ATM Technology in Detail Professor Richard Harris Objectives You should be able to: Discuss the ATM protocol stack Identify the different layers and their purpose Explain the ATM Adaptation Layer Discuss the key differences between ATM, Frame Relay and IP technologies Semester 2-2005 Advanced Telecommunications 143.466 Slide 2 Presentation Outline ATM Layering Adaptation layers in more detail Comparative reviews of ATM versus Frame Relay ATM versus LAN technologies ATM versus IP Semester 2-2005 Advanced Telecommunications 143.466 Slide 3 1
ATM Layering - 1 ATM Layering defined by BISDN Reference model Distinguish User, Control and Management planes Three layers of functionality Physical ATM ATM Adaptation (AAL) Semester 2-2005 Advanced Telecommunications 143.466 Slide 4 ATM Layering - 2 Control Plane User Plane Management Plane TCP/IP LMI, SNMP, Q.2931 FTP, etc CMIP SSCF - Service Specific Convergence Function SSCF SSCOP AAL AAL SSCOP - Service Specific Convergence Protocol AALCP SAAL AALCP - AAL Convergence Protocol ATM SDH, SONET, DS1, E1, etc Examples of Protocol Placement in the BISDN layers Semester 2-2005 Advanced Telecommunications 143.466 Slide 5 ATM Layering - 3 Layer functions Convergence Segmentation and Reassembly CS SAR AAL Layer Management Generic flow control Cell header processing VPI/VCI processing Cell muxing & demuxing Cell rate decoupling HEC header processing Cell delineation Transmission frame adaptation Transmission frame generation/recovery ATM Bit timing PM Physical medium Physical Medium Semester 2-2005 Advanced Telecommunications 143.466 Slide 6 TC Transmission Convergence PL Physical Layer 2
ATM Layering - 4 Upper Layer Protocols service definitions User Layer Protocols PDU s Upper Layer Protocols service definitions AAL Layer service definitions ATM Layer service definitions Physical Layer AAL PDU s ATM PDU s (cells) Cells - Framing? AAL Layer service definitions ATM Layer service definitions Physical Layer B-ISDN PDU s and Service Definitions Semester 2-2005 Advanced Telecommunications 143.466 Slide 7 = Service Access Point, SAP ATM - Cell Header - 1, first 5 octets Transmit Order Bit positions 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 GFC VPI VPI VPI VCI VPI VCI VCI VCI VCI PT C VCI PT C HEC HEC Header at UNI Header at NNI VPI = Virtual Path Identifier VCI = Virtual Channel Identifier GFC = Generic Flow Control PT = Payload Type C = Cell Loss Priority HEC = Header Error Control Semester 2-2005 Advanced Telecommunications 143.466 Slide 8 ATM - Cell Header - 2 Major Header Functions VPI & VCI information. Cell Loss Priority Bit - may discard cell if set to 1. Payload Type -3 bits. Generic Flow Control - GFC. For control signalling and OAM functions at the UNI. Header Error Control - HEC. An 8 bit field, error checking the first 4 octets (uses BCH code). Will auto correct single bit errors and identify most multiple bit errors. All resultant uncorrected/errored cells are discarded. HEC may also be used for cell delineation. Semester 2-2005 Advanced Telecommunications 143.466 Slide 9 3
ATM - Physical layer - 1 Physical Layer - Examples of Transmission Convergence (TC) functions. Cell Rate Decoupling Header Error Control - HEC Cell Delineation Semester 2-2005 Advanced Telecommunications 143.466 Slide 10 ATM - Physical Layer - 2 Transmission Convergence - Cell Rate Decoupling If no information is passed down from the ATM layer, the TC sub-layer in the transmitter generates Idle Cells to maintain the specified data rate. Some systems are Cell-based with no framing (eg. LAN s), others are Frame-based, eg. STM-1 frames in SDH. In the former case the user rate is 149.760 Mbps. In the latter case the rate is the well known 155.520 Mbps. ie. 149.760 = 155.520 * (26 : 27). In a practical case this means inserting a PLOAM cell after each 26 cells. See ATM LAN networks. Semester 2-2005 Advanced Telecommunications 143.466 Slide 11 Physical Layer - 3 Header Error Control (HEC) function The HEC protects the first 4 octets of Header only - not the cell! Designed to correct isolated single bit errors and detect a very wide range of multiple bit errors. The algorithm is conservative. Multi-bit error detected (Cell discarded) No error Detected (No action) Detection Mode No error detected (No action) Correction Mode Error detected (Cell discarded) Single -bit error detected (Correction) Semester 2-2005 Advanced Telecommunications 143.466 Slide 12 4
