ADSL - How adsl Works - 1
BT IPStream - Home to the Exchange.
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|Separates the analogue voice (phone) traffic from data (adsl)
Normal voice (phone) traffic is at a lower frequency (< 4 KHz)
than adsl data.
The "telephone" outlet on the splitter blocks the higher
frequency signals which prevents the data signals from interfering
with voice calls.
A splitter must be used for each telephone device that is connected
to the socket.
More information about
|Transceiver to connect the adsl line to the computer, or in
the case of a router to the local network.
Depending on the type of router, it can either be connected to
a network switch/hub or it may have its own networking switch.
The modem converts digital signals from your PC/Network into analog
signals that can be transmitted over telephone cable in the local
loop and vice versa.
This is the path your telephone line will take from your
house to the local exchange.
The cable consists of a "copper pair" and can either
be underground or via overhead cables to a distribution point
typically on a telegraph pole.
"Green CABs" or more correctly PCP boxes, are a
junction point where the telegraph pole or underground cable
joins a larger link back to the exchange.
The length of your line to the exchange will determine the maximum
speed that your line is capable of syncing to the exchange at.
- The further away from the exchange you are the more the adsl
More information about line length and
BT has about 4.7 million distribution points,
five million joint boxes, 4 million telegraph poles, 210,000
manholes, 98,000 PCP green cabinets and 5,500 exchange buildings with 25 million exchanges lines
BT's access network contains 121.7 million kilometres of copper
wire, enough to go around the world about 3,000 times.
Local Exchange Equipment.
Entering the adsl section at the local exchange, one of the
first things you cant help but notice was the huge amounts
of patch panels.
All lines that are adsl enabled will be attached to an adsl patch panel on the exchange side of the Main Distribution Frame (MDF).
These patch panels are are stacked in racks from floor to ceiling
and grouped in batches of 96/100 users and marked with the DSLAM it is attached to.
ADSL patch panels are attached to the Exchange side of the MDF (Main Distribution Frame) on the right and out to the DSLAM line card on the left.
Form part of the DSLAM, and are "slotted"
Each line card will hold connections from a number of users,
the multiples of which depend on the type of manufacturers equipment
and can vary from 8,16,32 etc.
A typical 3rd generation card from Alcatel will have 24 circuits
With older dslams it was assumed that the line cards set your maximum speed
is set by using jumpers to set the max speed. (say 2Mb).
Actual line sync speed up to this maximum can be set remotely
by BTw (576 kbps/1152 kbps/2272 kbps).
Integrated line cards filter adsl and POTs traffic and newer technology allows more flexibility for remote management to set speeds or IPStream ISP migration without an engineers visit.
- DSLAM - (Digital Subscriber Line Access Multiplexer)
Takes connections from a number of users and concentrates
them into a higher capacity connection along the ATM backbone.
The DSLAM is behind a storage cabinet and there is not much
that you can really see, but it is basically made up of numerous
The DSLAM has a number of line cards,
each line card itself holding connections from several users.
Typically a 3rd generation DSLAM will take 2 racks of 16 cards
giving an overall 'port' capacity of 768 circuits.
However the type of DSLAMs installed may vary from exchange
to exchange, and some makes of 4th generation units by Fujitsu
may initially support up to 1024 lines. (This
figure can later be doubled by adding another ETSI rack ).
DSLAM's segment traffic into several Virtual
Paths before forwarding to the ATM, most DSLAMs will control
When an exchange is said to be low on port capacity it means
that the exchange is running close to the maximum number of
users that can be attached to the line cards on the DSLAM.
BTw commonly talk about VPs which are "virtual"
paths or routes from the DSLAM, over the ATM backhaul.
It is at this point that contention at the local exchange takes
place as each VP will have a certain amount of bandwidth available
for its users to share.
Depending upon the size of the exchange, an IPStream VP will
be made up of a hundred or so customers who are a mix of various
IPStream ISPs. BTw are responsible for monitoring and running
the capacity on the VP. 20:1 Office and 50:1 Home users are
routed via separate VPs.
DataStream providers rent a Virtual Path from BTw and are responsible
for controlling the amount of users they put on the VP and the
amount of bandwidth they purchase. Some DataStream providers
may use their own or another telco's routing as the backhaul.
Therefore customers of a datastream ISP will share the available
bandwidth with other users of the same ISP or telco.
In 2003/2004 newly enabled exchanges* which required
400 pre-reg users, the common set up would be as follows:-
3 x 50:1 VP's with 4MB for 512k users (classed
by BT as USB service)
1 x 20:1 VP with 10MB for all 20:1 (classed
by BT as business users).
These days with the advent of higher speeds and more users
having access to adsl the amount of bandwidth allocated to the
VP has obviously had to be increased.
Future talk is that several existing VP's will be merged into
"Super VP's" with a larger amount of bandwidth and
*This information was correct mid-end 2003,
however these figures may have now changed with the advent of
** BTW had to update numerous exchanges during early 2004 when
it was found that the CISCO kit was unable to allocate bandwidth
proportionally between the 512k and the 1Mb users. Problems
started occurring say at peak times when there was little spare
bandwidth. As such a 512 user may not notice a drop to say 400kbps,
but the 1Mb users sure did.
Installation of Juniper ERX's ensured that 1Mb users got their
fair share of available bandwidth.