A BT Central is a "pipe" that provides the connection between the BT network and your ISP. There are several different types of BT Centrals, but I will concentrate on those that are most commonly used by most ISPs.
Information on how the Centrals form part of your overall connection onto the internet can be seen on my page how adsl works. This page aims to give more specific details on the BT Centrals.

All IPStream ISPs will use some sort of BT Central Pipes which is where all the ISP's customers traffic from all around the UK will merge before it goes out on to the ISP/Internet.
~ ISPs will have their "own Centrals" which are leased from BTw and therefore they can control and route their users traffic out on to the internet in a way which they deem the most efficient.
~ BTw also offer a product called BT Central Plus, where BTw does all the ISP's routing straight out on to the Internet on behalf of the ISP. At present this product is only used by the BT group ISPs.
~ Some ISPs don't own their own centrals - instead they may use capacity on a central pipe which is owned by a larger ISP. This type of reseller ISP is often referred to as a VISP (Virtual ISP).

ISPs have to purchase sufficient bandwidth on the centrals to make sure that all their customers get enough bandwidth to meet their needs, as all their customers traffic will traverse the Central pipes before it can either go on to your ISP or out on to the Internet. It is here that ISP contention is carried out - insufficient bandwidth on the centrals mean that customers will see slow speeds.

The most common "pipes" purchased by IPStream ISPs are either 155Mbps or 622Mbps.
622Mbps pipes are split into 4 x 155Mb segments. All of these segments do not have to be activated at the same time and the ISP can "flex up" by lighting individual segments at any time up to a year after installation of the pipe.

There are several differences between the 622Mbps and 155Mbps pipes.

Efficiency

The most major difference is that the 622's are provided over Ethernet whilst the 155's are provided over ATM.

• 155Mb centrals are provided over ATM (PPPoA).
• 622Mb centrals are provided over Ethernet (PPPoE).

ATM Centrals carry in increased overhead load which makes the 622's approx 23% more efficient as regards to the amount of end users traffic (throughput) that they can carry.
In broad terms this means that a 622Mbps pipe can carry the approximately* the same amount of IP traffic as 5 x 155Mb pipes.

*calculation (622 x 23%) = 143 = (765 / 155) = 4.94

Session Limits

A session limit is the maximum users that can connect to the centrals at any one time. (Concurrent Sessions).

• 155Mb central pipes has a maximum session limit of 8,000 users.
• 622Mb central pipes has a maximum session limit of 25,600 users.

Its odd to note that a 155Mb central can hold 8,000 subscribers, whilst a 155Mb segment of a 622Mb central can only hold 6,400 subscribers.
Even more peculiar is the fact that the 622's are more efficient bandwidth wise and can handle more traffic yet they cant handle anywhere near the amount of users.

LTS's (L2TP Tunnel Switches)

These tunnel endpoints are where the BT Colossus Network ends before traffic is transferred on to the ISPs Central pipes. These LTS's are clustered together and are known as a home gateway.

• 155Mb central pipes terminate on 2 LTSs.
• 622Mb central pipes terminate on a total of 12 LTSs (3 per 155Mb segment).

Differences between the tunnel endpoints of each of the 2 types of pipes can cause a problem if an ISP has a mix of 155's and 622's.

Round Robin.

BTw use what is known as the "Round Robin" method of distributing users on to the ISP's LTSs.
This method distributes each user in turn on to a different LTS in a strict rotation... 1,2,3,4,1,2,3,4 etc etc.
This is perhaps not always the best way to distribute users on to centrals as its entirely possible that certain pipes are more full than others.

Anomalies between 155Mbps and 622Mbps Centrals

Up to a couple of years ago most ISPs used 155Mb Centrals, these days with more and more users getting adsl and faster speeds the larger ISPs purchase 622's. ISPs can use a mix of 155's and 622's to meet their requirements. Having already purchased 155's in the past there are quite a few ISPs who do have a mix of the 2 different types of pipes. There are several anomalies between the two types of pipes which can cause problems for some ISPs, particularly if they mix the 2 different types of pipes.

Efficiency

I've already stated above that the 622's are more efficient than the 155's mixing the two types of pipes can cause an imbalance. BTw themselves are aware of this problem

quote ref SIN 412 page 5

BT will support a mixture of 155Mbits/s L2TP Passthrough and 622Mbits/sec Edgeless L2TP passthrough – however customers should note that such a mix will cause an imbalance in traffic between the two central types. This imbalance is due to the different numbers of LTSs used per unit of central bandwidth within the two central designs - as new sessions are evenly distributed over LTSs within a One to Many group. This imbalance is somewhat offset by the increase of bandwidth offered by 622Mbits/sec L2TP central. The 622Mbit/sec design offers a 23% improvement in bandwidth over the equivalent number of 155Mbit/sec BT centrals. The 155Mbps design uses 2 LTSs and provides approximately 56.5Mbits/sec (of end user IP traffic) per LTS. The 622Mbits/sec design uses a total of 12 LTSs and provides 46.3Mbits/sec (of end user IP traffic) per LTS. Consequently, when mixing 155Mbps and 622Mbps centrals in the same one to many group the 155Mbits/sec centrals will be approximately 18% under-utilised i.e. when traffic has reached the full rating of the 622Mbps central the 155Mbps centrals will have reached only 82% of their rated capacity.

Causing some pipes to become congested whilst other pipes may have plenty of spare bandwidth.

Round Robin

The round robin method of distribution of users onto centrals can cause problems particularly when there is a mix of 155 and 622 central pipes - due to them having different numbers of LTS's for the same amount of bandwidth. A few ISPs have already been caught out by this as users are put on pipes that may be full, yet other pipes have plenty of bandwidth to spare. Plusnet was one of the ISPs that suffered from these symptoms. The solution was ditching the 155's and purchasing all 622s.

Session Limits.

It seems odd that whilst the 622's have more available throughput, the amount of actual concurrent sessions on a 622 is proportionately a lot less.

The equivalent straight Mb for Mb is 32,000 v 25,600

However because of overheads† on ATM the throughput makes this differential even larger just 25600 for a 622Mb pipe whereas the true throughput† figure for the equivalent 155's would be nearer 40,000* subscribers. *equiv 155's = 5† x 8000 = 40,000

So now that some ISPs have upgraded to all 622's there are fewer users that can connect to them.
BT where meant to be introducing something called BT Central Extra which doubled the amount of concurrent sessions available, but somewhere over the past year it slid by the wayside and never actually materialised.

~ Update 18 January 2006 BT are to pilot increasing the maximum sessions on a 622Mb central to 30,000.

A Juniper ERX and the JunOS can handle up to 48,000 sessions.