Physical Layer - 4 Cell Delineation - Assuming no errors, HEC s in successive cells will only be correct if the cell boundaries have been maintained on reception. Assume the TC sublayer examines 40 bits in a shift register. If this is a valid header the last 8 bits will be the correct HEC for the first 32 bits, ie. the cell boundary has been maintained. If the HEC is incorrect, a one bit shift is made and the HEC recalculated. This bit by bit checking is defined as the hunt state. Alpha consecutive Incorrect HEC Cell-by-Cell check Bit-by-Bit Check synch hunt Delta consec. correct HEC Incorrect HEC Correct HEC presynch Cell-by-cell check Semester 2-2005 Advanced Telecommunications 143.466 Slide 13 Adaptation Layer - AAL AAL is the layer which adapts the cell-based ATM layer to packet, datagram, or bit stream oriented higher layers. Four different types of AAL. AAL is considered to consist of 2 internal sublayers: * Convergence Sublayer (CS) *Segmentation and Reassembly Sublayer (SAR) Semester 2-2005 Advanced Telecommunications 143.466 Slide 14 Adaptation Layer - 2 Service Parameters Timing Compensation Bit Rate Connection Mode Example Class A Class B Class C Class D Required Constant circuit emulation Connection- Oriented (CO) variable bit rate video Not Required Variable CO data transfer Connection- Less (CL) CL data transfer AAL Type Type 1 Type 2 Type3 Type 4 Type 5 Service Classification / AAL Types Semester 2-2005 Advanced Telecommunications 143.466 Slide 15 5
AAL - 3 A Virtual Connection (VC) endpoint consists of a physical medium interface, an ATM layer, and the Adaptation layer as shown. The figure shows multiple user services being simultaneously supported by the AAL, each providing for a different type of transfer: eg. simultaneous video and file transfers. Service P Service Q Service R RX VCI/VPI Convergence Sublayer Segmentation and Reassembly Sublayer TX VCI/VPI RX VCI/VPI TX VCI/VPI RX VCI/VPI TX VCI/VPI AAL ATM Layer ATM cell transport media Semester 2-2005 Advanced Telecommunications 143.466 Slide 16 AAL - 4 AAL Types AAL Type 1 (AAL1) - is specified for Constant Bit Rate (CBR) services for which clock signals at the receiver may need to be provided by the AAL from the incoming bit stream, eg. circuit emulation. AAL Type 2 (AAL2) - is specified for Variable Bit Rate (VBR) services, eg. certain video services. AAL Type 3/4 (AAL3/4) - is specified for packet transport services. This AAL is a merging of the earlier specified types 3 and 4 AAL Type 5 (AAL5) - Simpler than AAL3/4; provides best working with current LAN technologies. Semester 2-2005 Advanced Telecommunications 143.466 Slide 17 AAL - 5 For AAL3/4 and AAL5, the I.363 Rec. further divides the CS into an upper and lower half; ie. Service Specific Convergence Sublayer (SSCS) Common Part Convergence Sublayer (CPCS) Type 1 Type 2 Type 3/4 Type 5 SSCS SSCS CS CS CPCS CPCS SAR SAR SAR SAR ATM Semester 2-2005 Advanced Telecommunications 143.466 Slide 18 6
AAL - 6 AAL Convergence and SAR - a generic view, transmit side CS and SAR Higher Layer PDU AAL_SDU header CS_PDU trailer 44,47 or 48 H 48 H H T H 53 Semester 2-2005 Advanced Telecommunications Cells 143.466 Transmitted Slide 19 The Type 1 AAL - 1 Transfer of Constant Bit Rate Services Transfer of Timing Information Indication of Lost/Errored Information Functions: Segmentation and reassembly of user information Handling of cell delay variation Handling of lost & miss-inserted cells Source clock recovery at receiver Monitoring and handling of certain bit errors Semester 2-2005 Advanced Telecommunications 143.466 Slide 20 The Type 1 AAL - 2 AAL Type - Format 47 Octets SN SNP Payload 4 bits 4 bits Header SN = Cell Sequence Number SNP = Sequence Number Protection Semester 2-2005 Advanced Telecommunications 143.466 Slide 21 7
