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Switched virtual path connections (SVPC) are not currently supported by ATM Forum or Q.2931 signaling. SVPCs are anticipated to be available sometime near term as specifications are completed.
Emulation of time division multiplexing circuits, such as DS-1 and DS-3, is an easy task for the adroit ATM AAL1 layer. The AAL1 layer receives the bit stream and repackages the bits as cells. A 193-bit stream with DS-1 framing is converted to 53-bit ATM cells with hardly any effort as long as an accurate synchronizing clock signal is used in the physical hardware performing the bit shuffling.
There is a structured, meaning channelized, DS-1/NxDS-0 circuit emulation that provides an emulated point-to-point DS-0, fractional T1, and DS-1 circuits. Non-channelized DS-1 circuit emulation provides a point-to-point DS-1 circuit. Non-channelized DS-3 circuit emulation provides a point-to-point DS-3 circuit. Circuit emulation provides a means for suppressing the idle, or empty, frames, thus preventing the frames’ conversion to ATM cells.
Several coding and signaling schemes are available depending upon customer preferences.
A hybrid of permanent virtual path and switched virtual circuit, a user establishes a virtual path connection (VPC) to the ATM network. A signaling channel within the VPC is used to set up and tear down virtual channel connections (VCCs). The use of hybrid PVCs allows users with shared access the ability to have SVC service when the LEC does not support ATM signaling capabilities. Users with direct access must use this method of connecting instead of SVCs to communicate with users with shared access.
Transmission time is important in situations requiring the transfer of huge amounts of data. A survey of file types and typical sizes is listed in Table 7-4. As you would expect, the larger the file, the longer it takes to transmit it. Also, two transmission times are listed, 1 Mbps and 1000 Mbps. Again, as you would expect, the faster transmission time takes less time to transmit a file of any given size.
Example | Data | Transmission Time (at 1 Mbps) | Transmission Time (at 1000 Mbps) |
Page of Multimedia Text | 10 Mbytes | 100 seconds | .1 seconds |
CAD File | 12 Mbytes | 120 seconds | .12 seconds |
Digitized X-Ray | 15 Mbytes | 150 seconds | .15 seconds |
Integrated Circuit Design | 20 Mbytes | 200 seconds | .2 seconds |
Newspaper | 50 Mbytes | 500 seconds | .5 seconds |
Computer Simulation | 5 Gbytes | 10,000 seconds | 10 seconds |
Table 7-4. Typical transmission times
The message here is the transmission time should be adequate for the application. A company that produces animated computer images and simulations and requires remote connection to off-site locations probably should not be connected to an ISDN at 64 Kbps. At 1 Mbps transmission rate for a 20 Mbyte file, the flow of data seems like cold molasses clinging desperately to the bottom of the jar.
ATM is not without its competition. Leased lines, SONET over IP (X.25), Gigabit Ethernet (X.25), High Speed Frame Relay, and Switched Multimegabit Data Service (SMDS) each offer varying degrees of competitive pricing for ATM with similar port and PVC speeds. The next two years will produce an amazing amount of technology hype as technologists, including yours truly, harp on their favorite transmission technology for the 21st century.
Leased lines are legacy TDM networks that are slow and expensive. Just about everyone agrees these cretaceous dinosaurs are pretty much a thing of the past. While they will be around and serve useful purposes in a niche capacity, Frame Relay and ATM will gobble up this market as leased line equipment ages and must be replaced.
Gigabit Ethernet at 100 Mbps to the desktop is already a done deal. But as a backbone technology, well, 13 U.S. service providers have already installed ATM. Which do you think they will market over the next two years? The theoretical maximum transmission lengths for Gigabit Ethernet are 3 kilometers for single-mode fiber and 500 meters for multimode fiber. Since Gigabit Ethernet cannot replace ADSL to the home over the ubiquitous copper pair, ATM in partnership with ADSL is still the transmission technology of choice for delivery of video products to the home. The consumer video market is considerably larger than the desktop market, at least for now.
SMDS is already a dying niche technology with only one U.S. service provider. There are only about 200 national customers who provide an average of $125,000 revenue per customer. Don’t bet on this horse!
1997 Features | ATM | Frame Relay | X.25 | SMDS |
Packet/Cell Size | 53 octets | up to 4096 octets | up to 4096 octets | 53 octets |
Speed | up to 155Mbps | 56 Mbps - 2 Gbps | 9.6 - 64Kbps | 1-34Mbps |
Connectionless | Yes | No | No | Yes |
PVCs | Yes | Yes | Yes | N/A |
SVCs | Yes | Yes | Yes | N/A |
Multicasting | Yes | Yes | No | Yes |
Table 7-5. Comparison of ATM and competing technologies
Why is ATM advantageous? Because competing technologies are soon going to be niche markets. Remember, ATM is still an infant. Table 7-5 contrasts the features of ATM and the competing technologies. While Frame Relay, with up to 2 gigabits transmission speed, has ATM beat in 1997, Frame Relay’s speed advantage will disappear by the year 2000. As more businesses connect to the ATM network, the ATM port and PVC prices will fall. Additionally, ATM is the only technology that can seamlessly integrate all types of data transmission into a single global data pipe. This feature alone will make ATM attractive to global companies, who will drive the rest of the world to adopt the technology.
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