Network Capacity Planning
Internet service providers (ISPs) have historically talked about "contention ratios" when describing broadband. The contention ratio is the number of people sharing a given connection. Early ADSL services offered two levels. 20:1 for business and a cheaper 50:1 for consumers. Sharing a connection with only 20 people is clearly better than the higher number. Note that in this case the contention is at the telephone exchange – not on an individual’s line.
With the introduction of ADSL Max (and later ADSL2+ and FTTx), contention ratio disappeared from the language as BT changed to guaranteeing a certain throughput through the telephone exchange at peak times. Consumers could still pay more to get a higher throughput guarantee, depending on the package purchased.
Nowadays the contention has moved from the equipment in the telephone exchange to the backhaul into an individual ISPs core network as is described in this article. It isn't totally analagous to the contention ratio of old because instead of sharing 2Mbps on a 2Mbps connection you are sharing a connection that has much higher bandwidth than your local pipe. This difference is not visible to the end user, but is something that affects the overall quality of the customer experience. Perhaps more importantly, bandwidth sharing on the backhaul connection must be taken into consideration by an ISP when planning network capacity.
Once the initial capital investment has been made, the most expensive ongoing element of a broadband service is the backhaul. This is why a typical ISP provides packages with usage limits. To a rough approximation, the more GigaBytes you use the more it costs the ISP. Connectivity is however measured in bits per second not bytes so how does an ISP decide how much bandwidth it needs in terms of bps?
For an existing ISP planning to expand its market share and take on more customers this will be straightforward. It divides its existing peak backhaul bandwidth usage by the number of customers (known as tails) and gets an average usage per tail. It then uses this average figure to calculate the total additional bandwidth needed for a given number of new customers.
There are things to look out for here. First, consumers normally use less bandwidth than businesses. This is almost certainly down to the fact that there are likely to be more users sat behind a business broadband connection than a connection into someone’s home.
All ISPs will have different metrics, but a figure of 40kbps for consumers and 70kpbs for business is a reasonable average. It should be noted that these numbers are constantly growing in line with increased online usage. The rule of thumb has traditionally been is that internet usage grows by around 50% per annum. So 70kbps today is likely to be 100kbps in a years time. This is not necessarily an indicator of future growth. Increasing usage of HD video, for example, could completely change the metric.
The other factor driving usage is the speed of the local delivery technology. Someone with a 2Mbps ADSL connection is going to use less bandwidth than someone with 8Mbps ADSLMax, which in turn uses less than than a 24Mbps ADSL2+ connection, and so forth. Each jump in technology has resulted in a growth of usage of around 30 to 50%. These figures are all rough orders of magnitude because they will be different for each user community.
It gets worse - at least from the ISPs perspective. As local access speeds increase the minimum backhaul bandwidth required to service a community becomes far higher. For example a 2Mbps connection will require as a minimum a 2Mpbs backhaul otherwise it can’t possibly be a 2Mbps connection. Similarly a 100Mbps circuit needs a minimum of a 100Mbps backhaul.
However a 100Mbps backhaul is far more expensive to provide than a 2Mbps connection. The barrier to entry has just been raised. A 100Mbps Ethernet pipe will likely cost in the region of £20,000 per year, so the ISP needs to find a critical mass of users willing to sign up for the service to cover this cost.
The calculation of how much backhaul bandwidth you need to provision therefore starts at the maximum speed of a single local access circuit. A single 100Mbps backhaul serving 100Mbps fibre-to-the-home connections will potentially be sufficient for as many as 1,000 connected tails or more as they are not all using the network to its maximum rating at the same time.
It isn’t rocket science to work out that two 100Mbps users trying to use the maximum speed of the connection at the same time should only get 50Mbps each off a 100Mbps backhaul. Whilst this may be true, in practice users are never using the full capability of their connection, and this is always one of the determining factors of ISPs offering “up to” speed packages.
As users are added to the network the ISP will monitor usage, and if it sees maximum capacity being reached regularly with resultant network congestion it will increase the bandwidth available. As the number of connections grows, the required backhaul bandwidth can be based on a usage per tail figure that the individual ISP will have calculated based on its local experience.
Controls can be put in place to mitigate against usage abuse and growth. For example, someone continuously using a bit torrent to download files from the internet could permanently saturate a link and degrade everyone else’s service. Some ISPs will apply a limit to the amount of bandwidth that can be used by protocols such as bit torrent so that a satisfactory overall service is provided for other users.
The inclusion of this type of functionality in a network needs to be designed into the original architecture. It is also something that requires transparency in the commercial terms with customers.