The Type 1 AAL - 3 Sequence Number (SN) field Three Bit Sequence Count field CSI Bit - carries the CS Indication - from CS layer CSI Bit Sequence Count Field - 3 bits Sequence No. Field - 4 bits Semester 2-2005 Advanced Telecommunications 143.466 Slide 22 The Type 1 AAL - 4 Sequence Number Protection (SNP) field Provides error protection and some auto correction The SN field is protected by a 3 bit CRC code which provides single bit correction and multiple bit detection. The resulting 7 bit word is protected by an even parity check bit (simple error detection). Even Parity Bit CRC Field - 3 bits SNP Field Format Semester 2-2005 Advanced Telecommunications 143.466 Slide 23 The Type 1 AAL - 5 The CS Protocol for AAL Type 1 A number of functions may be handled - eg. Source Clock recovery at the receiver, may use Adaptive Clock or Time Stamp. Adaptive Clock The local CS reads the buffer of the incoming cells. The buffer fill level provides a mechanism to control the local clock. Synchronous Residual Time Stamp (SRTS) Uses a Residual Time Stamp (RTS) to measure and convey information to the receiver about the difference between a common reference clock derived from the network and the service clock at the sender. The RTS is transmitted in the CSI bits of successive s. Semester 2-2005 Advanced Telecommunications 143.466 Slide 24 8
AAL Type 3/4 - Overview Higher Layer PDU CPCS-H CPCS_PDU pad CPCS-T BOM SAR head COM SAR trail COM BOM = Beginning of Message COM = Continuation of Message EOM = End of Message ATM cell EOM SAR head SAR trail Semester 2-2005 Advanced Telecommunications 143.466 Slide 25 AAL Type 5 - Overview Higher Layer PDU CPCS_PDU AAU 0 AAU 0 AAU 0 AAU = ATM user-to-user Indication. It is inserted into the Payload type field of the ATM Cell header. ATM Cell AAU 1 pad CPCS trailer Semester 2-2005 Advanced Telecommunications 143.466 Slide 26 Comparing AAL3/4 and AAL5 AAL5 AAL3/4 CPCS-PDU Comparison Octets 1 1 2 1-65,535 0-3 1 1 2 CPI Btag BA Size CPCS_PDU pad AL Etag Length CPCS_PDU header CPCS_PDU trailer CPI = Common Part Indicator Btag = Beginning Tag BA Size = Buffer Allocation Size AL = 32 bit Alignment Etag = Ending Tag Length = CPCS_PDU Length Octets 1-65,535 0-47 1 1 2 4 CPCS_PDU pad CPCS -UU CPI Length CRC CPCS_PDU trailer CPCS-UU = CPCS user-to-user indicator Semester 2-2005 Advanced Telecommunications 143.466 Slide 27 9
Comparing AAL3/4 and AAL5 A comparison at the SAR sublayer is made. (The AAL type1 is also drawn for reference) Type 1 Type 3/4 SN SNP 1 octet S T SN 47 octets MID LI CRC 2 octets 44 octets 2 octets SN = Sequence No. ST = Segment Type MID = Multiplexing Ident. LI = Length Indicate CRC = Cyclic Redundancy Check Type 5 48 octets Semester 2-2005 Advanced Telecommunications 143.466 Slide 28 AAL5 - A final note A single bit in the PTI field of the cell header is termed the ATM-layer-user-to-ATMlayer-user parameter - AAU. AAU = 1 to mark the last of a CPCS_PDU (or the only of a small CPCS_PDU). AAU = 0 for cells carrying the beginning and continuation s ( for CPCS_PDU s spanning more than one cell. See Slide titled AAL Type 5 - Overview Cell Header 48 octets One Bit AAU in PTI Semester 2-2005 Advanced Telecommunications 143.466 Slide 29 Comparative Reviews Now that we have provided an overview of ATM and other forms of packet switching, we shall now revisit these earlier technologies and provide some comparative reviews of ATM versus Frame Relay ATM versus LAN technologies ATM versus IP Semester 2-2005 Advanced Telecommunications 143.466 Slide 30 10
