ACCESS & HND Local Area Networks

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WAN - Wide Area Networks

Wide Area Networks Wide area networks provide communications over large distances which can be national or global.  A wide area network often connects separate LANs in an organisation together.  They allow the electronic communication within a large organisation geographically spread apart. Whilst an organisation often owns the network in a LAN configuration in the case of a WAN this is not usually cost effective unless the organisation is very large.  It is more usual to use the switched or fixed network services provided by such network services providers as BT, Mercury or NTL.

WANs can be connected by submarine cable, Microwave and even satellite.  A satellite is usually launched into a geo-stationary orbit to ensure 24hr communication.  This means that it orbits at the same rate as the earth so that it always appears to be at the same relative position in space.
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WAN Topologies

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BACKBONE NETWORK Typically found in office or campus environments in which departments or buildings are interconnected over the backbone cables.  Bridges or routers manage traffic flow between attached subnetworks and the backbone.

MESH NETWORK Routers are interconnected with other routers.  The topology may be configured locally, but is often found in metroploitan or wide area networks.  The Internet is a mesh network.  These are very secure networks as they can provide alternative routing should congestion or network problems occur on a link.

HIERARCHICAL (Interlinked) STAR Most telephone networks use this principle. The exchanges are the hubs that form the switching part of the network.


WANs can be linked by satellite (usually geo-stationary), microwave (line-of-sight) or submarine cable.  This communication media is usually owned by a communications network provider such as BT or Mercury.

If a WAN is well designed then it is transparent to the user and they are unaware of the particular media in use.  An organisation would normally lease, rent or pay by access for use of such network facilities.  It is the network providers responsibility to provide sufficient bandwidth for the customers requirements.
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The types of technology available can be broken down into three basic categories.

Circuit-Switched Networking

A circuit-switched network is one where a circuit is set up for the duration of a session only. This type of network is pay-by-use and is flexible in being able to be repeatedly set up to provide connections to different locations as required. The connection has to be established and maintained for the duration of the transmission.

Both analogue and digital circuit-switching services are available such as:

DIAL-UP TELEPHONE LINES
The Public Switched Telephone Network (PSTN) is for voice transmission over analogue  telephone lines therefore a modem is required to MODulate digital signals to analogue for transmission over the network and then to DEModulate the received signal back into digital.  This is useful for remote access by laptops to a companies LAN.

This type of network is limited to 28,000bps speeds although compression techniques can increase this to 115,200bps.
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ISDN
Integrated Services Digital Network is a system of digital phone connections.This type of system allows both voice and a separate data channel.  A line connected to ISDN requires an ISDN adapter present at each end of the line.  This service offers two rates.
 

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Dedicated Circuit Networking
Is a secure fixed channel between two points.  This differs from dial up networks in that it never needs to be set up.  However it is inflexible becuase it remains connected on a point-to-point basis and cannot be set up to another point.  It is therefore restricted in its use to heavy traffic requirements between two points only.  It is however much more reliable, permanantly accessible as it isn't shared with other users and secure.  It offers up to 45Mbps which is over 800 times the speed of a Switched-56 connection.  Due to expense it is necessary to consider whether full-time connection is required before going for this type of connection.

There are several types of dedicated circuits available:

DEDICATED ANALOGUE LINES
Sometimes called Private wires are available for lease from some network services providers but are expensive.
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T-CARRIER SERVICES
T1 is basically a conditioned digital telephone line.  It is capable of transmission speeds up to 1.544Mbps and comprises of 24 channels each having a transmission speed of 64Kbps.  Each channel can carry voice or data.  The 24 lower speed channels are multiplexed into the one higher speed T1 line.  This system is a very expensive and can be transmitted over UTP (unshielded twisted pair telephone cable) it is now considered rather dated but a lot of such circuits are still in existence.

T3 lines are similar to T1 lines except that because of their higher transmission rates they require fibre-optic cables.  The T3 can transmit at over 64Mbps and consists of 672 channels each supporting 64Kbps.

These lines are very expensive and are commonly used by Internet Service Providers (ISPs) to connect to the Internet backbone.
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Packet-Switched Network
This is a protocol where data is fragmented into into smaller packages to be sent to the destination address.  These packages are sent across a large MESH network and can be routed via different paths and even arrive at different times and in different order.  On arrival at the destination the receiving network has to establish that it has all of the data and reassemble it in the correct sequence. Acknowledgement is sent back to the host that correctly sequenced packages have been receive.  After a period of time the host will retransmit packages that have not been acknowledged.  Two examples of this system are:

PLANNING A WAN
Planning a WAN is rather a specialised job and would usually be undertaken by a team of experts rather than an individual.  But there are four basic considerations that need to be addressed: [Top of page]
SCALE
How many sites are required to be linked? If there are only two sites then a point-to-point service can be selected such as: For multiple sites that need to be interconnected a multi-point-to-multipoint service may be required: [Top of page]
DISTANCE
Most Network service providers have charges relating to distance and bandwidth.  In the case of PSTNs, charges relate to local, and long distance rates.

SPEED & COST
This perhaps is one of the most important factors.  Sufficient bandwidth is essential for the effective operation of an organisation but the cost of speed has to be offset against the cost.  A balance has to be struck between operational needs and what you can afford.
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Brief Comparison
 
Type Speed Cost Method Notes
PSTN 28.8Kbps Inexpensive any-to-any Requires a modem
ISDN BRI 128Kbps Inexpensive any-to-any ISDN terminal adapter
ISDN PRI 1.5Mbps Medium cost any-to-any ISDN termianl adapter
T1 1.5Mbps Expensive point-to-point
T3 45Mbps Very expensive point-to-point
X25 64Kbps Medium cost any-to-any Requires a PAD
Frame Relay 56K-45Mbps Depends on which chosen any-to-any Requires a leased line

Usually WANs are a hybrid of technologies and LAN topologies to meet requirements of an organisation.  They have often developed over a period of time.

Other considerations have to include security and management of the system and the possibility of future upgrades.

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