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ATM Performance: Quality of Service (QoS)

ATM performance parameters are defined in ITU-T Recommendation I.350. Those parameters are used to measure B-ISDN performance of an end-to-end user-oriented connection in terms of Quality of Service (QoS), specific to each service class. The ATM Forum has extended this standard through the definition of QoS parameters and reference configurations for the User Network Interface (UNI).

Performance Parameters defined by the ITU-T address accuracy, dependability and speed. Accuracy parameters include Cell Error Ratio (CER) and Severely Errored Cell Block Ratio (SECBR). Dependability parameters include Cell Loss Ratio (CLR) and Cell Misinsertion Rate (CMR). Speed parameters address Cell Transfer Delay (CTD), Mean Cell Transfer Delay (MCTD) and Cell Delay Variability (CDV).

Quality of Service (QoS) objectives to be set by the network providers include Class of Service 1, 2, 3, and 4. QoS Class 1 addresses Service Class A traffic, providing QoS equivalent to digital private lines. QoS Class 2 supports Service Class B traffic such as audioconferencing, videoconferencing and multimedia. QoS Class 3 addresses Service Class C traffic, including connection-oriented data protocols such as SDLC and Frame Relay. QoS Class 4 supports Service Class D traffic involving connectionless data protocols such as SMDS.

ATM Advantages

ATM clearly offers the great advantage of lots of bandwidth, with access rates usually at T1 or T3, and OC-3 in some cases. The backbone operates at OC-3 (155 Mbps) or better. Further ATM is the first service to offer true bandwidth-on-demand. That bandwidth also is provided over high-quality, fiber optic facilities that provide excellent error performance. In combination, ATM’s strong error performance, access control and congestion control yield outstanding levels of throughput.

ATM also is highly flexible. It is the only technology that can operate equally well in LAN, MAN, and WAN environments. ATM also handles any form of data—voice, facsimile, data, image, video and multimedia—and with Quality of Service levels geared to match traffic requirements. Further, the traffic can be asynchronous, synchronous, or isochronous in nature, and in any combination. Although some standards have yet to be fully defined, ATM offers interconnection with Frame Relay, SMDS, and X.25 networks.

Much as we discussed relative to SMDS, ATM offers mesh networking without the need for complex and expensive leased lines. As a result, network configuration and reconfiguration are much simplified. Finally, ATM networks and associated costs are scalable. In other words, the cost of the network is very much in proportion to the scale of the network in terms of attached devices, networked locations, and bandwidth requirements.

Also, ATM is medium-independent to some extent. While the backbone ATM networks are designed around a SONET fiber optic transmission system, UTP and STP can be used to attach workstations to an ATM LAN switch. The local access facilities generally are copper or microwave at the T1 and T3 levels. Fiber is required at higher speeds such as OC-3 (155 Mbps) and above. Recently, ATM over VSATs was made available internationally in order to leapfrog national networks that do not support ATM. Rates are 8 Mbps or below, and 34/45 Mbps.

ATM Disadvantages

ATM suffers from limited availability, which is not surprising, given its youth and inexperience. LECs currently offering public ATM service in some metropolitan areas include Ameritech (CBR and VBR; options include T1 ATM, ATM-to-LAN, and Frame Relay-over-ATM), BellSouth, Cincinnati Bell (ATM and Frame Relay-over-ATM), GTE, Pacific Bell (CBR and VBR; ABR planned), Southwestern Bell (CBR and VBR), and US West (CBR and VBR; ABR planned by 1997). NYNEX announced an ATM-based Business Network Architecture to be built in 1996. In June 1996, Bell Atlantic announced trials in New Jersey and the Washington, D.C. area [11-41], and plans to offer ATM in six metropolitan areas on a flat-rate basis by the end of 1996. CAPs offering ATM services include MFS (CBR, VBR and voice-over-ATM) and Teleport Communications Group (CBR and VBR; options include LAN interconnection and broadband interconnection [11-42], [11-43], and [11-44].

ATM is particularly limited with respect to international availability, although Western Europe clearly is an exception. Connecting from the United States even to Europe is most unusual. However, Tandem Computer and several client organizations recently completed an 18-month trial of ATM as a replacement for Frame Relay connections between the United States and Europe. Through the PTAT submarine fiber optic cable, they established a T3 link leased from Cable and Wireless, using Stratacom switches. The test included ABR, VBR, and CBR services, including compressed voice over VBR. Results were reported to be highly positive [11-45].

Aside from issues of availability, it should be noted that ATM is not an inexpensive technology. Complete equipment upgrade is generally required in the form of LAN switches, routers, and DSUs. The cost of carriers services is not trivial, either. It also is worth noting that much work remains to be done on ATM standards. Standards for voice and video over ATM are not yet solidified, and network-to-network interfaces have yet to be fully defined. Network management standards are a long way from being complete, as one would expect with such a new technology.


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