ATM Versus Frame Relay - 1 The following diagrams compare the headers for Frame Relay and ATM respectively. Notice that they are actually very similar: Frame Relay ATM Flag 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 DLCI C/R E GFC VPI DLCI F BDE VPI VCI I (variable) VCI FCS VCI PT C FCS HEC Payload (fixed @ 48 bytes) Flag Semester 2-2005 Advanced Telecommunications 143.466 Slide 31 ATM Versus Frame Relay - 2 Key to acronyms: B = Backward explicit congestion notification bit (BECN) C = Cell loss priority bit (CLP) C/R = Command/response bit D = Discard eligibility bit (DE) DLCI = data link connection identifier E = Address extension bit F = Forward explicit congestion notification bit (FECN) FCS = Frame check sequence field GFC = Generic Flow Control field HEC = Header Error Control field I = Information field (user traffic) PT = Payload Type identifier VCI = Virtual Channel Identifier VPI = Virtual Path Identifier Semester 2-2005 Advanced Telecommunications 143.466 Slide 32 ATM Versus Frame Relay - 3 Comments: Each contains a virtual circuit ID Both contain bits to allow traffic to be tagged if there are any difficulties present in the network. (DE in Frame Relay and CLP in ATM) Both allow for congestion notification. For FR this facility is provided in the FECN and BECN bits. For ATM this is done in the bits residing in the type identifier PTI. Note: ATM provides no mechanism for identifying forward or backward congestion notification bits. The Frame Relay header is embedded in another PDU which is considered part of the overall FR header and trailer. By contrast, ATM doesn t have flag type fields and error checking is done in the fifth byte of the header. Semester 2-2005 Advanced Telecommunications 143.466 Slide 33 11
ATM Versus Frame Relay - 4 Attribute Frame Relay ATM Application support Asynchronous data Asynchronous, not designed for voice. synchronous voice, video, data. Connection mode Connection-oriented Connection-oriented Congestion management Yes, congestion notification, traffic tagging and possible traffic discard. PVC Yes Yes SVC (connection on demand) Congestion notification technique LAN or WAN technology PDU size ACK/NAK retransmissions Yes The FECN and BECN bits WAN based Variable (PDU is called a frame) No Yes, congestion notification, traffic tagging and possible traffic discard. Semester 2-2005 Advanced Telecommunications 143.466 Slide 34 Yes The CN bits in the PTI field Either Fixed at 53 bytes. PDU is called a cell. Only for signalling traffic on SVCs ATM Versus LAN Technologies - 1 Attribute Ethernet IEEE 802.3 ATM Application support Asynchronous data with some voice, but not designed for voice. Asynchronous data with some voice, but not designed for voice. Asynchronous, synchronous voice, video, data. Connection mode Connectionless Connectionless Connection-oriented Congestion management Method of identifying traffic Congestion notification technique Collision detection Collision detection Yes, congestion notification, traffic tagging and possible traffic discard. 48-bit MAC address 48-bit MAC address Virtual circuit ID: The VPI/VCI and an ATM address None None The CN bits in the PTI field PDU size Variable Variable Fixed at 53 bytes. PDU is called a cell. ACK/NAK retransmissions No No Only for signalling traffic on SVCs Semester 2-2005 Advanced Telecommunications 143.466 Slide 35 ATM Versus IP - 1 These two technologies use different identifiers and different encapsulation headers. ATM is connection oriented IP is connectionless. Semester 2-2005 Advanced Telecommunications 143.466 Slide 36 12
ATM Versus IP - 2 Attribute IP ATM Application support Asynchronous data Asynchronous, not designed for voice. synchronous voice, video, data. Connection mode Connectionless Connection-oriented Congestion management Method of identifying traffic Congestion notification technique None 32-bit IP address None Yes, congestion notification, traffic tagging and possible traffic discard. Virtual circuit ID: The VPI/VCI and an ATM address The CN bits in the PTI field PDU size Variable Fixed at 53 bytes. PDU is called a cell. ACK/NAK No Only for signalling traffic retransmissions on SVCs Semester 2-2005 Advanced Telecommunications 143.466 Slide 37 13