Cisco AG241 Manual de usuario descargar pdf (Pagina 6)

apcmag.com  june 06  apc labs

apc labs  june 06  apcmag.com

softwaresoftware

2006

JUNEDETAILED PRODUCT REVIEWS AND COMPARISONS

REVIEWS

BEST BUYS

LABS CHALLENGE

labs

apcmag.com  june 06  apc labs

apc labs  june 06  apcmag.com

softwaresoftware

2006

JUNEDETAILED PRODUCT REVIEWS AND COMPARISONS

REVIEWS

BEST BUYS

LABS CHALLENGE

labs

ome ADSL2+ ISPs let you

choose “line profiles” for your

connection. This is something that

Telstra-based ISPs can’t do; Telstra

doesn’t allow customers (or even

ISPs) to modify any DSLAM settings.

We did all our formal modem

testing using the default Internode

line profile MAX24 — replicating

what the majority of customers use.

But to satisfy our interest, we

tweaked a few settings here and

there — what we discovered was

staggering: huge performance

improvements are made by making

tiny changes to settings. (NB: Your

mileage may vary depending on

the quality of your phone line.)

There are two key settings

that can be changed on a DSLAM:

Target SNR — tells the modem to

aim for a signal-to-noise margin of

a certain amount. Expressed in

decibels, it’s the difference between

the loudness of the modem’s

signal and the loudness of

background noise on the line.

For each 3dB, the difference

doubles, making the connection

more resilient to spikes in line noise,

but also reducing the maximum

attainable line speed. Internode

offers a line profile for gamers

called “MAX24 — HS” which shaves

target SNR right down. People with

very good phone lines close to the

exchange will get a faster

connection. But if there is

fluctuating line noise, the

connection could be unstable.

Interleaving — mixes the packets

of data together, along with error-

correction data, so if line noise

causes packets to corrupt,

your modem can recreate them

(without the packets having to be

retransmitted). Interleaving is

described in milliseconds — eg.

4ms of interleaving means 4ms

worth of packets mixed together.

We discovered during testing

that interleaving can considerably

reduce modem performance. For

example, the Cisco 877W modem,

which clocked in at the leading

speed of 19.1Mbit/s actual

throughput with interleaving

disabled, lost a full 33 per cent of its

speed when we switched

interleaving on at the DSLAM. (It

dropped back to 19th place at an

unimpressive 12.6Mbit/s.) Given

Cisco’s rep for speccing its boxes

up with enough grunt to handle

features being enabled (without

performance degradation), this

was quite surprising — perhaps

a sign of immaturity in the

877W’s firmware.

Other modems that suffered the

most with interleaving on were

LevelOne WBR-3407A (29% drop in

performance), Dynalink RTA1335

(27%), Dynalink RTA1046VW (25%)

and Draytek Vigor 2800VG (24%).

On average, having interleaving

enabled saw performance drop by

17 per cent. However, Internode

managing director Simon Hackett

says that interleaving plays a very

important role on sub-optimal

phone lines. “Any packet loss at all

can have a huge effect on

performance,” he says. “When a

phone rings, it’s a 50 volt signal

heading down the line; at a low

frequency that’s not supposed to

interfere with ADSL, but it does

produce a broad spectrum impulse

spike. Interleaving does exactly

what CD players do; takes data and

spreads it over time so if there’s a

short spike in line noise (like a

scratch on a CD) a number of

corrupted packets can be recreated

on the fly.”

Indeed, one modem in the test,

the low-end Zyxel Prestige 660HW,

performed considerably better with

interleaving switched on, going

from a mediocre 10.5Mbit/s to

eighth-fastest — 14.5Mbit/s

throughput (a 38% boost).

Because interleaving combines

a few milliseconds’ worth of data

together, which must be decoded

at the other end, it affects ping

time. This can be a downside for

gamers who value fast response

in their games.

Given that small settings

can make such a major difference,

getting the most from your

ADSL2+ connection is easiest if

you sign up with an ISP — such

as iiNet and Internode and others

— that have selectable profiles

(usually on their web site once

you have logged in to your

account settings.)

Give your

DSLAM a tweak

Did you know you can probably tweak your ISP’s DSLAM to get extra speed

from your connection?

Internode and other ISPs let you change the settings of

your DSLAM port online via their web site.

A batch of Internode’s Ericsson DSLAMs at

a Telstra exchange. The spindly yellow

cable is the fibre-optic feed.

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