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ADSL - How adsl works - 2

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BT's IPStream Network.
Page 2 of 3 (1 2 3)

adsl network kitz Home Gateway BT Central Pipe ISP Edge Routers logon authentication RAS ATM Backhaul ATM Cloud L2TP Tunnel BTs Colossus IP Network


ATM Backhaul .

The "pipe" between your local exchange and where it becomes part of the ATM Cloud.

The ATM backhaul from the DSLAM/MSAN at the exchange usually consists of a 155Mb pipe, although not all of this 155Mb capacity may be in use.

Some small exchanges may be subtended /"piggy backed" to a larger exchange in a nearby town, and as such will have a lesser capacity of 30Mb. It is assumed that these pipes are segmented with various amounts of bandwidth as allocated by the Virtual Path.

Back to diagram.

ATM Cloud (MSiP).

BT's ATM network between various points, which can take many different paths/routes.
It carries traffic in pure ATM form and is called the Multi-Service intranet Platform (MSiP). Since it is impossible to know exactly which route traffic will traverse, an ATM network is depicted as a cloud.

BT's ATM network was originally built in the 1990's to handle traffic for business customers who required such services as intranets on leased lines. Today it also carries IP packets as an ATM stream for internet customers to and from the Colossus backbone.

ATM (Asynchronous Transfer Mode) is a networking technology based on transferring data in cells or packets of a fixed size over a dedicated connection.
The cell used with ATM is relatively small compared to units used with older technologies. The small, constant cell size allows ATM equipment to transmit video, audio, and computer data over the same network, and assure that no single type of data hogs the line.

In the UK BT uses the networking protocol PPPoA (Point-to-Point Protocol over ATM) over this part of the network for the transmission of data packets.

As a side note some other countries (such as the US) use PPPoE (PPP over Ethernet) for adsl.

Back to diagram.

PoP (Point of Presence).

Location or point where a network can be accessed. - See also Co location Centre.

RAS / B-RAS (Broadband Remote Access Server).

Although the RAS could be thought of as a "super exchange" for adsl, in actual fact it is hardware that is located in strategic points throughout the UK. In general networking terms the RAS would be known as a L2TP Access Concentrator (LAC).
You could think of the RAS being a big hub, which has thousands of connections from hundreds of exchanges coming into it, which then forwards the connection on towards your ISP across the main Colossus backhaul.

The RAS is the first major hop of your connection, and it is here that the login authentication to your ISP is carried out, and you are allocated your IP address.

It is also at this point that you use the bt_tests such as bt_test_user@startup_domain to bypass your ISP and route you directly to the intranet.

There are currently 10 RAS's that serve all the various local exchanges:-
Birmingham, Bletchley (MK), Ealing, Edinburgh, Ilford, Kingston, Manchester, Reading, Sheffield, Faraday.

Due to the way BTw perform their routing, you may find that the RAS that you connect via is not necessarily the nearest to you location wise - its also quite common for different DSLAMs at your exchange to be routed to several different RAS's.

Anyone whose ISP uses a BT Central Plus product should be able to see the RAS as their "first hop". Some ISPs however purchase their own central pipes from BT and as such this hop in the routing is hidden. Users with ISPs who have their own centrals may be able to find out which RAS they are connected to by following the instructions here.

Back to diagram.



Authentication of your adsl connection is carried out at the RAS.
A very simplified explanation would be to say that a BT RADIUS server at the RAS would establish an L2TP connection to the ISP's RADIUS server.

The BT RADIUS server authenticates the domain part of the username and from this determines the ISP's Central and associated router. The ISP's RADIUS then authenticates the end user by their username and password. Once both of these have been verified an IP address is assigned to the user. The whole authentication process is actually a lot more complicated, and a more in-depth view can be seen here.

RADIUS default timers:- Home gateway router timeout = 10, no of retries =2.

SSB (Service Selection Barring) is implemented at the RAS during the authentication process.
It is SSB that stops users from being able to access two different ISPs at the same time, and BT Wholesale set up your connection so that it can only connect to your particular ISP's realm.

When you migrate, BTw will change the SSB details, by removing your old ISP details and inputting your new ISP information so that you can only connect to the new ISP.

Back to diagram.

Colossus IP Network.

BT's main Internet Backbone built in the late 1990's to carry IP traffic. It consists of a massive network of fibres and powerful (gigabit) routers which can operate at over a billion bits per second. Every day it carries many GBs of internet traffic around the UK and linking to international internet backbones. (Central Plus ISPs only).

Back to diagram.

Home Gateway.

The tunnel endpoints on which PPP sessions from the Colossus Network are terminated and where it joins the ISP Central Pipes is known as a home gateway.

The Home Gateway is a cluster of LTS's (L2TP Tunnel Switches). The amount of switches required depends upon the size and type of Central Pipe used by the ISP - for example a 155 Mbps Central terminates on 2 LTS' whilst a 622 Mbps pipe will terminate on 12 LTS'.

The test login is carried out at this level.

Back to diagram.

BT Central Pipe.

A BT Central connection provides the link between the ISP's network and the BT network.
An IPStream ISP who manages their own network traffic will lease one or more Central Pipes from BT, most commonly being multiples of 155 Mbps or 622 Mbps. If an ISP uses 622 Mbps pipes, they have an option of activating these in 155Mbps segments.

A 622Mb central is actually 2 x Gigabit Ethernet Fibre Optic cables (1000 BaseSX) that terminate on 12 tunnel endpoints (Home gateways/LTS's).
The 622Mbps design uses a total of 12 LTS's and provides 46.3Mbps (of end user IP traffic) per LTS.

A 622Mb central pipe can hold up to a maximum of 25,600 simultaneous sessions, and can sustain up to 622 Mbit/s of downstream traffic plus up to 311 Mbit/s upstream throughput.
However it should be noted that because its presented as IP over PPP over L2TP over UDP over IP over Ethernet typical EU downstream throughput will be 557Mit/s.

There more in depth information on my BT Centrals Page.

Back to diagram.


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