ATM Standards
ATM standards are set by the ITU-T. Beginning in 1988 with the first set of B-ISDN standards recommendations, ATM and B-ISDN were inexorably linked. In 1992 the ATM Forum was formed as a voluntary organization of manufacturers, consultants, and interested parties to develop interoperability specifications based on member consensus. The Internet Engineering TaskForce (IETF) also has gotten involved, as ATM has significant implications relative to the Internet, as least at a backbone level. While a lot of work remains to be done in the arena of ATM standards, the following provides a view of some of the more important standards and standards under development and review.
ITU-T Standards of significance include:
- I.113: B-ISDN Vocabulary
- I.121: Broadband Aspects of ISDN
- I.150: B-ISDN ATM Functional Characteristics
- I.211: B-ISDN Service Aspects
- I.311: B-ISDN General Network Aspects
- I.321: B-ISDN Protocol Reference Model
- I.327: B-ISDN Functional Architecture Aspects
- I.361: B-ISDN ATM Layer Specification
- I.362: B-ISDN ATM Adaptation Layer Functional Description
- I.363: B-ISDN ATM Adaptation Layer Specification
- I.413: B-ISDN User-Network Interface
- I.432: B-ISDN User-Network Interface-Physical Layer Specification
- I.555: Frame Relay and ATM Internetworking
- I.610: B-ISDN Operations and Maintenance Principles and Functions
ATM Forum Implementation Documents of significance include the following:
- ATM User-Network Interface (UNI) Specification for PVCs
- ATM Broadband InterCarrier Interface (B-UNI) Specification
- ATM Data Exchange Interface (DXI) Specification
Internet Engineering Task Force (IETF) RFCs (Requests for Comment) include:
- RFC 1483: Definition of Multiprotocol Encapsulation over AAL 5
- RFC 1577: Definition of Internet IP over ATM
ATM Access
Access to an ATM backbone network is at rates equal to or greater than DS-1. Whether access is from an end-user CPE environment or from a Frame Relay, X.25, or SMDS network switch, the access rate is DS-1, DS-3, or perhaps at a higher rate such as OC-3 (155 Mbps). Access is provided through a User Network Interface (UNI) specified by the ATM Forum; in each case the interface address is 20 octets in length.
- User Network Interface (UNI)
- specifically refers to a private UNI between a user device and an ATM network. At this level, the ATM network assumes all responsibility for conversion of the user Protocol Data Unit (PDU) into ATM PDUs and cells. A variation of the UNI concept also is employed by ATM users to access a SMDS network. This SIP Connectionless Service Protocol is via ATM Adaption Layer (AAL) 3/4. In the future, the user interface usually will be through a SDH/SONET connection, although FDDI connectivity will be supported, as well.
- Data Exchange Interface (DXI)
- is a private UNI for end user access to an ATM Network from DTE/DCE such as a bridge, router, or ATM DSU. The DXI concept allows for the sharing of protocol responsibility between the user and the network provider. At the Physical Layer, the DXI allows for connection via V.35, EIA 449/530, or EIA 612/613 HSSI connection. A variation of the HDLC protocol is used at the Data Link layer. The user information is encapsulated within a DXI (HDLC) frame and converted to the appropriate, class-specific ATM protocol at the DCE. The DCE assumes responsibility for functions through Segmentation and Reassembly (SAR), presenting the data to the ATM network switch in ATM cells. There exist several DXI modes, which correspond to AAL 3/4 and AAL 5.
- Network-to-Network Interface (NNI)
- also known as B-ICI (B-ISDN InterCarrier Interface), is a public UNI for interconnection of public networks via PVCs. NNIs exist for interconnection of ATM networks; NNIs also exist for interconnection of ATM networks and Frame Relay networks. Interoperability issues that must be addressed include protocol conversion, mapping between virtual circuits, alignment of traffic management parameters, and mapping of local network management information.
- Frame UNI (FUNI)
- is a derivative of the DXI standard. FUNI was developed by the ATM Forum in early 1995 to extend ATM access to smaller sites at rates from 56 Kbps to 1.544 Mbps (T1). Low-speed data enters a router, which forwards the data to the ATM switch as frames similar to Frame Relay frames. Those frames then are converted to cells.
ATM Network and Equipment
The ATM Network consists of Customer Premise Equipment (CPE), Broadband Switching Systems (BSSs) and interconnecting transmission facilities (Figure 11.7) [11-9] and [11-38].
Figure 11.7 ATM Network. Source: TA-NWT-001110, Copyright © 1993, Bellcore. Reprinted with Permission.
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