 SAR Header payload SN SNP 47 bytes CSI Sequence. count CRC-3 Parity 1 bit 3 bits 3 bits 1 bit The AAL 1 CS functions: 1. Handling of cell variation» Due to queueing delays, inter-arrival times of cells vary (jitter). Sender Receiver cell i-1 cell i cell i+1 ATM cloud cell i-1 cell i cell i+1 t t s s i-1 i i-1 i Inter-departure gaps Inter-arrival gaps» CS writes received data into a buffer, and then delivers the information to the application at constant bit rate. 34

35 The AAL 1 CS functions: 2. Processing of the sequence count» The sequence count values are processed by CS in order to detect lost or misinserted cells. Detected misinserted cells are discarded. In order to maintain bit count integrity of the AAL user information, it may be necessary to compensate for lost cells by inserting dummy SAR-PDU payloads. 3. Forward error correction» For video and high quality audio, forward error correction may be performed in order to protect against bit errors. This may be combined with interleaving of AAL user bits to give a more secure protection against errors. The AAL 1 CS functions: 4. Transfer of timing information a. Synchronous residual time stamp (SRTS): CS conveys to the receiver in the CSI field the difference between a common clock derived from the network and the sender s clock b. Adaptive clock method: The receiver writes the received information into a buffer and reads out from the buffer. If its clock is fast/slow the occupancy in the buffer will be below/over the median 35

36 The AAL 1 CS functions: 5. Structured and unstructured data transfers Two CS-PDU formats have been defined: a.cs-pdu non-p format: Constructed from 47 bytes of information supplied by an AAL user b. CS-PDU P format: Constructed from a 1-byte header and 46 bytes of information supplied by an AAL user. The header consists of a 7-bit pointer (SDT pointer) and 1 even bit parity. ATM Adaptation Layer 2 - AAL 2 Variable Bit Rate (VBR); time sensitive traffic.» Packetized voice and video (compressed) Allows a cell to be transmitted before the payload is full to accommodate an application's time requirement. Specifications completed only in

37 ATM Adaptation Layer 2 - AAL 2 At the sender, AAL 2 multiplexes several streams onto the same ATM connection At the receiver, it de-multiplexes the date from the connection to the individual streams. AAL2: The SSCS and CPS sublayers The AAL 2 services are provided by the convergence sublayer, which is subdivided into the» Service Specific Convergence Sublayer (SSCS)» Common part sublayer (CPS). Sometimes called Common Part Convergence Sublayer (CPCS) 37

38 Functional model of AAL 2 (sender side) Each stream is served by a separate SSCS which is associated with a CID AAL-SAP SSCS SSCS CID=Z SSCS CID=Y CID=X CPS ATM-SAP AAL2 CPS-PDU A CPS-PDU consists of 48-bytes and forms the payload of the ATM Cell The CPS-PDU has a 1 byte header (called Start Field) The remaining 47 bytes are filled by multiple CPS-packets and by padding bits 38

39 AAL2 CPS-PDU: An example with only 1 CPS-Packet 48 bytes Variable OSF SN P CID LI UUI HEC PAD Start Field CPS-Packet Header CPS-Packet Payload OSF: Offset - Bits to Payload SN: Sequence Number P: Odd Parity Check for the Start Field CID: Channel ID - Identifies User Traffic LI: Length Indicator - Bytes in the Payload UUI: User-to-User Indicator - User Defined Field HEC: error check on the 3 byte CPS-packet header AAL2: The Offset Field (OSF) Used to identify the beginning of a CPSpacket. It points to the first new CPSpacket in the CPS-PDU payload In the absence of a new CPS-packet, it points to the beginning of the pad The value of 47 indicates that there is no beginning of a CPS-packet in the CPS- PDU. 39

40 AAL2: The CPS-Packet header fields Channel identifier (CID) - 8 bits:» Identifies a channel. Same value is used for both directions.» CIDs are allocated using the AAL negotiation procedures (ANP) Length indicator (LI) - 6 bits:» Default maximum length of the CPS-Packet payload is 45 bytes. Header error control (HEC) - 5 bits:» Pattern is: x 5 +x User-to-user-indication (UUI) - 3 bits:» Used to transfer information transparently between the peers. AAL2 example: 2 PDUs containing multiple CPS-Packets S PH G.729A PH G.729A PH G.729A PDU 1 S G.729A PH G.729A PH G.729A PAD PDU 2 Example: G.729A coded voice with RTP timing» 4 bytes compressed RTP header» 10 bytes G.729A for 8 kbps coding» AAL fills the cells until there is room» The third G.729A packet is splitted in two cells» The S (OSF) field in the second cell points to the PH (CPS-Packet Header) field of the fourth packet 40

41 ATM Adaptation Layers 3/4 - AAL 3/4» Bursty (VBR) connection-oriented traffic (AAL3). -Error messages, large file transfer like CAD or Data Backup-» Bursty (VBR) connectionless traffic (AAL4)(short but bursty transfer). -LANs, Frame Relay-» Error detection on each cell.» Support cell multiplexing -AAL3/4 have same SAR- AAL 3/4: The CS-PDU format 41

42 ATM 3/4: The SAR-PDU Format Header CPS-PDU payload Trailer CPS-PDU SAR-PDUs Begin, cont., end, single message ST SN RES MID 47-Bytes SAR-PDU payload LI CRC SAR-PDU ST segment type, 2bits SN sequence number, 4 bits RES MID reserved field or multiplexing identifier, 10 bits LI length indicator, 6 bits CRC cyclic redundancy check, 10 bits ATM Adaptation Layer 5 - AAL 5» Simple and efficient adaptation layer (simplified version of AAL 3/4, uses 5 bytes header). -For connectionless or connection-oriented VBR traffic-» High speed LANs» Assumes that higher layer protocol provide for error recovery -Simplifies SAR-» Assumes that only one message is crossing the ATM UNI at a time -No support for cell multiplexing- CS User data from higher layer protocol CS-PDU CS-PDU CS-PDU Header CPS-PDU payload Trailer Header CPS-PDU payload Trailer SAR SAR PDU Payload 42

43 ATM Adaptation Layer 5 - AAL 5» Simple and efficient adaptation layer (simplified version of AAL 3/4, uses 5 bytes header). -For connectionless or connection-oriented VBR traffic-» High speed LANs» Assumes that higher layer protocol provide for error recovery -Simplifies SAR-» Assumes that only one message is crossing the ATM UNI at a time -No support for cell multiplexing- CS User data from higher layer protocol CS-PDU CS-PDU CS-PDU Header CPS-PDU payload Trailer Header CPS-PDU payload Trailer SAR SAR PDU Payload 43