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4. ATM (Asynchronous Transfer Mode)ATM is a high-performance, fixed-size cell-oriented switching and multiplexing technology to convey multiple service types, such as voice, video, and data. The ATM uses a 53-bytes fixed size cell. Each cell has 48 bytes of data and 5 bytes of header information. ATM networks are connection-oriented. A connection-oriented networking protocol establishes a specific route for traffic flows based on some connection identifier. The ATM structure is composed of three layers; Physical Layer, ATM Layer, and ATM Adoption Layer (AAL) as shown in Figure 4-1. The physical layer is divided into two sublayers. The ATM physical medium sublayer is responsible for transmission of data over the Physical Medium (PM), regardless of the type of medium used. ATM was originally designed to operate over fiber optics but modified to operate over copper and coaxial cables as well. The physical medium sublayer is responsible for receiving and transmitting bit streams in a continuous method. The Transmission Convergence (TC) sublayer is responsible for the transmission and reception of frames over framed transmission facility. ATM cells are packed into these frames and unpacked at the remote end. This sublayer also performs error detection/correction on the ATM header to prevent the cells from being sent to the wrong destination. The main task for the ATM layer are General Flow Control (CFC) function, cell's header generation/extraction, cell multiplexing and de-multiplexing, and Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI).
The ATM Adaptation Layer (AAL) relays ATM cells between ATM Layer and higher layer. The AAL makes the ATM layer services more adaptable to specific services. The specific services may include user services, control services and management services. The Segmentation and Reassembly (SAR) sublayer performs segmentation of the higher layer information into a size suitable for the payload of the ATM cells of a virtual connection and at the receive side, it reassembles the contents of the cells of a virtual connection into data units to be delivered to the higher layers. Different AALs were defined in supporting different traffic or services. AAL Type 1 supports constant bit rate (CBR), synchronous connection oriented traffic. AAL Type 2 supports real-time Variable Bit Rate (RT-VBR) of connection-oriented, synchronous traffic such as voice. AAL Type 3/4 supports non real-time (NRT-VBR) such as data traffic. AAL Type 5 is similar to AAL 3/4 with a simplified information header scheme. Examples of services are classic IP over ATM, Ethernet Over ATM, SMDS, and LAN Emulation (LANE). AAL 5 is a widely used ATM adaptation layer protocol. The functions of the transmission Convergence Sublayer (CS) differ depending on the medium being used. For instance, if SONET is the medium, the physical layer requires a different set of functions than a DS-3 medium would require. This sublayer provides whatever services are needed by each type of medium. The CS sublayer encapsulates the data arriving from upper levels, appends small headers, CRC, and trailers and prepares the data to be segmented in cells. ATM Related Standards ITU Recommendations: General Aspects I.121: Broadband Aspects of ISDN. This Recommendation states the basic principles of broadband aspects of integrated services digital network (B-ISDN). ATM is the transfer mode for implementing B-ISDN. I.320: ISDN protocol reference model. This Recommendation defines the ISDN Protocol Reference Model (ISDN PRM) to model the interconnection and exchange of information ??including user information and control information ??to, through or inside an ISDN. I.311: B-ISDN General Network aspects. This Recommendation describes ATM transport network structure composed of Physical Layer and ATM Layer. I.321: B-ISDN protocol reference model and its application Recommendation. This Recommendation describes B-ISDN PRM, which is based on the ISDN PRM defined in Recommendation I.320. I.326: Functional architecture of transport networks based on ATM. This Recommendation defines the functional architecture of ATM transport networks using the generic rules defined in ITU Rec. G.805. It includes the specific aspects regarding the characteristic information, client/server associations, the topology, the connection supervision and multipoint capabilities of ATM transport networks. I.327: B-ISDN functional architecture. The general functional architecture model, i.e. reference configurations, functional group, and reference points for the ISDN is described in Recommendation I.324. The objective of this Recommendation is to provide a basic functional architecture of the B-ISDN to complement Recommendation I.324. ATM Layer I.150: B-ISDN asynchronous transfer mode functional characteristics. This Recommendation addresses specifically the functions of the ATM layer. This layer is common to all services including signaling and OAM. I.356: B-ISDN ATM layer cell transfer performance. This Recommendation defines speed, accuracy, and dependability performance parameters for cell transfer in the ATM layer of a broadband ISDN. I.361: B-ISDN ATM layer specification. This Recommendation specifically addresses the cell structure and the ATM cell coding and the ATM protocol procedures. Service Aspects I.211: B-ISDN service aspects. This Recommendation is a guideline to the objective of providing detailed Recommendations on specific standardized services to be supported by a B-ISDN. AAL (ATM Adaptation Layer) I.362: [Withdrawn] B-ISDN ATM adaptation layer (AAL) functional description. Deleted, since the service classes defined therein were no longer appropriate and were in conflict