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A STRUCTURED APPROACH AVOIDS THE MESS OF WIRES

This latest article from Gardiner & Theobald looks at the structured cabling that forms part of the office IT network. It gives an insight into current technologies and compares the costs of delivering copper and fibre to the desk.

CABLE VS WIRELESS The decision to go 'wired or wireless' depends on many factors, including:

budget - wireless networks are generally cheaper to install;

data transfer - transferring large volumes of data reduces the speed/quality of wireless networks far more noticeably than wired networks; and - building characteristics - thick walls or large metal structures degrade wireless network signals.

We will assume for the purpose of this article that the decision is a structured wired network. Wireless network technologies have been covered previously (AJ 27.04.06).

STRUCTURED CABLING Not so long ago, structured cabling entailed a mix of multiple media topologies ('daisy-chain', 'bus', etc) and used varying types of cable, such as coax and twinax. The vendor selected the cabling system used, and the users - i. e. the clients - were therefore entirely dependent upon the vendor's decisions.

Structured cabling originated in the telephone cabling industry. As data requirements became increasingly critical it was necessary to adopt a structured approach that avoided the traditional mess of wires. These days structured cabling comprises standards-compliant cabling (there are too many standards to list).

It is normally copper rather than fibre running along the floor from the desk (horizontal cabling) to the patch panels and switches/hubs. From these a multi-mode/single-mode fibre-optic cable is used for the backbone/vertical riser of the building down to the main/basement equipment room. Structured cabling is relatively cheap to produce and quick to install. It is a standard media platform that supports multiple applications. The use of structured cabling allows many topologies to be accommodated, but the star topology is the one that is generally used.

TOPOLOGY The 'topology' of a network refers to the configuration of cables, computers and other peripherals used in that network. 'Physical topology' should not be confused with 'logical topology', which is the methodology used to pass information between devices (computers, servers, switches, etc).

As previously mentioned, the topology used in structured cabling today is most often a star topology. This topology is expandable at any time within its life, unless the cabling requirements change - for example, if a higher speed of data transfer (bandwidth) is required. (In this case the cabling would be replaced with CAT6 or CAT7. ) With this topology, network administrators can reconfigure their networks constantly to meet ever-changing business requirements.

ADVANTAGES OF A STAR-BASED NETWORK:

it establishes an orderly hierarchy with clear lines of responsibility;

failure of one of the 'sub-star' (or leaf ) networks does not inhibit other networks, so the system is robust;

problems can be isolated quickly and easily and dealt with;

upgrades are easier. Any individual LAN, backbone, or any item of equipment can be upgraded without significantly affecting other portions of the network; and it is suitable for large businesses.

DISADVANTAGES OF A STAR-BASED NETWORK:

it is more expensive than other topologies;

more maintenance is required than for other topologies; and a higher quality of maintenance staff is needed.

When considering the layout of a network, the maximum distance between servers, hubs and nodes (computers, printers, etc) needs to be kept in mind. For example, the distance between desk and patch panel cannot exceed 90m. Multiple comms cabinets/ rooms may be required if the building has, for example, one long side. Consideration needs to be given to how the cable will be run to the desk outlets, eg underneath a raised oor.

OUTLETS PER DESK There is normally one outlet per service (voice/data) per user, plus some spares. However, this ratio can change depending on the client's business and the application of the users. If the system is for a commercial bank, for example, the number of outlets per user will increase. IP telephones may also change this ratio, normally causing the number of points per person to decrease as a result of the telephone handset interfering with the PC's outlet. This is not always the case, as some clients do not allow this, and stipulate a separate outlet for voice communication.

TYPES OF CABLE The two main types of cable used in structured cabling are copper and fibre.

The 'category' of a copper cable is prescribed by the Telecomms Industry Association and the ISO (International Standards Organisation). It specifies the maximum bandwidth achievable over a particular distance (normally 90m with CAT5e).

Categories 5 & 6 are cable types, which use all four pairs of wire to both send and receive data.

The CAT5 cable is a 4-pair high-performance cable that consists of twisted pair-conductors. The CAT5 cable is designed to run up to 100MHz, and is typically used for Ethernet networks running at 10 or 100Mbps. The CAT5e cable, also known as Enhanced CAT5, has its performance requirements raised. Like the CAT5 cable, CAT5e is designed to run up to 100MHz;

however, it has the capacity to run 100 Mbps or 1Gbps. The CAT 6 cable provides higher performance than CAT5e and has more stringent specifications. CAT6 systems have a bandwidth of 200Mhz (characteristics are defined to 250Mhz).

The improved performance of a CAT6 system is able to support Gigabit Ethernet transmission using only two pairs of the cable rather than all four. CAT 6 is a bigger cable and requires larger cable trays, and the bending radius is also greater.

There are three types of fibre available for cable use in data networking:

50/125 multi-mode fibre - 50microm diameter of the inner glass core, 125microm diameter of glass outer cladding;

62.5/125 multi-mode fibre - 62.5microm diameter of the inner glass core, 125microm diameter of glass outer cladding; and - 9/125 single-mode fibre - 9microm diameter of the inner glass core, 125microm diameter of glass outer cladding.

Multi-mode fibres have a large core which allows for less critical alignment and can be used with low-cost LED (light-emitting diode) technology. However, because of the core diameter, the bandwidth is limited. Single-mode fibre, on the other hand, has almost unlimited bandwidth because the small single core supports only one light mode. However, this requires very high-precision alignment in both joints and connectors, and expensive laser technology is needed to drive the fibre. These factors combine to make a single-mode installation approximately four times more expensive than a multi-mode installation.

COSTS TO THE DESK The costs shown in the table above are based on using either a fibre backbone and copper (CAT6) to the desk, or a fibre backbone and fibre to the desk. For the purpose of this exercise average layout, lengths, quantities, components, manufacturers and costs have been used to equate to the end-unit costs. The costs should therefore be used as a general differential for guidance purposes only. Finally, bear in mind that the costs of fibre-related components are falling steeply and are becoming closer to the costs of standard copper-related components.

DESIGN CONSIDERATIONS Taking into consideration the aforementioned, the architect should note that:

the office floorplan should allow for multiple comms closets/ rooms if the 90m rule is exceeded;

there should be one comms room of approximately 30m 2 for every 90m of length of the building in both directions per level;

comms rooms need to be an equal distance apart, i. e. if you need three, don't group them together;

risers should be placed between comms rooms;

there should be at least one central equipment room of approx.

100m 2; and - there should be allowance put in for cable passage, e. g. a false floor of at least 150mm depth (this does not take into account other services).

Grace Mair is a surveyor at Gardiner & Theobald.

See our online briefing on desktop technology at www. ajplus. co. uk/ICT

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