 |
Comm
Corner
Telecommunications Connections: Asymmetric Digital Subscriber Line (ADSL) The Southwest Bell Solution by John Woody |
|
|
Comm
Corner
Telecommunications Connections: Asymmetric Digital Subscriber Line (ADSL) The Southwest Bell Solution by John Woody |
| Two transmission technologies touched on in our August
1998 and September 1998 Comm Corner column need further discussion. Both
have to do with broadening a user’s connectivity bandwidth well beyond
our familiar POTS analog service. Both technologies are just over the horizon
as service for individual/SOHO use.
One is the cable modem connection we have heard and read about and is offered by the cable TV companies when available. The other is the low end of the xDSL technologies, Asymmetric Digital Subscriber Line (ADSL). Both technologies give near T-1 bandwidth (1.44 Mbps) to users. ADSL is one of several Digital Subscriber Line (DSL) technologies which have evolved over the past few years via company and standards group development. They are known as the xDSL family of technologies.
The xDSL Family of TechnologiesThe xDSL technologies are derived from the need to transmit large amounts of data over existing twisted pair telephone cable. This technology is based on the use of existing POTS copper twisted pair telephone cable and is known as subscriber lines. Subscriber lines are those telephone lines which are run from the building or home into the telephone company (telco) switching equipment, usually within 20,000 feet. This point is important because the technology is meant to work within the bounds of the existing twisted copper pair POTS wiring. POTS service usually contained at the subscriber end of these lines includes voice telephone, fax, and voice band modems (analog). The telephone switching equipment allows one end user to call one other as a end user. The analog (POTS) bandwidth allocated to each subscriber is usually from about 200 Hz to 3,200 Hz and has a capacity to carry a maximum of up to 64 Kbps. This bandwidth is not always available to analog data users due to many factors, mainly line noise and other interference within the system and lines.The Asymmetric Digital Subscriber Lines (ADSL) technology provides a method of transmitting digital information on the existing twisted pair subscriber lines. This technology is designed to use the existing copper installed media base, a very important consideration due the cost of installing any new media capability. The ADSL processing power required to pass data over the existing copper communications base is large and costly. The main reason to use the existing copper media base is that it is in-place. Any new media base is much more costly than developing a technology to use the existing media base. The existing copper media base is the only means of reaching large numbers of subscribers. ADSL is one of about seven such technologies which have been designed to provide extremely high bandwidth data rates over the existing telco copper infrastructure. ADSL data capabilities include 100 to 800 Kbps upload (upstream) and 1 to 8 Mbps downloads (downstream). This uneven data flow is the asymmetric meaning given to it as the data rates are different from the provider to the customer (download) as compared to the customer to provider (upload). It works over the existing copper lines. The main forces which are driving requirements for high data rate transmissions include nearly all uses of the Internet, including the World Wide Web (WWW), video services, on-line consumer services, e- commerce, and other entertainment services. Each of these categories brings a need for lots of data to be moved in a short period of time, ie., high bandwidth. In the past, high bandwidth was strictly the preview of the business world. Businesses had to have a large data transmission need for the T-1 (1.54 Mbps) bandwidth in order to justify the cost at approximately $2,500.00 per month to be able to send data at high bandwidth. Individuals could never justify the cost of T-1 for home use. Even the telco tariff at $67.00 per month on ISDN (128 Kbps) has kept this digital service out of the reach of individual users. ADSL is being priced at near POTS rates as will be seen below. Other factors which influence this motivation for higher bandwidth includes the growth and capacity of the PC wherein it has become nearly a household necessity as a communication tool. The PC is powerful enough to provide a means of sending very large amounts of data from household to household. The very powerful generation of computers available today are able to send video on demand, for example. Video on demand requires around 1 to 3 Mbps data rates. This was once only available to those with very expensive computers and connections. ADSL is being priced at a rate which will allow individuals to be able to send this kind of data. The Small Office, Home Office (SOHO) is becoming more sophisticated and is requiring a need for large amounts of data bandwidth. This is another opportunity for ADSL communication and expands the SOHO into the really big leagues.