with F-Series Recommendations. I.363: [Withdrawn] B-ISDN ATM adaptation layer (AAL) specification . This Recommendation was subdivided into ITU Recs. I.363.1 to I.363.5 when revised in 1996. I.363.1: B-ISDN ATM Adaptation Layer specification : Type 1 AAL I.363.2: B-ISDN ATM Adaptation Layer specification : Type 2 AAL I.363.3: B-ISDN ATM Adaptation Layer specification : Type 3/4 AAL I.363.5: B-ISDN ATM Adaptation Layer specification : Type 5 AAL Congestion Control I.371: Traffic control and congestion control in B-ISDN. This Recommendation provides a general description as well as objectives and procedures for traffic control and congestion control for the B-ISDN at the ATM layer. UNI (User Network Interface) I.413: B-ISDN user-network interface. This Recommendation gives the reference configuration for the B-ISDN user-network interface (UNI) and examples of physical realizations. It describes physical layer information flows according to the B-ISDN Protocol reference model and identifies interface functions and addresses OAM issues. I.432: [Withdrawn] B-ISDN User-Network Interface - Physical layer specification. This Recommendation was subdivided into ITU Recs. I.432.1 to I.432.5 when revised in 1996 I.432.1: B-ISDN user-network interface - Physical layer specification : General characteristics I.432.2: B-ISDN user-network interface - Physical layer specification : 155 520 kbit/s and 622 080 kbit/s operation I.432.3: B-ISDN user-network interface - Physical layer specification : 1544 kbit/s and 2048 kbit/s operation I.432.4: B-ISDN user-network interface - Physical layer specification : 51 840 kbit/s operation I.432.5: B-ISDN user-network interface - Physical layer specification : 25 600 kbit/s operation QAM (Operation and Maintenance) I.610: OAM principles of the B-ISDN access. The scope of this Recommendation is to identify functions required to operate and maintain the physical layer and ATM layer aspects of the B-ISDN. The functions apply to the Virtual Path (VP) and Virtual Channel (VC) connections that may be routed through the B-ISDN. I.751: Asynchronous transfer mode management of the network element view. This Recommendation covers the ATM NE plane management requirements and information model supporting the ATM layer, including any management requirements for the adaptation between the physical layer and the ATM layer (transport path/VP level adaptation). Functional capabilities of the ATM NE are given in Recommendation I.731, and in Recommendation I.732. ATM Equipment I.732: Functional characteristics of ATM equipment. This Recommendation describes the functional requirements to enable interoperability between ATM Network Elements (NEs). I.731: Types and general characteristics of ATM equipment I.732: Functional characteristics of ATM equipment I.761: Inverse multiplexing for ATM (IMA). This Recommendation defines a protocol for inverse multiplexing of an ATM cell stream over multiple physical links, and to retrieve the original stream at the far-end from these links. Multiplexing of the ATM cell stream is performed on a cell-by-cell basis across the multiple links. Interworking I.741: Inter-working and interconnection between ATM and switched telephone networks for the transmission of speech, voice-band data and audio signals. This Recommendation provides requirements for the interworking and interconnection functions between public ATM and other types of public switched telephone networks (including wireless networks) for the transmission of speech, voice band data and audio signals. Physical Link I.762: ATM over fractional physical links. This Recommendation specifies the requirements on how to map ATM on a "circuit-mode connection" supporting unrestricted information transfer rates at integer multiples of 64 kbit/s up to the maximum rate of the interface (ITU Recommendation I.231.10). Signaling AAL Q.2100: B-ISDN signaling ATM adaptation layer (SAAL) - Overview description. This Recommendation briefly describes the various components which make up the AAL functions necessary to support signaling (SAAL). Q.2144: B-ISDN signaling ATM adaptation layer - Layer management for the SAAL at the network node interface. This Recommendation provides Layer Management function for SAAL (Q.2100) at the NNI, which include the interfaces to the SSCOP, (Q.2110), to the SSCF at the NNI (Q.2140), and to systems management. SSCOP (Service Specific Connection Oriented Protocol) Q.2110: B-ISDN ATM adaptation layer - Service specific connection oriented protocol (SSCOP). This Recommendation specifies SSCOP, a transport protocol that provides guaranteed in-sequence delivery of messages to the signaling protocols and performs flow control, error reporting to the management plane, and a keep-alive function. Q.2111: B-ISDN ATM adaptation layer - Service specific connection oriented protocol in a multilink and connectionless environment (SSCOPMCE). The SSCOP (q.2110) in a Multilink and Connectionless Environment (SSCOPMCE) is an extension of the SSCOP. Q.2119: B-ISDN ATM adaptation layer - Convergence function for SSCOP above the frame relay core service. This Recommendation specifies a function that allows B-ISDN data communication applications utilizing protocol stacks which include SSCOP (Q.2110) to be deployed in an HDLC-based environment such as Frame Relay. UNI (User Network Interface) Q.2120: B-ISDN meta-signaling protocol. This Recommendation specifies the B-ISDN Meta-signaling protocol and procedures. The Meta-signaling protocol and procedures define the activity that assigns Signaling Virtual Channels on the B-ISDN User Network Interface (UNI). Once the signaling channels have been allocated, the signaling protocol stack can be created and signaling can proceed.
