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BT DataStream and In Span Handover - the low down
Friday 04 April 2003 19:55:00 by Andrew Ferguson

BT DataStream as a product has existed for some time, but only during 2002, after various Oftel rulings, has In Span Handover become a reality. Now a number of licence holders are moving towards market or have footholds, in either high speed consumer lines, or ultra low contention DSL lines for businesses.

BT Datastream still makes use of the normal BT DSLAMs and exchanges. Which explains why the three speeds of service available are 512kbps, 1Mbps and 2Mbps and all with 256kbps upstream. The 512kbps service is rate adaptive. For every BT exchange that a provider wants their Datastream product to be available from, they must rent a virtual path back from each DSLAM they want to connect to at that exchange. Classical BT Datastream takes this path all the way to the ISP using BT networks. Each virtual path can hold a number of users, the contention obviously can be controlled by the number of users on the path and its size. In Span Handover(ISH) is where it gets interesting. The virtual path leaves the exchange on a BT network and travels to the point of presence (POP) nominated by the provider, this POP is where the handover actually occurs.

The handover process is the method by which the BT owned network is handed over to a network owned by another provider. Obviously you will want to connect multiple virtual paths to this private network, or else the product becomes very expensive. This private network now delivers the connection to the service provider's own traditional network.

The clever part is that your TCP/IP data is never visible to BT at any point, as it remains hidden in an ATM network. This explains why if you try to tracert a connection that uses DataStream, you will not see evidence of all this magic.

The reason why providers want to be able to sell connections that work like this is because the virtual paths from the exchanges and private network are under control of the provider. This control means the provider controls the Quality of Service and the contention levels. This means 1:1 or 5:1 contention networks are possible, athough for consumers the trend is for 1 & 2Mbps lines with 50:1 contention. Also by careful planning and deciding which POPs are used and by building highly resilient networks, a more reliable service than the currently available ADSL products is possible. The negative side is that it requires capital investment in networks and technical skills to run the networks effectively.

The references to 74 exchanges per ATM connection relate to the ability to connect multiple virtual paths to the private network at the point of the In Span Handover. Each virtual path has a minimum size of 2Mbps, assumming you are selling 2Mbps services. The problem is the presentation for the private network is normally an STM-1 link, or a 155Mbps connection, and BT will not allow providers to connect more virtual path bandwidth to this than 155Mbps. This is because of the way the Quality of Service parameters work. Here in lies the problem. This results in only 74 x 2Mbps paths being able to connect (155 - 6Mbps for ATM overheads, gives 74*2). This technical restriction will remain in place until larger links, like an STM-4 (622Mbps) become available in the near future. Several commentators have mentioned that the limit of 74 virtual paths was readily understood by them, so it is not clear at this time who has mislead who, in relation to Internet Centrals problem.

Some providers may already be building their networks with the 74 path limit in mind. Others may now be frantically looking for more ATM lines that can be used at various BT POPs to allow a further 74 exchanges to be connected. It is believed that a month ago only 12 suitable ATM lines had been ordered, or were actually installed and running. As the lead time can be 6 months in the worst case scenario, either a lot of extra lines have been ordered quickly, or a number of providers are gambling on most users coming from a small subset of BT's exchanges.

Updates: It should be pointed out that the above does not mean Service Providers are restricted to 74 exchanges across the country. The actual number of exchanges covered will depend on how many ATM pipes they are able to use. The limits are likely to be alleviated by the introduction of 622Mbps SDH ATM links in the near future, the larger size supporting more exchanges and working out cheaper per Mbps of bandwidth.


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