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No communications system has infinite capacity

Traffic capacity

The busy hour

Cellular telephone systems

Vehicular traffic

The internet

Telephone exchange technology

Preferential telephone switching

No communications system has infinite capacity

Public and private telephone exchange systems can get overloaded with calls.
Private business radio systems can get overloaded with calls.
Public cellular (mobile) telephone systems can get overloaded with calls.
On the day of the London terrorist bombings in July 2005, the cellular telephone networks became unusable.  Some people said that the companies had turned off the cell phone base stations to prevent the possibility of terrorists detonating bombs remotely using a cell phone.  This was wrong.  They can't easily turn off their base stations.  The problem was overload. 
Teesside was badly affected by black ice,  resulting in all vehicles either travelling at only a few miles per hour or experiencing complete gridlock.  There were many accidents, causing people to be very late getting home from work.  The cell phone systems soon became overloaded and few people could make any calls.
At the Lockerbie PanAm flight 103 disaster, part of the aircraft hit at the side of a main road.  BT trunk cables are laid at the side of main roads.  To a large extent, Lockerbie exchange was isolated from the rest of the world. The exchange overloaded as so many people tried to make calls at the same time.  The emergency services and media journalists found that the cell phone system was also overloaded and most calls failed.

BBC News on traffic gridlock

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Traffic capacity

The mathematical theories of traffic flow can be applied to several situations.  They apply to telephone exchanges, roads, railways, computer networks and RAYNET message handling.

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The busy hour

Telephone traffic is measured in Erlangs, which equate to the number of telephone calls made in an hour.  This helps BT and other telephone companies to plan new telephone exchanges and check that existing ones can handle expected demands.   Each exchange has a busy hour and a busy day.  The exact hour and day varies, depending on if the exchange is in a major city or a small village and if there is a regular large public event on one day of the week.

Knowing the busy hour, when the most calls are made through a particular exchange, the telephone equipment must have the capability of handling that amount of simultaneous calls.  It must also have some extra capacity to cope with future growth and unusual circumstances.  The problem is deciding on how much extra capacity should be provided.

If an exchange has 5,000 telephone lines connected to it, there is no way that there can be 5,000 telephone calls through it at the same time.  Only a small proportion of calls can exist at the same time.  First come, first served.  BT and other companies can't be expected to waste money providing huge exchanges with tons of equipment which will never get used.

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Cellular telephone systems

The above also applies to cellular (mobile) telephones.  A cell phone company may provide cell site base stations with large capacity in a city.  It may provide many cells to cover one city.  It is a different matter in the North York Moors, the Pennines or The Cheviots.  Sheep don't usually need to make many cell phone calls.

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Vehicular traffic

As above, it all ties in.  Streets and roads are virtually devoid of traffic at 04:00 hrs.  You can dash about and get where you want to go without getting held up by other traffic.  In major towns and cities it can be a nightmare around the busy hours of 09:00 hrs and 17:00 hrs.  If 50% of the vehicles in the UK decided to drive them at exactly the same time, many of them just wouldn't fit.  The roads simply don't have the spare traffic capacity.

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The internet

People get home from work, have something to eat, then many use the internet.  It may run slow between 18:00 hrs and 21:00 hrs.  The modern broadband internet access speeds of 8 Mbps and 40 Mbps remove the delay between your house and your telephone exchange.  Once your access speed reaches these values, the noticeable delays are only due the main network paths becoming overloaded.

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Telephone exchange technology

In the UK, it was in the 1920s when the many individual telephone companies were bought out and taken over by the government to form a single nationalised service called the GPO (General Post Office).  In those days and even as recently as the 1960s, telephone exchanges were manually operated.  Operators in the early days not only had to be female, but also had to be an unmarried female.  By the 1960s, a proportion of operators were male, especially those on the night shift.

Automatic exchanges eventually replaced the manual switchboards.  This gave telephone users the amazing facility of making their own calls connect by using a dial to select which other telephone to connect to.

Middlesbrough "new" automatic telephone exchange was opened in 1959.  The last large manual exchange in the area to go automatic was Whitby.  The last small village exchange to change from manual to automatic was probably Forest-in-Teesdale in the late 1960s.

The original UK automatic telephone exchanges used what is called Strowger technology (clunk, click, whirrr, mechanical step by step), then came crossbar exchanges, then the original electronic exchanges which used reed relays and finally, the modern computerised electronic exchanges.

