ATM has become a very hot topic in the internetworking world. One of the important aspects of an ATM network is the interface between user equipment and the ATM network. This article covers the development efforts in the ATM community regarding this important interface.
To promote the rapid development of ATM standards and consequently to hasten product acceptance, a vendor consortium known as the ATM Forum discusses ideas and methods, culminating in "recommendations". These are not official standards, but are submitted to the ITU-T (formerly the CCITT) where they are given weighty consideration as part of the overall B-ISDN umbrella. The final standards usually differ in some small way, which requires the ATM vendors to modify pre-standard products, usually as a software update. This approach keeps ATM standards development on the "fast track". The Forum began with four members in 1991 (Cisco Systems, NET/Adaptive, Northern Telecom, and Sprint) and now numbers over 550!
A great deal of effort has been focused on the definition of the User-to-Network Interface (UNI) specification. The "user" is most often an ATM client (roughly analogous to the DTE) and the "network" is the ATM switch. When connecting a private ATM network to a public (carrier) network, the UNI also applies, with the switch on the private side taking the role of the user. UNI is an asymmetrical protocol (e.g., only the network side may allocate VCs). ATM is a connection-oriented service -- all connections are identified by the tuple of a virtual channel identifier (VCI) plus a virtual path identifier (VPI).
UNI 2.0, published in June 1992, supports only PVC operation which implies manual configuration. This is fine for limited testing but it certainly does not scale. However, this is the status of all ATM products today, unless the vendor has developed a proprietary SVC signaling technique. UNI 2.0 also recommended the use of a new ATM adaptation layer, AAL5, which is less complex and has less overhead than the AAL3/4 which is targeted for data transport.
The Interim Local Management Interface defines how certain basic operations are performed across the UNI. These include exchange of status, configuration, and control parameters of the link and physical layers, as well as an ATM address registration mechanism. It is initially based on SNMP directly over AAL (no UDP or IP).
The main thrust of UNI 3.0 is support for SVCs through a signaling protocol based on B-ISDN's Q.93B. Ratified at the October 1993 meeting of the ATM Forum, it is both a subset and superset of Q.93B -- the basic protocol was simplified but support for point-to-multipoint connections was added, essential for efficient multicasting and for video distribution. UNI 3.0 permits use of all AALs (1-5).
Q.93B allows for a single ATM physical address called E.164, devised to pass telephone numbers as BCD digits. UNI 3.0 also permits a private network address format based on the OSI layer 3 NSAP. It is intended that the ATM switch will supply the higher-order portion of the address while the ATM client will fill in the six-byte station ID, usually an 802-style MAC address. This provides hierarchy and unique automatic address assignment, even for mobile stations.
Another important development from the UNI Signaling Working Group is the definition of quality of service parameters which are passed in the connection setup message for each VC. Ones pertinent to LAN traffic include peak cell rate, peak burst length, and sustainable cell rate. The ATM network (series of switches over the path) confirm that resources are available and committed for the life of that VC. If not, the connection is rejected. LAN traffic connections are normally requested as a "best effort" service with no guarantees of end-to-end delivery. This "available bit rate" (ABR) category is appropriate for bursty LAN traffic which can tolerate varying delay and even low cell loss. This "contract" is expected to be honored by the ATM user/client and may be subject to enforcement within the switches (traffic policing). An early draft of UNI 3.0 has been available since August 1993.
The ITU-T subsequently modified UNI 3.0 "slightly" and issued it as official Recommendations Q.2931 and Q.2110. The ATM Forum is incorporating these modifications along with other embellishments into UNI 3.1, which is expected to be released by the end of 1994. Q.2110 defines the Service-Specific Connection-Oriented Protocol (SSCOP) which provides for reliable transfer of signaling (*not* network management or user data). Since the two UNI versions employ incompatible SSCOPs, care must be exercised to speak the same version over each physical link (not end-to-end).
UNI 4.0 is expected to provide for the negotiation/renegotiation of quality of service parameters. Presently the ATM network either accepts or rejects the clients parameters; a refusal leaves the client in a quandary -- try again later, play 20 questions, etc. And once accepted, the contract is frozen for the life of that VC. UNI 4.0 adds the needed flexibility at a cost of greater protocol (software) complexity.
Part of Cisco's broad support of this expanding ATM industry is the LightStream 2020 Enterprise ATM Switch. This second-generation scaleable ATM platform provides a tightly integrated architecture for ATM switching, bridging, LAN integration and wide-area networking. Cisco is committed to the acceleration of the LightStream 2020 product line developments to ensure that the rapid adoption of ATM is not slowed by lack of interfaces or IOS functionality.