Chapter 5 Asynchronous Transfer Mode ATM 1 Introduction
- Slides: 39
Chapter 5 Asynchronous Transfer Mode (ATM) 1
Introduction ATM Protocol Architecture l Logical connections l ATM cell structure l Service levels/categories l ATM Adaptation Layer (AAL) l Chapter 5 ATM 2
Introduction l ATM evolved from B-ISDN development efforts – Frame Relay: high-speed WAN (1. 5+ Mbps) – ATM: very high speed WAN (155 Mbps and 622 Mbps) l ATM, like Frame Relay, was built on the assumption that the underlying physical media was reliable and flexible – minimal error and flow control capabilities – even more streamlined, therefore faster, than Frame Relay l Specifications developed by ITU-T and ATM Forum Chapter 5 ATM 3
ATM Protocol Architecture l Fixed-size packets called cells l 2 primary protocol layers relate to ATM functions: – “cell switching” like packet switching – Common layer providing packet transfers, logical connections (ATM) – Service dependent ATM adaptation layer (AAL) l AAL maps other protocols to ATM – like IP (AAL 5) Chapter 5 ATM 4
Protocol Model has 3 planes User – provides for user information transfer and associated controls (flow control, congestion control) l Control – performs call control and connection control functions (signaling) l Management – provides plane management and layer management and coordination functions l Chapter 5 ATM 5
ATM Protocol Reference Model Map data to the ATM cell structure Framing, cell structure & Logical Connections Various data rates (155. 52 Mbps, 622. 08 Mbps) over various physical media types (Fiber Optic, SONET, UTP, etc. ) Chapter 5 ATM 6
User Plane Layers User information AAL ATM ATM PHY PHY … End system Chapter 5 ATM Network End system 7
User Plane Layers User information Chapter 5 ATM User information 8
Logical Connections l VCC (Virtual Channel Connection): a logical connection analogous to a virtual circuit in X. 25, or Frame Relay data link connection – full-duplex flow between end users – user-network control signaling – network-network management/routing l VPC (Virtual Path Connection): a bundle of VCCs with the same end points (not necessarily same endusers) – and switched along the same path Chapter 5 ATM 9
ATM Connection Relationships Virtual Channel: basic logical communications channel Virtual Path: groups of “common” virtual channels Physical Transmission Path: physical communications link Chapter 5 ATM 10
Advantages of Virtual Paths l l Simplified network architecture – allows separation of functionality into individual logical connections and related groups of logical connections Increased network performance and reliability – network consists of fewer aggregated entities l Reduced processing and short connection setup time – complex setup tasks are in virtual paths, simplifies setup of new virtual channels over existing virtual path l Enhanced network services – supports userspecified closed groups/networks of VC bundles Chapter 5 ATM 12
Virtual Path/Virtual Channel Terminology Virtual Channel (VC) A generic term used to describe unidirectional transport of cells associated by a common unique identifier Virtual Channel Identifier (VCI) A unique numerical tag for a particular VC link Virtual Channel Link A means of unidirectional transport of cells between the point where a VCI is assigned and where it is translated or terminated Virtual Channel Connection (VCC) A concatenation of VC links that extends between two connected ATM end-points Chapter 5 ATM 13
Virtual Path/Virtual Channel Terminology Virtual Path Identifier Virtual Path Link Virtual Path Connection Chapter 5 ATM (VP) A generic term which describes unidirectional transfer of cells that are associated with a common unique identifier (VPI) Identifies a particular VP A group of VC links identified by a common identifier between the point where the identifier (VPI) is assigned and where it is translated or terminated (VPC) A concatenation of VP links that extends between ATM end-points where the VCIs are assigned and where they are translated or terminated 14
ATM VPC/VCC VP 3 a b c d e ATM Sw 1 a VP 5 ATM Sw 2 ATM DCC ATM Sw 3 VP 6 b c VP 2 VP 1 Sw = switch ATM Sw 4 d e DCC = Cross-connect switch Chapter 5 ATM 15
ATM Connection Relationships Chapter 5 ATM 16
VPC/VCC Characteristics Quality of Service (Qo. S) l Switched and semi-permanent virtual channel connections l Cell sequence integrity l Traffic parameter negotiation and usage monitoring l – average rate, peak rate, burstiness, peak duration, etc. l (VPC only) virtual channel identifier restriction within a VPC Chapter 5 ATM 17
