fibre optics

Role of fibre optics within a structured cabling environment

Terence D Lockyer examines fibre optic transmission for LANs

Structured cabling is an important issue faced by most Information Technology departments today. Cable plant is expected to have a 10 to 75 year lifetime and must withstand the pressures imposed by frequent office moves, and user demands for ever more sophisticated communications fuelled by a rapidly advancing technology. Fibre optics can provide the performance requirements of the next generations of networks, but apart from a minority of specialist applications a widespread application of fibre to the desk has not been economically justified today. However, the judicious selective use of fibre can provide the 'future proof' evolution required in a cost effective way. This paper looks at the issue of fibre optic transmission for local area networks, the fibre optic basics every IT manager should know, what cabling systems should be installed and what are the fibre optic options. Advice on fibre installation policy is also given.

Keywords: local area networks, fibre optic cabling, FDDI, structured cabling environment, fibre optic transmission

For almost the past two decades fibre optics had been heralded as a communications technology about to revolutionize the LAN market place. However, the truth today is that this still remains an unfulfilled promise for the vast majority of users. That the much predicted fibre LAN era has not arrived is undoubtedly due to the higher cost of installing fibre compared to conventional copper solutions. Intrinsically, this is not inevitable: mass production techniques have brought the cost of

BICC Data Networks Ltd, Brindley Way, London Road, Hemel Hempstead, Herts HP3 9XJ, UK Paper received; 29 November 1990

raw fibre to comparable levels with copper. However, this is not true of other component technologies. Fibre optic source and detector manu- facturers have not invested in the manufacturing technology which can provide the sort of low cost packaging which is characteristic of the semiconductor industry. This is at least in part due to their historical preoccupation with the cost insensitive performance demands of the telecoms market. Similarly, lack of standards and wholesale demand has inhibited the development of high performance low cost connectors. Installation is still a specialized skill requiring an investment in training

and equipment hardly justified bythe market demand. It is thus not surprising that fibre optics attracts a premium from suppliers, with a consequent reluctance by users to invest.

Things are changing, however, and with the introduction of a 100 Mbit/s Fibre Distributed Data Interface (FDDI) LAN market we could at last be seeing the last of the chicken and egg cost scenario and the beginning of the drive of fibre to the desk. Analysts predict the blossoming of the FDDI workstation market by 1993, enabled by the availability of a $1000 PC adaptor. Until recently, the cost of a set of FDDI transceiver optics has been in the region of $300, however, a major fibre optics supplier has of late announced a part targeted at a cost of $100, making the prospect of the $1000 adaptor more than a possibility.

FIBRE IN LANS: ADVANTAGES AND DISADVANTAGES

Fibre optic's major benefit is that it will support very high data rates such as the 100Mbit /s FDDI standard. Additionally, it has low attenuation, allowing maximum link lengths ranging from 2 km to 20 km to be implemented. It provides full

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immunity to the generation of radio frequency interference (RFI) and susceptibility from other interfering sources disrupting traffic. With the adoption by 1992 of Europe-wide regulations governing these aspects, the electro-magnetic compatibility benefits can be very real. Since it provides electrical isolation it cannot propagate electrical hazards which could result in safety risks or equipment damage. Fibre gives the lightest and most compact of cables. Also it is more secure than the other cables because it is virtually impossible to tap without being detected.

Fibre optic cable's main weakness is that it is more expensive to employ than other cables, and it is generally only suitable for data and video transmission so it does not provide a universal wiring solution for all services.

BASIC PRINCIPLES OF FIBRE TRANSMISSION

Optical fibres' light transmission property is due to the phenomena of refraction or bending, which occurs when light impinges on the interface between materials of different refractive index. Refractive index is a measure of the relative speeds of light in the media. If the light strikes the interface at less than a particular critical angle, total internal refraction occurs and the light is 'deflected' back into the incident media. Light striking at greater angles passes into the other media but suffers a small change in its direction. We are all familiar with this effect which occurs when we look at a pool of water. We can see the bottom of the pool close to our feet, although the depth appears shallower due to the bending, however, further away we can only see a reflection from the water where the angle of view is less than the critical angle. Optical fibre consists of a circular core of glass surrounded by a concentric cladding with a lower refractive index. This geometry causes light which is launched into the fibre core below a particular angle to be guided along the fibre by total internal refraction at the core cladding interface.

