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SMDS Costs

Equipment costs include SIP-capable hosts, bridges, routers, and DSUs, in some combination. The DSU, also known as a SMDSU, typically is contained under the skin of a DS-1 or DS-3 MUX.

Network access costs include a dedicated access line to a SMDS network node. The dedicated circuit may be FT1 (N x 64 Kbps), DS-1 (T1 at 1.544 Mbps or E1 at 2.048 Mbps, or DS-3 (T3 at 45 Mbps or E3 at 34 Mbps). Network costs also typically include a port charge and monthly usage charge, per address. Table 11.3 shows an example of network charges, courtesy of MCI Communications, the only IXC offering SMDS. This plan sets fixed prices of 4.1 cents per MB of data at speeds < T1 and 3 cents per MB at greater speeds; in every case a usage cap is set [11-32] and [11-33].

Table 11.3 MCI SMDS pricing, March 1996

Monthly Port Charge/Address Maximum Monthly Usage Charge/Address

Bandwidth RBOC SMDS Access Private-Line Access RBOC SMDS Access Private-Line Access

64 Kbps $50 $180 $293 $164
128 Kbps N/A $336 N/A $327
256 Kbps N/A $394 N/A $654
384 Kbps N/A $578 N/A $980
512 Kbps N/A $735 N/A $1,307
768 Kbps N/A $946 N/A $1,547
1.024 Mbps N/A $1,178 N/A $1,620
1.526 Mbps $250 $1,470 $3,037 $1,740
4.0 Mbps $750 $3,000 $3,940 $3,500
10.0 Mbps $1,500 $5,500 $5,833 $6,600
16.0 Mbps $2,000 N/A $8,813 N/A
25.0 Mbps $2,500 N/A $12,960 N/A
34.0 Mbps $3,250 N/A $16,524 N/A

Source: Network World, April 8, 1996

SMDS Advantages

The advantages of SMDS are considerable. Intended for LAN-to-LAN internetworking over the MAN, it is extendible to the WAN and has application to a broad range of data communications requirements. As shown in Table 11.3, its wide range of bandwidth options at lower speeds position it well against Frame Relay and ATM; further, it currently offers higher speeds that does Frame Relay.

Where available, an SMDS network is relatively simple to implement and reconfigure. Unlike Frame Relay, there is no requirement to establish PVCs with a connectionless service. Rather, any-to-any connectivity can be established based on SMDS addresses. The lack of PVCs and the scalability of SMDS make networking highly cost-effective compared with either a leased-line mesh network or a PVC-based Frame Relay network.

Further, SMDS offers excellent access control and congestion control, thereby avoiding the latency issues and exposure to data loss which are characteristic of X.25 and Frame Relay networks. These characteristics also make SMDS an excellent alternative for isochronous communications, which is intolerant of latency and loss. Finally, SMDS provides a smooth migration path to ATM via software upgrades—in fact, it was designed with ATM in mind.

SMDS Disadvantages

SMDS suffers from limited availability. Not all of the RBOCs and CAPs offer SMDS. Primarily intended as a MAN network technology, it can be extended to the WAN, although performance becomes something of an issue when distance increases. In any event, MCI stands alone in the IXC world as supporting SMDS. Unlike Frame Relay and ATM, SMDS is limited to a public offering—it just doesn’t make sense as a private network technology. The limited carrier support for SMDS also translates into limited manufacturer support. While large numbers of manufacturers build Frame Relay and ATM gear, SMDS equipment vendors can be counted on your fingers and toes, with digits to spare. This lack of vendor support also translates into a less competitive market with small manufacturing runs and, therefore, higher equipment costs that are not likely to come down [11-29].

Assuming that conventional wisdom is correct, ATM is the network of the future. Therefore, the future of SMDS is likely to be a short one. While it performs beautifully, is cost-effective and available, it just doesn’t have the horsepower (bandwidth) to compete with ATM in a B-ISDN scenario. Until ATM takes over, however, SMDS will continue to enjoy a small, but loyal, following. Even into the world of ATM, SMDS can continue to serve as an ATM access technology.

SMDS Applications

SMDS applications primarily are data in nature. Although intended for LAN internetworking in a Metropolitan Area Networking scenario, SMDS also serves to connect LAN-to-host, controller-to-host, terminal-to-host, host-to-host, and for host channel extension. SMDS also supports image networking beautifully, as well as point-of-sale networking, distributed process control, and desktop publishing. The service also can support isochronous data such as realtime digitized voice and video, although such application is unusual.

Bell Atlantic in March 1996 announced an interesting application for SMDS involving Internet Service Providers (ISPs). While still restricted to intraLATA service provisioning Bell Atlantic’s IP Routing Service (IPRS) will offer a means of providing large numbers of dialup users with analog or ISDN access. In each LATA a single CO switch will be equipped with the capability to accept that IP-addressed traffic and concentrate it through 34 Mbps SMDS pipes, accomplishing the cell segmentation process in the CO switch. At the receiving end, the ISP or intranet managers must place a high-capacity switch or router with a SMDS interface. The advantage is in the high-capacity network access, and elimination of ISDN termination equipment and banks of modems. [11-34].

Calgon recently installed a multisite SMDS network from MCI to link a number of systems, including IBM mainframes and token ring LANs. Applications supported include e-mail, order processing, and inventory updates. The 56-Kbps lines connect to Cisco routers, MultiTech DSU/CSUs, a Cabletron hub and TCP/IP and IPX network protocols. ATM was rejected for lack of standards; and Frame Relay due to the high cost of multiple PVCs. Plans are to extend the network throughout the entire corporation, and upgrade to 45 Mbps circuits [11-35].


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