|
September 1997
By Kathleen Cholewka
Product Leaders: HDSL Modems
End-to-end HDSL 2 Conserves Copper
Telmax's HDSL 2 modems let net managers shunt T1 data over a single copper pair
|
|
|
Net managers connecting sites on campus networks typically have to pull new fiber, an expensive, time-consuming task. Telmax Communications Corp. (Fremont, Calif.) thinks they have better things to do: Its Webxpress2300 and Xpress Central220 HDSL 2 modems can shunt data at 1.544 Mbit/s over a
single copper pair.
"HDSL 2 does away with fiber installation," says Kieran Taylor,
broadband analyst at Telechoice Inc. (Verona, N.J.). "Instead of
starting from scratch, net managers can use wire from their phone
network to get a T1-speed connection between servers or LANs."
So what's the catch? HDSL 2 can only hit T1 speeds over 26-gauge
copper at distances of 12,000 feet or less between modems. And HDSL 2
is still prestandard, so the Telmax gear is a single-vendor solution.
Telmax says it will offer a free upgrade when the ANSI (American
National Standards Institute) T1/E1.4 committee finishes up the spec
sometime next year.
Get Wired
|
MORE INFO
Product Summary
|
HDSL 2 (high-speed digital subscriber line 2) works over one
wire-pair, unlike HDSL or S
DSL (symmetrical digital subscriber line),
which use two pairs to transmit data. Its ability to make the most out
of copper connections should make it very attractive to carriers,
although services are scarce for the time being.
Integrated Technology Inc. (Eugene, Ore.), an ISP (Internet service
provider), offers Internet access and LAN connectivity via Telmax's
Webxpress2300. "Right now, it's the highest speed for the longest
distance," says Jim Lustgarten, president. "HDSL's going to
revolutionize the HDSL industry."
"When it comes to carriers, being able to conserve some copper is a
big deal," says Taylor. "Especially since deregulation means they'll
have to give up a lot of their lines. Copper is becoming especially
valuable."
In Its Place
The Webxpress2300 is a network modem that's deployed at the edge of
the LAN. The Xpress Central220 is a central-site rack that can hold as
many as 20 hot-swappable Webxpress2300 cards.
|
On The Wire
|
The Webxpress2300 transmits 1.544 Mbit/s each way over one copper
pair. Connections are point to point between modems (see the figure).
Other network devices tie to the Webxpress 2300 through one of three
interfaces. The DSX-1 serial interface accommodates T1 (1.544-Mbit/s),
E1 (2.048-Mbit/s), or PBX connections. The V.35 (2-Mbit/s) interface
connects to a router or multiplexer. The Ethernet port bridges devices
at the MAC (media access control) layer using IEEE 802.1D spanning
tree.
The Xpress Central220 usually sits in a data center. Each of the
Webxpress2300 cards it accepts packs 32 kbytes of first-in-first-out
(FIFO) buffers to prevent dropped packets.
The Webxpress2300 and Xpress Central220 can be managed via SNMP.
Network managers also can make use of Telmax Xpressview Manager, which
tests line quality, troubleshoo
ts configurations, monitors
signal-to-noise ratios, and performs bit-error tests. The modems can
be configured remotely.
The Big Squeeze
HDSL 2 can squeeze a full-duplex T1 onto a single copper
pair--that's because of its very efficient spectral shaping, which
prevents cross-talk with adjacent transmissions. The enabling
technology--Optis (overlapped pulse-amplitude modulation transmission
with interlocked space)--incorporates line-encoding techniques like QAM
(quadriture amplitude modulation), CAP (carrierless amplitude
modulation), and DMT (discrete multitone transmission). It allows
incoming and outgoing bit streams to partially overlap.
HDSL 2 can save networkers quite a bit of cash. T1s go for roughly
$1,225 per month, while a plain copper pair with no services on it
costs about $15 a month. The Webxpress2300 with one HDSL2 interface
costs $995. A DSX-1 with a T1 or V.35 version costs $1,295; an
Ether-net module is $1,495. The Xpress Central220 goes for $695. The
modems are $1,2
95 each. By comparison, the Copperaccel SDSL and HDSL
modems from Performance Telecom (Rochester, N.Y.) cost roughly $1,200
apiece.
|
|
|
 |
|
|
