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.
SDSL Differences
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.