Control Signaling A mechanism to establish and release VPCs and VCCs (per ITU-T Rec. I. 150) l 4 methods for VCCs: l – Semi-permanent VCC: no control signaling required – Meta-signaling channel: permanent, low data rate channel for setting up signaling channels – User-to-network signaling virtual channel: set up between user and network – User-to-user signaling virtual channel: set up between users within a VPC, allowing users to set up and tear down VCCs, without network intervention Chapter 5 ATM 20
ATM Cells l Fixed size – 5 -octet header – 48 -octet information field Small cells may reduce queuing delay for high-priority cells (essential for low delay) l Fixed size facilitates more efficient switching in hardware (essential for very high data rates) l Chapter 5 ATM 22
ATM Cell Format (p. 98) Chapter 5 ATM 23
Header Format Generic flow control (more ->) l Virtual path identifier (VPI) l Virtual channel identifier (VCI) l Payload type (3 bits: identifies cell as user data or network management cell, presence of congestion, SDU type) l Cell loss priority (0: high; 1: low) l Header error control (more ->) l Chapter 5 ATM 24
Generic Flow Control l Used to control traffic flow at usernetwork interface (UNI) to alleviate short-term overload conditions – Note: not employed in network core l When GFC is enabled at the UNI, two procedures are used: – Uncontrolled transmission: not subject to flow control – Controlled transmission: flow control constraints (using GFC mechanism) are in force Chapter 5 ATM 25
Header Error Control l 8 -bit field - calculated based on the other 32 bits in the header – CRC based on x 8 + x 2 + x + 1 -> generator is 100000111 error detection l in some cases, error correction of single-bit errors in header l 2 modes: l – Error detection – Error correction Chapter 5 ATM 27
HEC Operation at Receiver Based on recognition of fact that bit errors occur in bursts. Chapter 5 ATM 28
ATM Service Categories l Real-time service – Constant bit rate (CBR) – Real-time variable bit rate (rt-VBR) l Non-real-time service – Non-real-time variable bit rate (nrt-VBR) – Available bit rate (ABR) – Unspecified bit rate (UBR) – Guaranteed frame rate (GFR) Chapter 5 ATM 31
ATM Bit Rate Service Levels Chapter 5 ATM 32
ATM Adaptation Layer (AAL) l Support higher-level protocols and/or native applications – e. g. , PCM voice, LAPF, IP l AAL Services – Handle transmission errors – Segmentation/reassembly (SAR) – Handle lost and misinserted cell conditions – Flow control and timing control Chapter 5 ATM 33
ATM Adaptation Layer (AAL) Chapter 5 ATM 34
Applications of AAL and ATM Circuit emulation (e. g. , T-1 synchronous TDM circuits) l VBR voice and video l General data services l IP over ATM l Multiprotocol encapsulation over ATM (MPOA) l LAN emulation (LANE) l Chapter 5 ATM 35
AAL Protocol and Services Basis for classification: • requirement for a timing relationship between source and destination • requirement for a constant bit rate data flow • connection or connectionless transfer Chapter 5 ATM 36
AAL Protocols l AAL layer has 2 sublayers: – Convergence Sublayer (CS) Supports specific applications/protocols using AAL l Users attach via the Service Access Point (like a port number) l Common part (CPCS) and application service -specific part (SSCS) l – Segmentation and Reassembly Sublayer (SAR) l Chapter 5 ATM Packages data from CS into ATM cells and unpacks at other end 37
AAL Protocols and PDUs Chapter 5 ATM 38
AAL Protocol Descriptions Chapter 5 ATM 39
Segmentation and Reassembly PDUs Chapter 5 ATM 40
AAL Type 1 Constant-bit-rate source l SAR simply packs bits into cells and unpacks them at destination l One-octet header contains 3 -bit SC field to provide an 8 -cell frame structure l No CS PDU structure is defined since CS sublayer primarily for clocking and synchronization l Chapter 5 ATM 41
AAL Type 1 Chapter 5 ATM 42
AAL Type 2 l Intended for use with applications with variable bit-rate service on multiple channels (multiplexing), or low bit rate, short-frame traffic AAL Type 3/4 l l l Intended for variable bit rate applications that generate bursty data and demand low loss Originally, connectionless (AAL 4) or connection (AAL 3) oriented, now combined into single format (AAL 3/4) Provides comprehensive sequencing and error control mechanisms Chapter 5 ATM 43
AAL 3/4 Chapter 5 ATM 45
AAL Type 5 l Streamlined transport for connection oriented protocols – Reduce protocol processing overhead – Reduce transmission overhead – Ensure adaptability to existing transport protocols – primary function is segmentation and reassembly of higher-level PDUs Chapter 5 ATM 46
AAL 5 Chapter 5 ATM 48
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