by Arthur Cooper and David Welk
Switched 56 is another neat little name used to denote the use of 56Kbps circuits as a Wide Area Net (WAN) technology or strategy. It is really just a souped-up version of plain old telephone system (POTS) service. The main differences between POTS service and switched 56 service are the speeds that can be obtained. Most POTS circuits are still only capable of providing between 28.8Kbps and 33.6Kbps. Granted, there are new technology modems such as USR's X2 technology modem that claim to provide 56Kbps on a standard POTS line, but the fact remains, the speed of 56Kbps is not a guaranteed thing. If the POTS system in your area still has old copper wire or carrier facilities, you may not obtain the higher speeds capable with these types of modems. For that reason, it is sometimes necessary for network managers to avoid the use of POTS lines and make the jump to a switched line capable of providing a clean, consistent rate of 56Kbps. When this occurs, we are then entering the world of switched 56 technology.
What exactly is switched 56 technology? Is it just an alternative leased line service or is it more? Switched 56 service is a digitally switched or "dial-up" technology that has the capacity to provide a single channel for dependable data connectivity. This single channel has a clock speed of 56,000 bits per second, which means that theoretically, it can pass about two pages of documentation per second.
To get an idea of where switched 56 service fits in the big picture of carrier speeds and services, let's look at a brief description of T-1 service and where switched 56 fits into the picture.
A T-1 circuit is divided into 24 channels of 64Kbps. Control signaling typically requires the isolation of the upper 8K bits of the channel from the data transport section of the bandwidth. Hence, you are left with 56Kbps of useable data on each channel. This 56Kbps of useable data is typically used on a per channel basis to move data from one point to another. Now remember, a T-1 circuit refers to a carrier mechanism. In other words, there does not necessarily have to be a switch or switching device associated with T-1 carrier systems. Many times, there may be Point-to-Point T-1 circuits used, and it is not necessary that any switching be performed on the terminating ends of these carrier circuits. It is when the individual channels are pulled out of a T-1 and route these signals to other channels in another T-1, or over to a user location, that some type of switching must be performed. When this 56Kbps of data has been switched out of a T-1 and is sent to a specific user location, the resulting circuit is referred to as a switched 56 circuit. Some people refer to the process of allocating the bandwidth of a T-1 channel as dropping-and-inserting. When a 56Kbps circuit is dropped into a customer or user location and the circuit is terminated, that customer or user must be provided with a switched 56 circuit. The term switched means that the other end of the 56Kbps circuit will generally ride a T-1 circuit into the cloud of the Public Switched Telephone Network (PSTN). Within the PSTN, the individual switched 56 circuits can be routed or switched to any other termination point connected to the PSTN.
NOTE: The term cloud is a general term that network people will sometimes use to refer to the entire network. The network would of course be comprised of network switches, wire, carrier systems, and anything else needed to move signals from one place to another.
Switched 56 access lines were some of the original digital circuits installed by the telecommunication carriers. The low cost of switched 56 relative to digital leased lines makes it ideal even today for supporting sporadic high-speed applications that are extending into new or existing locations. The fast data transmission capabilities and low error rates, relative to analog dial-up, which is just now gaining this quantity of throughput, make switched 56 access an ideal solution for today's wide area environments.
A savvy network manager must know when it is right to choose this solution versus direct T-1 lines or other methods of WAN interconnectivity. Cost is always a factor when deciding on a specific technology, and switched 56 is a very cost-effective solution. Because the connections are only made when actual data must be transmitted, this is a cost-effective means of transferring data. Further, because switched 56 has been around a lot longer than other newer technologies, such as frame relay or ATM, the costs for terminating equipment, circuit turn/up and installation fees, and the monthly access charges are much lower for switched 56.
If T-1 lines are used to interconnect sites, there will be high flat-rate charges each month, whether any data is transmitted over the lines or not. With a switched 56 solution, the costs for termination equipment are cheaper than those of T-1 terminating equipment. Also, because it is an on-demand connection, there is less wasted bandwidth than there would be if you were using T-1 connectivity. There are many vendors who are doing on-demand switched frame relay connectivity over T-1 circuits, but once again the cost for this type of service far exceeds the cost of a switched 56 connection.