Southwest Bell ADSLSBC Communications, Inc., the parent of Southwest Bell (SWBell) announced in January 1999 that it has plans to “launch an emerging technology service” for data and Internet access by deploying ADSL in its five state region with accompanying price adjustments to make the service available to the general public. SWBell expects to offer ADSL in 526 central offices which would make ADSL available to the majority of it’s customer base by the end of this year. This is part of a broad plan by SBC Communications, the SWBell parent company to provide high bandwidth service throughout it operating companies. SBC has deployed ADSL in its Pacific Bell subsidiary and is planning on expansion of that service on the West Coast.The SWBell ADSL service will be offered at $39.00 per month (one to three year contract) and will provide downstream speed of 1.5 Mbps and 128 Kbps upstream speed connections. SWBell states that customers will be able to simultaneously use a phone or a fax machine while getting downstream data connections of up to the 1.5 Mbps speeds. This service is described by SWBell as a “always on” dedicated connection. The downstream throughput speed is dependent on the customers distance from the central office (CO). SWBell will guarantee the downstream speed at 384 Kbps for this tariff rate at a distance of 17,500 feet from the CO. It is expected that this $39.00 per month tariff will be directed at individual and small office customers. The $39.00 ADSL service would have a one time equipment and installation charge of $212.00. ADSL terminal adapters will be required and is part of the $212.00 charge. The ADSL service will handle both data and voice. The voice connections will require a splitter at both the customers location and at SWBells CO. The customer would be responsible for providing and maintaining the splitter equipment on its premises. This capability of routing voice and data is handled by directing the voice traffic to the serving wire center while the data is directed through a multiplexer (DSLAM) for connection to SWBell’s ATM/Cell Relay network. Southwestern Bell Internet Services will offer ADSL Internet service for a combined price of $49.00 per month plus the one time equipment and installation charge. Other ISPs are gearing up to provide this service as well. For customers in need of speeds higher than 1.5 Mbps, SWBell will offer ADSL at rates of up to 6 Mbps downstream and upstream speed of 384 Kbps. The guaranteed throughput downstream speed for this service will be 1.5 Mbps. The actual tariff for this service is $129.00 per month (one to three year contract) with an installation charge of $511.00. The throughput downstream speed of 1.5 Mbps is dependent on the distance between the customer and the central office. SWBell is guaranteeing a downstream throughput speed of only 384 Kbps. I assume this is the 20,000 feet from my technical review above. The SWBell distance limitation is being stated as 17,000 feet from the central office. This could be a major limitation to the service. Anyone who lives at or near the distance limit would have to live with the 384 Kbps throughput speed, which is faster than an analog modem, but not the blistering 1.5 Mbps downstream throughput speed as advertised. I have some experience with the distance limitation discussed above. SWBell recently installed an amplifier on my ISDN line. When I quizzed the technician about he installation, he stated that I was probably near the ISDN distance limit for ISDN connections and that the amplifier was needed to maintain throughput speeds. I had seen only that I seemed to be connecting at 56 Kbps on my data B channel instead of the previous 64 Kbps speed. I assumed that the differences were due to changes I had made internally in my computer. The distance from my central office, Wetmore, was really the problem. I did not request the service upgrade, SWBell made the change form their end. Distance from the central office may be the major consideration for individuals who subscribe to ADSL service. SW Bell is guaranteeing only 384 Kbps and stating that customers will most likely experience the higher bandwidth of up to 1.5 Mbps speeds. The distance will greatly affect the final downstream bandwidth. SWBell will make several checks before deploying ADSL service. These checks include a distance check, cable check, number of subscribers in the neighborhood, and the condition of the copper wire for the connection. Fiber optic media will not work. The ADSL terminal adapter (TA) being offered by SWBell is from Alcatel. This TA is for single computer connections. The TA is an external unit. It connects to the computer via a Ethernet port and to the telephone line via a standard RJ11 connector. This is a limitation of the SWBell provided hardware. Another piece of equipment might be required to connect a small office network. A router might be required to handle multiple workstation nodes. This Alcatel equipment comes with the necessary Ethernet connections to connect it into a network. The physical proportion of the TA is about the size of the APCO ISDN Ascend Router. The TA is software upgradeable and follows the IEEE solid RFC 1483 standard. This equipment is called Customer Premises Equipment (CPE). The Alcatel TA supports the dial-through PPTP tunneling for secure communications. I was assured by SWBell that the “always on” (read direct connection) is secure from intrusion. It uses TCP/IP protocols and fully supports IP per RFC 1483/RFC 1577, Network Address Translation (NAT), PPP, DHCP, SNMP and web managed (HTTP) protocols.
Internet Service Provider ConnectionThree ISPs were part of the testing by SWBell in Austin. Other ISPs are gearing up to provide the ADSL service and are purchasing the necessary server/bridge equipment to handle ADSL. The ISPs have other issues to work out as well. They must also have an ADSL connection to SWBell. The ISPs are working hard to obtain the equipment, which interfaces with the CPE equipment.The ISP ADSL/telco connection will probably be one pipe instead of the many we have seen in the past. Texas Networking, for example, will obtain DS3 155 Mbps Asynchronous Transfer Mode (ATM) pipes. This bandwidth will allow Texas Networking to provide its customers “unspecified bandwidth”, which means that Texas Networking will be able to price its ADSL service at $20.00 to $25.00 per month. This is the price of their Lone Star analog account, but with up to 1.5 Mbps download capability. The only other cost is the cost of the SWBell service charge of $39.00 per month plus the one time Installation charge of $212.00.
ConclusionComputer communication is again at the edge of a major bandwidth breakthrough. ADSL and Cable Direct connections are about to bring really high speed computer communications to the rest of us. I will attempt to cover the technical aspects of Cable modems and the Cable connection next month. I am looking forward to extending my outside world to these high speed connections.John Woody is a networking communications consultant specializing in small office, home office networks, training setup, and internet connectivity.
|