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ADSL - How adsl Works - 1

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BT IPStream - Home to the Exchange.
Page 1 of 3 (1 2 3)

adsl network kitz adslfilter patchpanel DSLAM DSLAM DSLAM adsl modem voice data local loop Virtual Path

Home Equipment

  • ADSL Filter/Splitter
Separates the analogue voice (phone) traffic from data (adsl) traffic.
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 adsl filters.
adsl splitter
  • ADSL Modem/Router
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.
adsl modem

Local Loop

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 signal deteriorates.
More information about line length and sync speeds.

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.

telegraph poles

Local Exchange Equipment.

  • Patch Panels.

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.

patch panel
  • Line Cards.

Form part of the DSLAM, and are "slotted" in.
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 per card.

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.

line card
  • 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 circuit boards.

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 several VP's.

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.

Fujitsu FDX xDSL
commonly used in a lot
of exchanges.

GeoStream Access Gateway Hub 64


Exchange Backhaul

  • VP - Virtual Paths.

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.

virtual paths

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 users.

*This information was correct mid-end 2003, however these figures may have now changed with the advent of 1Mb 50:1.
** 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.


patch panels
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