T1 Background Information - HDSL, SDSL
Comparing hicap t1 to HDSL and SDSL
T1 is an old but extremely well supported and reliable technology that was standardized in the late 60s. T1 as changed over time to not specify the layer 1 implementation at the electrical level so much as it specifies the service and interface that must be delivered. Thus, many creative companies will deliver a "t1" that is emulated using DSL technology. A standard t1 uses 2 pair of twisted copper wire (4 wires) and B8ZS line coding (Binary 8 zero substitution). B8ZS allows t1 technology to be resistant to other pairs running other line codings (crosstalk) in the same copper plant. T1s require specially conditioned lines (no taps or analog signal boosters) and they require digital repeaters every 6000 feet of cable. A pure t1 is also called Hicap service (High Capacity Digital service by telco engineers). In the mid 90s many telcos began using HDSL to deliver t1 loops. HDSL can go up to 12,000 feet w/o a repeater. HDSL uses ISDN's 2B1Q line coding--2 bits encoded into a four-state (quaternary) symbol. This is the exact same line coding that a Basic Rate ISDN line (2x64Kbps B channels + 1x16Kbs signalling channel - D channel) uses. HDSL became so popular that in many areas over 70 to 80 percent of all t1s are delivered using HDSL technology. The smart jacks (line power and termination cards) convert the 2B1Q coding to B8ZS such that our CSUs that we plug the t1 into still understand the service as a standard t1 facility. HDSL also uses 2 pairs of copper wire, though it is not quite as tolerant as standard t1 to crosstalk from other pairs, as I understand it. The literature conflicts on this point somewhat. The HDSL 1 spec is what is widely deployed. The HDSL 2 spec states that the same capabilities of HDSL 1 can be delivered over 1 pair of wire. A lot of the literature talks about SDSL as being basically HDSL 2.
Now, SDSL is where things diverge. SDSL is rate adaptive. That means that if line conditions are not favorable the bit rate will be lowered to try to maintain at least some data flow. HDSL is *not* rate adaptive. The other big difference is that SDSL again uses only one pair. This makes it far cheaper to provision, and it also makes it far more likely to crash or have downtime figures that exceed HDSL and certainly t1. SDSL is also much more vulnerable to crosstalk. This means that as new facilities are added a working SDSL line may suddenly not work so well b/c of proximity in the bundle to some other service. It is critical to understand the weaknesses of SDSL when choosing providers. A hicap or HDSL t1 to a tier 1 provider (Worldcom/UUnet, Sprint, ATT, C&W, Qwest are the teir 1s) will usually cost at least $1000/month. Some exceptions might occur if your business is in a park where fiber to the building is in place. If that is the case, often times there is a local provider willing to run MAN (metropolitan area network access) ethernet, which offers greater speed and inexpensive connections (VPN server with a firewall and an ethernet NIC). Many tier 2 providers (regional ISPs, and national ISPs who lease fiber from tier1) sell SDSL connections. Many of these companies have also filed for chapter 11 in the last year because these connections were priced at a level that made profits impossible. Any t1 that is sold for less than $800 a month is almost assuredly an SDSL t1. Caveat emptor. One final note: ADSL also uses 1 pair. This is what drives residential DSL. The bit rates are adaptive and the uplink is usually 20 to 30 percent capacity of the downlink.