Fibre is broadly categorized by two numbers (50/125), which represent the core and cladding diameters in microns (millionth of a metre), and also by its numerical aperture (NA) which is related to the angle of the acceptance cone of the fibre. Fibres are further categorized by being single- or multi-mode and step or graded index. Single- or multi-mode refers to the number of modes of light transmission supported by the fibre which is basically determined by the diameter of the core relative to the wavelength. Typically, single mode fibres have cores of approx 8 microns and are operated at 1300 nm and longer wavelengths, whereas multimode typically has core diameters of greater than 50 micron, and can be operated at lower (850nm) wavelengths. Graded or stepped index refers to the profile of the refractive index at the core cladding boundary. Stepped index fibres have an abrupt change, whereas graded index have a specially profiled gradual change. The advantage of graded index fibres is that they have higher bandwidths than step index for the same geometry. This is because different modes of propagation travel different path lengths as they travel down the fibre, and hence become separated in time resulting in a reduction in bandwidth. Graded index fibre solves this problem by profiling the refractive index (higher in the centre and reducing towards the cladding) so that the higher modes will travel through regions of lower refractive index than will the modes which travel a more direct route at lower angles. Single mode fibres are step index but are of so small a diameter that, as the name suggests, only a single mode can propagate resulting in no bandwidth reduction due to differential mode propagation effects.

A fibre optic link consists of a light emitting transmitter connected over a fibre transmission line to an optical detector receiver. Typical emitter devices are light emitting diodes (LED) or laser diodes. LEDs emit light over a band of optical wavelengths, whereas lasers emit at a single optical wavelength. LEDs centred at 850 nm are considerably cheaper than at

1300 nm. Laser devices are generally more expensive still, as they require complex circuitry to drive them, and because of their high optical output they can present eye damage hazards.

Typically, detectors are pin diodes which are relatively cheap, unless the nigh sensitivities associated with long link lengths are demanded.

FIBRE LANS STANDARDS

The principles of Open Systems Interconnection (OSI) and the benefits of having multivender interworking are well understood in the IT market. Purchasing fibre networking or structured wiring solutions is no exception, and you should look for products from companies which support standards. You will then have at least a reasonable expectation that inter- working will be achieved, and your investment will be supportable for a long time to come.

So let us look at what fibre standards exist today.

IEEE 802 LANs

In 1980 the US-based Institute of Electrical Electronics Engineers (IEEE) established the 802 project to produce LAN standards covering the physical and data link layers of the International Organization for Standardization (ISO) OSI Reference Model. IEEE 802 LAN standards are, in general, categorized by the media access (MAC) technology used. The standards which currently have fibre optic interfaces standardized or in development are:

• CSMA/CD 'Ethernet' • Token passing bus • Token ring

Ethernet In 1988 CSMA/CD 'Ethernet' was the first LAN standard to include a fibre element, the Fibre Optic Inter- Repeater Link (FOI RL). The FOI RL was developed to interconnect coaxial cable segments via repeaters and fibre optic transceivers. In essence it defines a fibre optic transceiver for

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use with a repeater. However, many vendors have developed adapter card and multi-port repeater products using this fibre optic transceiver in a full fibre or mixed media network. These networks are configured as star topologies. There are at least a dozen venders of inter- operable products supporting this standard.

Meanwhile, the 802.3 committee has been working on a fibre optic network standard '10 BASE F'. This contains three sections describing a fibre media, a passive star system and an active star system. It has been several years in the formulation, and looks to still have some way to go. Critics of this standard question the fundamental need for an active star standard other than the existing FOIRL standard. FOIRL is already an ISO standard and, although only officially intended as an inter- repeater link, has been implemented by LAN vendors for computer attachment. Furthermore, in recog- nition of this standard the active star 10 BASE F is committed to support FOIRL for inter-working with the established FOIRL base. It is now questionable that there is a need to create a new architecture for fibre optic active stars different from multiport repeaters. These today are used as the 'glue' hub device in structured wiring schemes for 10 BASE T twisted pair and CSMA/CD coaxial standards, and are supported economically with components from semiconductor manufacturers.

The original justification for needing the active system was based on the contention that repeater- based star topologies did not give sufficient levels of hierarchy to construct topologically large net- works. However, many network designers today point out the desirability of using bridging to break up single networks into more manageable smaller networks. Furthermore, the availability of low cost and high performance bridges, as well as the emergence of FDDI as a backbone network, renders the need to construct large Ethernet topologies obsolete. I subscribe to the view that the 10 Base F active star system is effectively obsolete today.

The lOBaseF Passive Star network

remains a specialized solution for those applications which demand the benefits of a passive hub and are able to accept the limited port count, increased complexity and lower robustness of this approach.

Token bus The 802.4 Token bus standard was developed principally for factory automation and manufacturing applications, and is of little interest outside this environment. It does have a standardized fibre station interface defined; however, because of its minority interest it is not further described here.

Token ring The 802.5 token ring is the standards embodiment of the token ring LAN endorsed by IBM. 802.5 is currently developing a fibre 'station attachment' specification. This has been some time in the definition stage (at least three years), and is still only at draft status. One of the weaknesses of token ring is that the network itself is generally undefined, or only defined in terms of signalling interface specification. This does not help ensure that vendors will implement equivalent or compatible network devices, although they should be compatible with the station attachments. Hence a user's investment in station equipment would be protected bythis approach, but supply of network components probably would be single vendor sourced.

