Vodacom clarifies HSDPA contention ratios
SEACOM recently announced that they will launch operations in mid-2009, promising to cut the cost of international bandwidth by around 80%.
This cost reduction should see far higher broadband usage limits and improved quality of service. It may also bring about improved contention ratios on broadband services like ADSL, HSDPA and iBurst.
According to SEACOM the current international contention ratios for ADSL and HSDPA is most likely in the region of 100:1, something which they believe will improve to 50:1 in 2009 and 20:1 in 2010.
Contention ratios – an indication of the number of people connected to an ISP who share a set amount of bandwidth – are often used as a measure of the throughput users can expect from a broadband service.
This measure is particularly useful when dealing with unlimited broadband services, but in a ‘capped environment’, like South Africa, with low monthly usage contention ratios are however not always be a good indication of quality of service.
Vodacom hits back
Vodacom says that the SEACOM claim that 3G and HSDPA users today see a contention ratio of 100:1 gives the general public the impression that services such as HSDPA are oversubscribed by a factor of 100 and that subscribers can therefore expect to only see 1/100 of the service levels expected.
“Practically they can expect speeds of only 18Kb/s! This clearly is not the case,” said Vodacom. “Vodacom does not agree with this method used to calculate contention ratios as it is based on fixed parameters and does not take real-world usage patterns into account.”
The cellular operators says that taking the available bandwidth and dividing it by the number of subscribers multiplied by the maximum throughput each subscriber requires, seems at face value to be correct but in reality does not reflect what happens in the 3G broadband space where the number of concurrent users, their access devices and their time spent on-line are very dynamic and can radically change from moment to moment.
“GPRS, 3G and HSDPA users do expect a maximum speed close to the theoretical limit of the relevant service but do not access this service on a 24×7 basis. Rather the typical mobile broadband user will access his service on an ad-hoc basis and this typically will be a function of his specific data bundle.”
“If a data user on 3.6 access technology can download a total of nearly 40Gb per day, it stands to reason that a 1Gb bundle, stretched over a month, will result in a fraction of on-line time over the course of the month.”
Mobile services allow the number of on-line users to vary dynamically as users join and leave the network. There are also many different devices that are used to access the service, from 40Kb/s GPRS devices to 384Kb/s 3G devices, right up to HSDPA 3.6 devices.
“As there are a number of these variables and they all can change dynamically it becomes impossible to calculate a contention ratio based on the methodology used for fixed-line services in the past,” Vodacom says.
Real time contention ratios
Vodacom says that they employ a real-time contention ratio measurement to ensure subscribers achieve an acceptable level of service. This is done by constantly measuring the real-world throughput seen on the network and calculating a contention ratio based on this measurement. As the measured variable changes, so does the contention ratio seen at that point in time.
Recent testing indicated that Vodacom's average international download speed was 787 Kbps with a peak speed of 2218 Kbps while the average local download speed was 1711 Kbps with peak speeds of just over 2200 Kbps.
In the upload department Vodacom performed constantly with an average speed of 227 Kbps, regularly peaking over 300 Kbps.
Vodacom says that it can therefore be argued that any user on 3G or lower (<384Kb/s) will see the maximum throughput of his access device at all times and thus his contention ratio is fixed at 1:1, i.e. he will always get the maximum speed. From the user perspective he is not 'contending' with anyone else for his throughput.
“Users on HSDPA 1.8 will vary from between 1:1 to as high as 1:2.2 in the download direction while upload contention ratios varies between 1:1.3 to 1:1.7,” the cellular operator argues.
“This number of 1:1 to 1:2 is the actual experience of a HSDPA 1.8 user on the Vodacom network and is a far cry from the theoretical 1:100 calculated in the attached presentation.”
Vodacom concluded by pointing out that the ASA ruled that Vodacom and other broadband providers may not make any maximum speed claims.
“While Vodacom internally still employs the methodology described above, the ruling practically implies that all Vodacom users, by ASA definition, achieve a 1:1 contention ratio.”
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