Internet Speeds and Contention Ratios Explained

It's all a matter for Contention

FOR & AGAINST

A common point of confusion amongst internet users is connection speed and how that translates to performance. There are multiple offerings in the market. This document is an attempt to shed some light on the different alternatives, so users can make an educated selection of the different services available.

In Detail

A communications link has various characteristics, one of them being speed. The speed of a link is measured in bits per second. One thousand bits per second is 1 Kilo bit per second and is usually seen as 1Kbps. One million bits per second is 1 Mega bit per second (1Mbps), while 1 billion bits per second is 1 Giga bits per second (1Gbps). As file size is most often measured in Bytes, speed is measured in Bytes per second (with all the higher units listed above as 1KBps, 1MBps or 1GBps). As 1 Byte is 8 bits then 1 KBps = 8 Kbps. Knowing the speed of a line one can calculate the amount of time it would take for a file, of a known size, to travel through the link. So a 10MB file (10 Mbyte file) being downloaded on a 1 Mbps link, will take 10x8=80Mbits (file size in bits) divided by 1 Mbps = 80 seconds. The same file on a 256Kbps link would take 4 times the amount of time or 320 seconds. The same file on a 128Kbps link (2 ISDN lines) would take 640 seconds, while on a 64Kbps link it would take 1280 seconds.

Communications links are usually “full duplex”, meaning you can use them both to send (upload) or receive (download) data. Many internet lines are not symmetrical. In other words, the speed in the upload direction is not the same as the speed in the download direction.

Because of the nature of the internet links used for internet access usually have higher speed (higher capacity) in the download direction than the upload direction. In other words a user can receive data faster that they can transmit data.

The simplest and unfortunately the most expensive option of a communications link for internet access is that of dedicated capacity. Dedicated capacity guarantees that all data travelling on the link will go to a single user. In other words if a single user connects their PC to the internet with a dedicated (some times referred to as a 100% Committed Interface Rate, CIR) 1Mbps line, then the user will be able to download a 10MB file in 80 seconds every time they try to download.

For the purposes of the following discussion the dedicated link described above can be defined to have a 1 to 1 (1:1) contention ratio, or no contention.

Building a communications link is an expense. If it is a fibre optic line then a trench needs to be built, cable laid and termination equipment must be installed. If it is a satellite link then a satellite must be built, launched and operated etc. The higher the speed of the link the higher is the cost of the link.

Internet access usage is a process that is not of a continuous nature. If you think about it, you access www.cnn.com by entering the address on the browser. For a few moments, the link has to work really hard to download a couple of MBs of data. Once the screen is displayed on the browser, the user reads the news, looks at the pictures,etc. The link is then completely idle.

One could consider allowing a second user access to this link. If the two users were completely coordinated and used the link in sequence, then they would both experience the same 1Mbps performance and at the same time could share the cost. If the two users could not coordinate their download patterns and they both accessed the internet at the same time, then they would both experience the performance of a 1/2Mbps line (500Kbps).

The line now has a 1:2 contention ratio and can be defined as a 1:2 contended 1 Mbps line.

As described previously, there is no way to define the exact speed a user would experience at any given moment, as that would depend on the other user's usage pattern.

Worst case scenario is, the experienced speed is exactly the division of the full link speed and the contention (1Mbps/2=500Kbps). A properly defined contended service should state the peak speed as well as the contention ratio. It is due to the high cost of providing high speed lines that most broadband internet access technologies use contention, in order to make the broadband service affordable.

Fair Access Policy.

Performance depends on the usage patterns of other people sharing the same resource. Therefore, the service providers try to identify and penalise people that use the internet in a manner different from the non-continuous pattern described above. If someone programs a PC to constantly download movies or other multimedia content, then that user occupies a slice of the resource all the time. The user should purchase(and pay for) a dedicated link.

