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ADSL - How adsl Works - 3

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The ISP Network
Page 3 of 3 (1 2 3)

All ISPs run their networks slightly differently as the amount of capacity and requirements will vary depending upon their type of users.

BT Wholesale offer a service called Central Plus whereby BTw do the majority of the routing for ISPs and takes customers of these ISP's adsl traffic straight out to the Internet. Customers of ISPs that use Central Plus will see BT routing before their ISP details when they do a tracert.

Larger ISPs tend to lease Central Pipes from BT, so that they can route and manage users traffic themselves.

Below is a diagram of Plusnet's network. The diagram is interactive - move your mouse over various points to identify the equipment and a link to more information about the equipment.

Plusnet Network Diagram KEY Central Pipes Central Pipe Juniper Edge Routers Juniper Edge Routers gateway routers gateway routers gateway routers gateway routers gateway routers Transit Providers Transit Providers Colocations Colocations Peering Peering Customer Support Customer Support Servers Servers Servers Servers transit provider Internet Internet Internet

BT Central Pipes

Generic Information on BT Central Pipes can be found on my page How adsl works 2.
Current information on Plusnet's Central Pipes in use can be found on its own page here.

Edge Routers

These powerful routers terminate the ISP end of the Central and control the traffic flow in and out of the internet.

Plusnet currently have 6 Juniper ERX 1440's and 1 x E320 and are the first hop you will see when performing a tracert.

These Junipers act as the aggregation point where service providers accept and authenticate connections for tens of thousands of customers. The E-series allows authenticated users access only to the bandwidth, QoS, services and features that they have subscribed to. Therefore, it is the key place in the network to control, deliver and account for customer services.

Each 14-slot Router contains a 40Gbps switch fabric and supports full redundancy. 12 slots are dedicated to line modules.

A 155Mb line card on a Juniper can sustain 8000 subscribers up to a maximum of 48,000 therefore a Juniper could take traffic from more than a single 622 Mb pipe.

Back to Network diagram.

Juniper ERX-1440

Full size images:
Front    Rear   PDF

More info at

Gateway routers

An ISP will have a multitude of gateway and internal routers whose task it is to direct traffic on to the next part of our journey.

Depending upon where you have requested data from, these routers will pass you on to the best path to ensure that your data gets to its destination.

Back to Network diagram.

Juniper routers

Transit Providers

These are companies that specialise in forwarding data between various countries and overseas. An ISP may use several different transit providers for different routing. The ISP will have to pay the organisation transit costs for data sent and received.

Well known transit providers include Level 3, BT net, Cogent, Sprint.

Back to Network diagram.

transit providers


Colocation Centre

A colocation centre (or colo) is a large data centre where ISPs and other telecommunication companies site their networking equipment on secure premises.

These premises provide air conditioned rooms which are vital for powerful servers and routers. They offer high standards of security against disasters such as fire, intrusion, and power outages.

Telehouse (North and East London) and Telecity (London and Manchester) are 2 organisations in the UK which offer colo facilities.

Back to Network diagram.

telecity London

Telecity London

Peering Points

Peering Points offer ISPs a point where they can exchange traffic with each other. Peering is offered by organisations located in a colocation centre.

The idea of peering is that ISPs can reduce their transit costs on the internet backbone. It also has an added benefit that it can provide a more direct access point to another ISP.

LoNAP, LINX, XchangePoint (XPE) and MaNAP are some of the companies in the UK that offer peering services.

Back to Network diagram.

peering services



Powerful computers that are dedicated to a specific task. An ISP will have many different types of Servers. The server will host a specific type of information and then "serve" the data to the user. Some essential Servers are Mail, Web, File and database, other types of servers that an ISP may have are dedicated games servers, CGI servers, SQL etc.

ftp webstats mail servers
netapp storage cluster
ftp, web stats and mail servers.
NetApp Storage Cluster
more geeky stuff

Back to Network diagram.


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