Many disdainful things have been said in the UK about Strowger equipment and the person who invented it.  It was invented by an American undertaker (mortician).  So what?  Who cares about the profession of the inventor?  It couldn't have been invented by a professional automatic exchange engineer as there weren't any automatic exchanges.  That's why Mr. Strowger invented them.

The GPO changed into Post Office Telephones, which changed into British Telecommunications, which changed into British Telecom, which changed into BT.  Somewhere along this route, an amazing con trick occurred.  The government, i.e. the people, owned all of the telephone system.  Somehow, the government persuaded the people to buy something which they already owned.  So the people bought the national telephone company from themselves for millions of pounds in the form of shares.  There aren't many confidence tricksters who could get away with making people buy things they already own, but governments are cleverer and have fewer scruples than confidence tricksters.

The crossbar exchange was a stop-gap measure while we developed the electronic exchange for use in the UK.  It was still mechanical, but a lot less mechanical (and quieter) than Strowger.  It had a major disadvantage which went undiscovered for a while.  The Americans didn't tell us about it, either because they didn't know or because they did know and thought it was funny and so kept quiet.  You couldn't put one near a railway line.

This was discovered when Eaglescliffe exchange was built using crossbar technology.  It was built 30 yards from a railway line and everything worked fine until the first train passed.  The vibrations dropped all the switches out.  This meant that all telephone calls through Eaglescliffe got chopped off.  Each person with a chopped call assumed that it was only their call which had died.  They didn't have the imagination (or the stupidity) to assume that every single call had been killed.  No huge problem.  Everybody dialled again and all was fine.  Until the next train.  And the next.  And the next... 

BT fixed the problem quite easily, once the extent and seriousness had been recognised.  They bought a big sheet of thick rubber, lifted up the exchange, shoved the rubber underneath and dropped the exchange back down on it.  Job done.  All fixed with low technology and a big crane.

In the days of mechanical Strowger telephone exchanges, it needed one engineer to maintain a few small exchanges serving several villages, perhaps twenty engineers to maintain a large town exchange and around fifty for a large city exchange.

When the first electronic exchanges, the TXE2s (what happened to the TXE1s?) came into service, the manning requirements changed drastically.  Compared with Strowgers and crossbars, they were totally silent and with no moving parts.

Each TXE2 exchange was maintained by one engineer and one dog.  The function of the engineer was to feed the dog.  The function of the dog was to prevent the engineer from touching or trying to adjust any part of the electronic exchange equipment.

The modern computerised electronic exchanges also resulted in a large reduction in engineering manpower.  Nowadays, one hundred exchanges are maintained by one engineer and one dog.  They both live in a pub.  If the engineer's PC says that part of an exchange has failed, his function is to type "Well mend yourself." on his PC.  The function of the dog is to bite the engineer if he falls asleep from boredom.

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Preferential telephone switching

The exact name for this function has varied over the years, but it amounts to the same thing.  It has been used in the days of all automatic exchange technology.  It originally required switches on uniselector racks to be operated.  Nowadays it is done by computer commands.

It has been indicated above that if an exchange gets overloaded due to too many people wanting to make telephone calls at the same time, the first people to make a call will be successful and all others won't even hear a dialling tone.  During a major disaster (which causes telephone overloading), it is very inconvenient and inefficient if the people and organisations who deal with major disasters can't make telephone calls to co-ordinate a response.

All telephone lines are allocated as priority 1, 2 or 3.  Priority 3 is the normal state of telephone preference working.  Everybody can make an outgoing call.

If a telephone exchange or a cellular base station gets overloaded, a senior manager of the telephone company may authorise an engineer to switch it to priority 2.  This means that the only people who can make outgoing telephone calls are those who have priority 1 or 2 telephone lines.

If things don't soon recover and the exchange is still overloaded, a senior manager may authorise an engineer to switch to priority 1.  This means that the only people who can make outgoing telephone calls are those who have priority 1 telephone lines.  This will definitely remove the traffic overload and allow all priority 1 telephones to work properly.

Note that preferential switching does not prevent any line from receiving a call, just from making a call.  You can still receive a call even if you can't hear a dial tone.

All of this is public knowledge, so of course we all want to be priority 1 as we are all important, although others may choose to disagree.  Because of individuals and organisations all wanting to be on a priority 1 or 2 telephone line, which completely defeats the object of preferential switching, the decision is taken at a high level.  The Deputy Prime Minister's Office decides.

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Page updated on 09 January 2017

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