SSCF (Service Specific Coordination Function) Q.2130: B-ISDN signaling ATM adaptation layer - Service specific coordination function for support of signaling at the user-network interface (SSCF at UNI). The SSCOP provides an assured data transfer service and the Service Specific Coordination Function (SSCF). The SSCF at the UNI performs a coordination function between the service required by the signaling layer 3 (Q.2931) user and the service provided by SSCOP. Q.2140: B-ISDN ATM adaptation layer - Service specific coordination function for signaling at the network node interface (SSCF at NNI). This Recommendation describes for the SSCF at the NNI the mapping of primitives from layer 3 to signals of the SSCOP (Q.2110) and vice versa.
MFA Forum Specifications: There are a number of specifications published by MFA Forum. The ATM Forum and the ITU publish standards on ATM technology. Normally, the ATM Forum adapts the ITU recommendations and creates implementation agreements. The list of specifications is located at the Technology: ATM FORUM SPECIFICATIONS page. Key specifications are as follows:
af-arch-0193.000: ATM User-Network Interwork Interface (UNI) Specification Version 4.1. af-sig-0061.002: ATM User Network Interface (UNI) Signaling Specification version 4.1 af-bici-0013.003: B-ICI 2.0 (integrated specification) af-pnni-0055.002: Private Network-Network Interface Specification v.1.1 af-aic-0178.001: ATM-MPLS Network Interworking, Version 2.0 af-cs-0197.000: ATM-MPLS Network Interworking Signaling Specification 1.0 af-lane-0021.000: LAN Emulation over ATM 1.0 af-mpoa-0114.000: Multi-protocol Over ATM Specification, Version 1.1 af-pnni-0055.000: Private Network-Network Interface Specification V. 1.0 af-sec-0100.002: ATM Security Specification Version 1.1 af-sig.0061.000: UNI Signaling 4.0 af-tm-0121.000: Traffic Management 4.1 af-vmoa-0145.001: Loop Emulation Service Using AAL2 Rev 1
IETF RFCs: IRTF RFC specify methods for transporting packets and signaling in IP network over ATM network and vice versa. RFC 1755: ATM Signaling Support for IP over ATM. This document describes the ATM call control signaling exchanges needed to support Classical IP over ATM implementations as described in RFC 1577 (replaced with RFC 2225). RFC 2225: Classical IP and ARP over ATM. This RFC defines an initial application of classical IP and ARP in an Asynchronous Transfer Mode (ATM) network environment configured as a Logical IP Subnetwork (LIS) RFC 2684: Multiprotocol encapsulation over ATM adaptation layer 5. This RFC describes two encapsulations methods for carrying network interconnect traffic over AAL type 5 over ATM. RFC3336: PPP Over Asynchronous Transfer Mode Adaptation Layer 2 (AAL2). The Point-to-Point Protocol (PPP) provides a standard method for transporting multi-protocol datagrams over point-to-point links. This RFC describes the use of ATM Adaptation Layer 2 (AAL2) for framing PPP encapsulated packets. RFC3355: Layer Two Tunnelling Protocol (L2TP) Over ATM Adaptation Layer 5 (AAL5). This RFC is intended for implementations of L2TP that use ATM to provide the communications link between the L2TP Access Concentrator and the L2TP Network Server. RFC4454: Asynchronous Transfer Mode (ATM) over Layer 2 Tunneling Protocol Version 3 (L2TPv3). This RFC defines the specifics of how to use the L2TP control plane for ATM or transporting various ATM services over an IP network. RFC4717: Encapsulation Methods for Transport of Asynchronous Transfer Mode (ATM) over MPLS Networks. ATM Pseudowire (PW) is used to carry ATM cells over an MPLS network. This RFC specifies methods for the encapsulation of ATM cells within a PW to enable service providers to offer emulated ATM services over existing MPLS networks. |
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