Although very widespread, especially in populous areas, switched 56 circuits are quickly becoming outdated with the acceptance and propagation of ISDN. The type of connectivity provided by a switched 56 service is ISDN-compatible and versatile, with the ability to carry switched digital signals such as video, voice, and data. The main difference between switched 56 technology and ISDN technology is the dependence of ISDN on D-channel signaling. This difference creates the absolute limitation of 56Kbps in switched 56 as opposed to ISDN, which is capable of utilizing the entire channel of 64Kbps. The technical explanation for the limitation of switched 56 service is that the communication control signaling protocol uses only 7 of the 8 frame bits that are available in a 64Kbps channel. See Figure 15.1 for an illustration of a frame.
ISDN services tend to have higher capabilities as far as aggregating circuits when compared to switched 56. Aggregating circuits or inverse multiplexing is a technology that consolidates various telecommunications circuits for extremely high-speed applications such as video-conferencing or bulk data transfers. One example might be CAD/CAM/CAE (high-end drafting) applications. A CAD system dials through the telco cloud, as shown in Figure 15.2.
FIGURE 15.1. A graphical display of the signaling bits that compose character framing.
FIGURE 15.2. CAD/CAM systems exchanging bulk data such as an image transfer.
One of the uses of inverse multiplexing technology can even be to aggregate both ISDN services and switched 56 services to create a single communication link. This is an excellent way of utilizing existing equipment and services in conjunction with the new capabilities and equipment requirements of ISDN.
Information that does not require data speeds greater than 56Kbps through the enterprise network can utilize a switched 56 service. The ability of the carrier's equipment to support leased digital lines and provide dial-up capable digital lines requires extra processing power. This additional processing feature, added to CSU/DSUs for support of switched 56 dial service, tends to make the equipment more expensive.
NOTE: A CSU/DSU is an acronym that was created by combining the two terms Channel Service Unit (CSU) and Data Service Unit (DSU). A channel service unit is a device that can take the quasi-analog signal coming in on a data circuit and derive intelligence from it that will be passed to the user. Likewise, in the opposite direction, the data coming from the user is converted into a quasi-analog signal by the channel service unit in order to enable it to be transmitted out on the data circuit. The data service unit is between the channel service unit and the user. This unit provides for things such as proper termination specifications, data isolation, and proper mechanical connection specifications. In other words, the data service unit would be the actual physical connection between a user and the data circuit's channel service unit. In the past ten years, these two devices have been combined into one small appliance that looks similar to a modem. This new combined channel service unit and data service unit is almost always referred to as a CSU/DSU. Old-timers will sometimes call them a CSU, but in most cases there is a DSU attached as well.
Telecommunications equipment that provides solutions used in a backup and restoration situation or in a dial-on-demand situation usually deploys switched 56 circuits and equipment. As a wide area technology with most of the pricing based on usage, switched 56 service is also a viable solution for digital audio and video desktop-conferencing applications. In fact, for many years this was the only technology available for wide area video conferencing. Most systems utilize one channel for transmitting data and one channel for receiving data (see Figure 15.3).
FIGURE 15.3. Audio and video conferencing utilizing switched 56 for connectivity.
Other sporadic high-speed applications, such as image transfer and viewing, datafax transfers, and disaster recovery situations, require the accommodation of varying network traffic patterns.
Switched 56 service handles these requirements adequately.
CAUTION: Determine the availability and interoperability of technologies before settling on any disaster recovery or dial-on-demand applications. Most of the regional Bell operating companies interoperate well, but there are differences going across national borders such as into Canada.
After the application processing, the binary information traverses the enterprise network through the telco-provided, wide area "cloud." It is by way of these digital service access points, sometimes called switch points, that information flows through digital facilities from end to end.
This quality of service ensures the highest level of data accuracy and throughput that most corporate networks, as well as Internet access, require. In looking at the following five carriers, which provide a good cross section of capabilities and operating domains, you can start to get a feel for their level of deployment of switched 56 technology. You can also start to understand the level of ISDN implementations and how that service offering is affecting the market for switched 56 services.
Overall, a dramatic price drop has occurred in switched 56 service offerings during the last two years. ISDN technology is growing in popularity, but a new analog technology that is promising 56Kbps throughput on file downloads is gaining acceptance.