Different service providers devise different methods for protecting the users, that match the contention ratio usage profile, from service abusers. The Hughes system measures the amount of data continuously downloaded by a site and reduces their throughput, should it detect ‘continuous-download’ type of behaviour.

The Tachyon system defines a set amount of data per month, that can be downloaded, without any restriction but then throttles the amount of excess data.

This abusive type of behaviour is not only limited to a single user doing constant high speed downloads. Taking a particular grade of service, that a service provider defines for 10 local users and using it to provide internet access to 100 users, results in similar type of traffic patterns. This causes the service provider to take action, in order to protect the rest of the users. When sites reach the limits of traffic patterns, that are “normal” for contention based systems, they should upgrade to a dedicated service.

In addition to contention between the various site's sharing the common resource (satellite terminals sharing a DVB download channel for example) there is contention within the single site. If a dedicated 1Mbps link is used by 10 people, then each PC will receive its own share of the 1Mbps resource. This contention is governed by the same rules and patterns described above.

What is it all about?

The speed of transmission is one characteristic; the faster the speed of the link, the faster the speed when downloading a file. Another characteristic is Contention Ratios.

At Servicesat we offer differing services, from different suppliers. The main difference, between these supplier's services, being how they treat contention and how they punish the user for over use. Hughes Network Systems offer a great service for Home Users; relatively inexpensive, affordable and reliable.

It is however, a heavily contended service, so for large downloads or multiple user requirements, it does have draw-backs. Other suppliers, Tachyon Networks for instance, offer a “dedicated-like” service (MVG - Monthly Volume Guarantee). With this, ‘packets’ of data are paid for and as long as you have not gone over your ‘packet’s’ limits, there is a guarantee of 95% of the rated speed, 95% of the time. With a normal contended service, this does not happen.

Even for a single user one problem with contended service is ‘throttle’. This occurs when a user is excessively downloading large files. To explain this,imagine a leaky bucket. imagine a leaky bucket.

The user is the bucket. He receives data (e-mails, browsing, music, etc) and puts it into the bucket. As he uses the system the bucket fills up. At some point the user may fill up the bucket, it is full and ‘data’ spills out. This is all he now receives and is ‘throttling’.

To help the user, Hughes opens a small hole in the bucket to allow ‘used’ or ‘seen’ data to leak out and make space for new data. The speed by which data enters the bucket (burst rate), the size of the bucket and the size of the leak hole depend upon the individuals Grade of Service. The higher the GoS, the larger each of these parameters are.

Contended services are also affected by the numbers of users trying to access the service.

Whether in the same office or other users who are part of your contended service; if lots of people are trying to access at the same time, speeds become slower.

One analogy, is the water in our homes.

If one turns on the tap, the water comes out at an expected rate of flow. Sometimes however it comes out slower than normal.

This may be due to someone else probably having another tap on in the house, at the same time. While this is not so prevalent in lots of homes today, it does occur and as more 'users' attempt to open more taps and use the same source of water, the slower the water flows at each individul tap.

This is what happens with contended services. There is never any change to the supply of water entering, there is just a differing number of 'users' trying to access that water supply, at different times of the day. Unfortunately, we usually all want to use the bathrooms and taps at similar times, just like when we are travelling to work.

If a man has to travel from A to B and has to do it in 'rush-hour' that journey will take longer than outside of these times - there are more 'users' trying to use 'the service'. If he goes home late one night and he is the only one on the road, his service has become uncontended or dedicated. So, if you are more than likely to be travelling every day in the rush-hour traffic, that's when you look for an alternative. First option is of course to travel at different times, not usually an option available to most of us. Second option is to look for an alternative form of transport unaffected by the extra traffic during rush-hour.

The alternative for the traveller maybe the train, ferry, bicycle or (for some) a helicopter but for the internet user it is uncontended service. Or like being able to drive on an empty Autobahn, ALL the time.

If you want to be the one with the steady flow of water, whenever you need it OR to always drive on an open motorway, then you need services, like those provided by, Tachyon Networks and Satlynx.