Caps, Traffic Shaping and Contention Ratios
Broadband services have revolutionized the Internet space, mainly because of the high speeds and affordability of these offerings. ADSL, Cable and Fiber-to-the-Home services are generally associated with multiple Mbps speeds, flat rated pricing and unlimited usage.
Although operators rarely offer bandwidth guarantees for broadband services, the quality of service is generally high.
The quality of a broadband offering is often linked to the contention ratio, a figure that indicates how many users share a certain amount of bandwidth. Internationally ratios of 20:1 and 50:1, which means that 20 users or 50 users respectively share the same bandwidth, are typically used.
Shared bandwidth – in comparison with dedicated bandwidth often encountered with leased line services – allows operators to provide a more affordable service.
Local broadband providers are reluctant to publish their contention ratios, even though ICASA’s ADSL regulations require them to do so. Telkom has indicated that their contention ratio for ADSL is 20:1, but they have previously said that this figure is for the local access network only and that the figure ‘fluctuates as the service grows’.
Traffic shaping and bandwidth caps
A common complaint from broadband providers is that a small group of high bandwidth users deteriorate the network which negatively impacts on other subscribers. ‘Excessive usage’ is often ascribed to P2P file sharing applications which can quickly cannibalize all available bandwidth on a network.
To combat the negative impact of excessive P2P usage, broadband providers often employ strategies like imposing bitcaps on users or shaping traffic. These restrictions are generally met with user dissatisfaction and in South Africa some broadband providers use both these methods to protect their networks.
Telkom, Internet Solutions and iBurst impose traffic shaping on their subscribers while Vodacom, MTN, CellC and Sentech do not shape their internet traffic.
One of the biggest complaints for most local broadband subscribers is however not traffic shaping but rather the low bitcaps, or bandwidth caps, that characterize South African broadband offerings.
Many international ADSL offerings provide unlimited usage but local consumers are generally restricted to caps ranging between 1 GB and 3 GB per month. While higher usage accounts are available, the cost is so high that it places the luxury of unlimited bandwidth out of reach for most subscribers.
These very low monthly usage allowances have a detrimental effect on the local IT industry and the subscriber’s broadband usage experience as it inhibits him/her from taking full advantage of the benefits of a fast, always on connection. It can therefore be seen as a crippled service.
It is not surprising that the ICASA ADSL regulations called for uncapped local bandwidth, but despite these regulations not much has changed in the local ADSL landscape.
New standard needed
With bandwidth caps the contention ratio value becomes less significant. While it is important that enough bandwidth is available during peak times for subscribers to experience acceptable speeds, the limitations placed on bandwidth usage over the full month means that operators have far more bandwidth to play with.
In an ‘unlimited usage’ environment it means that users can affectively use the service at full speed for the duration of the month, but with capped services it translates into a ‘limited usage period’ which benefits network operators.
The OECD has recognized that contention ratios for networks with bitcaps will vary and have coined the term ‘Implied Contention Ratio’ to shed some light on the matter. The Implied Contention Ratio is an indication of how many users can theoretically be accommodated over a one month period using the maximum speed of the broadband service.
While this is obviously only a theoretical measurement, it is still insightful to evaluate the differences between standard contention ratios and Implied Contention Ratios associated with capped broadband services.
Telkom’s DSL 4 Mbps service with a standard 3 GB cap, for example, is advertised as having a 20:1 contention ratio. Used at full speed a subscriber will deplete their usage allowance within 102 minutes. This gives an Implied Contention Ratio of 422:1, a far cry from the advertised 20:1.
The table below this article gives a full run down of Implied Contention Ratios.
Informing Consumers
The detrimental effects of low monthly bandwidth caps are often documented, and when combined with traffic shaping it cripples a broadband service to such a degree that the benefits of true broadband vanishes quickly.
Unsurprisingly South Africa is lagging behind the rest of the world when it comes to bandwidth intensive Web 2.0 services like video sharing and social networking.
The OECD further warns that traffic shaping and bitcaps create an information gap. Consumers need to be fully informed about exactly what they are buying.
With the various network protection strategies employed by broadband providers like iBurst and Telkom, uninformed consumers seldom know about the differences between the various services and what they are actually paying for.
| Service | Speed (in Kbps) | BitCap (Megabytes) | Minutes to reach cap | Implied Contention Ratio |
| iBurst – 40 MB | 1024 | 40 | 5 | 8100:1 |
| HSDPA – 1 GB | 1800 | 1024 | 78 | 556:1 |
| DSL 4 Mbps – 3 GB | 4096 | 3072 | 102 | 422:1 |
| iBurst | 1024 | 2500 | 333 | 130:1 |
| DSL 384 | 384 | 1024 | 364 | 119:1 |
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