Switched 56 service is part of AT&T's ACCUNET Global Switched Digital Services (GSDS) service offering. AT&T is the largest domestic long-distance carrier and a formidable competitor in the global arena. The model 4ESSTM nodal switching machines offer a four-wire configuration that uses the well-known 700 56X XXXX number for switched 56 addressing. AT&T also uses a standard dialing capability known as RS-366 and a terminal interface unit, which converts V.35 signaling to the DDLC format for digital data. The global network makes use of fiber optic cabling and satellite technology to connect distant "phone calls." The installation of a Northern Telecomm (Nortel) data unit at the customer's premises is all that is required to support two-wire switched 56. Using audio and video bridges allows AT&T to offer Desktop Visual Collaborative Conferencing. Additionally, using multiport audio bridging enables audio broadcast conferencing.
Table 15.1 is compiled from information on AT&T's Web page (http://www.ATT.com).
| Company | Naming Convention |
| Ameritech | PGSDS |
| Bell Atlantic | Switched 56 |
| Bell South | Accupulse 56 |
| GTE | Switched Data |
| Nynex | Switchway 56 |
| Pacific Bell | CenPath or Centrex |
| Southwest Bell | MicroLink 1 |
| US West | Switchnet 56 |
After completing searches of various telcos on the Web, I was only able to find that California-based telco, Pacific Bell, posted the most straightforward pricing information. Pacific Bell's claim, "to have switched 56 capabilities available to 90% of their business and residential customers," is impressive. Pacific Bell also touts the flexibility of serving two-wire and four-wire configurations, which "enables installation to locations that are farther in distance from the Central Office."
NOTE: The preceding quotations are from Pacific Bell's WWW page:
http://www.pacbell.comThe following pricing is outdated and almost certainly will have been changed:
SDS 56 = $500 one time installation (waived if service contracted for two years) + $45 per month + usage (voice rates with time-of-day discounts)
(800)PAC-BELL
Ameritech serves seven major metropolitan areas and has advertised very limited switched 56 capabilities. Pushing to implement ISDN throughout their five-state region has been an ongoing effort.
NOTE: The URL for Ameritech's Web page is
http://www.ameritech.com
MCI provides Digital Gateway T-1 service and bases their pricing on usage. As an interexchange carrier (IXC), MCI requires access from the local exchange carrier (LEC) for the DDS loop and switched digital access. Their network uses 64Kb bandwidth and rate adaptation to conform to switched 56 standards.
NOTE: MCI's Web page can be found at
http://www.mci.com
The SprintNet Global Data Network provides Switched Data Services (SDS).
NOTE: Sprint's Web page is located at
http://www.sprint.com
Switched 56 is an important WAN technology. Because the traffic of a network is often localized on a Local Area Network (LAN), it is not necessary to pass data from one LAN location to another. When it does become necessary to pass data from one location to another, the amount of data is usually sporadic and relatively small. If the amount of data to be passed is not a particularly large amount, it makes no sense for a network manager to purchase and maintain leased connections between the LANs, thus comprising the entire network.
When this is the case, the network manager must come up with a wide area networking topology that will be cost-effective and able to handle the sporadic data needs of the network. For some networks, this may mean only a few POTS lines would be needed between network LANs. However, when a POTS line alone will not suffice, switched 56 is the technology of choice. It will handle a fairly large amount of data quickly and keep costs down, because the only time the connection will be up is when it is actually needed.
The switched 56 line is the best dial-up data circuit available today. ISDN lines, it may be argued, can handle higher bandwidths than switched 56, but the cost of ISDN has not come down to match the cheaper rates available on switched 56 circuits in most parts of the United States. There is also an abundance of equipment that will readily interface with switched 56 circuits. ISDN requires costly modems and/or interface equipment. When the cost of doing long-range ISDN comes down, then, and only then, will switched 56 circuits disappear. Until that time, any network manager with sporadic, smaller amounts of inter-LAN data traffic should always consider switched 56 when deciding on a WAN topology for the network.
© Copyright, Macmillan Computer Publishing. All rights reserved.
){kind=link}
){kind=link}
){kind=link}