Fibre optics are well suited to ring topology in that each station is in series with the signal, and hence can act as a retiming repeater. Thus each fibre connection in a ring system will consist of a point-to-point link, thus issues of passive optical splitting do not arise. The preferred media for 16 Mbit/s is fibre.

In token rings, stations are attached to the main ring by a 'looped' drop cable from a wiring concentrator 'star' connection called a Trunk Coupling Unit (TCU), or Multistation Access Unit (MAU) in IBM parlance. The TCU contains a mechanism which shorts the ring, by- passing the station lobe if a station fault occurs (power off, cable removal or station detected fault).

The main area to resolve with a fibre network is how this function is to be implemented optically.

Apart from what is happening at standards committees, a number of vendors provide fibre optic ring repeater units for spanning between TCUs separated by distances in excess of 250 metres. The specification of these devices are not governed by standards, so inter- working between vendors maybe an issue.

ANSI FDDI LAN

FDDI was designed as a fibre LAN from the outset. It is based on a dual 'counter rotating' token ring topology. The protocol is an append token technique which allows several frames to be in transit at the same time, as opposed to 802.5 which only allows one. FDDI maps onto a structured wiring system using a similar topology to token ring, where lobes (or loops) extend from the central ring to the office workstation to give a star shaped ring. Generally, centralized backbone equipment such as concentrators, bridges and routers, etc., are dual attached (connected to both rings), but workstation outlets are single attached via a concentrator for resilience and cost reasons.

Today, most FDDI networks are almost exclusively used as high speed backbones for lower speed Ethernets or token rings, being connected to them by bridges or routers. Workstation connections are provided for applications requiring large quantities of data transfer such as CAD and graphical systems, however, as discussed earlier, FDDI will become prevalent for networked workstations as prices fall.

Commercial building wiring standards

In 1985 a number of US-based companies started a standards effort to produce a building tele- communications wiring specification, conducted by the Electrical

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Industries Association. It has now been issued as the Commercial Building Telecommunications Wiring Standard (SP-1907B) by the Telecommunications Industry Assoc- iation. This embraces several elements from major proprietary cabling systems, defining a hierarchical structured wiring approach based on a star topology.

In Europe the ISO/IEC JTC1 committee SC25 WG3 is working on a generic standard for Customer Premises Cabling (CPC) which is using the EIA/TIA document as source material, although the position of fibre and unshielded twisted pair (UTP) within CPC is more prominent. SC25 WG3 is also engaged on providing a badly needed Planning and Installation Guide for a Token Ring Network, which principally addresses the mapping of token ring onto a structured cabling system. Additionally, it specifies recom- mended distances and character- ization of different media for 4 and 16 Mbit/s. Generally, it recommends the use of fibre as the media to use in the trunk (backbone) of a 16 Mbit/s system, but STP to the work area data outlet.

CABLING SYSTEMS EVOLUTION

Let us look at the way in which cabling systems have evolved. Traditionally, the choice of mainframe of minicomputer dictated what cabling was needed. Typically, this would be a star wired system with dumb terminals connected directly into a single host. Cable was generally not considered from the user's viewpoint, and was selected on its technical merits for that application, the result being a proliferation of differing cable types with each new release of computer hardware often requiring a new cable type. For example, a site with an IBM 370 mainframe, System 36, 3270 terminals and WANG word processor would have to support two types of twin-axial cable and two coaxial cables. Moreover, terminals were wired point-to-point, making moves expensive and disruptive.

In the mid/late 1970s in order to

solve these problems, many people proposed a LAN approach based on Ethernet or broadband networks. Multidrop cabling meant ease of user connection, facilitating moves. However, early implementations of LANs still required special cable. For Ethernet, thick coaxial cable was used for the backbone cable from which the user connected via a cable- mounted transceiver and a multiway drop cable to theirworkstation. Users later got even more freedom to configure their own connections with the introduction of the low cost Thin Ethernet (RG 58), which could be used to string between computers configured in work groups.

Traditional point-to-point star cabling systems, as described above, have a number of major shortcomings. First, user moves and changes are time-consuming and costly to implement. With users becoming more mobile and with changing terminal needs, getting the right cable to the right desk at the right time is an impossible task. Second, if IT departments have to support a number of cable types, this adds to the cost of support. Third, with traditional cabling systems, keeping up with the growth in user connections is difficult and costly. Very often new cables have to be installed outside normal working hours, which adds to the cost. Finally, the control and documentation of which cable goes where is difficult with old cabling systems. Few people remove unused cables and trunking quickly becomes full.

Multidrop LANS had their own problems. Ethernets were limited to data communications. Ethernet coaxial systems tended to distribute network equipment haphazardly wherever the linear run of the network demanded, and multiple access points needed to be provided to the cable to allow attachment. The transceiver drop cable attachment of Ethernet, although more expensive than Thinnet, had the benefit of isolating the user from disrupting the network. The very flexibility of Thinnet Ethernet cable is also a disadvantage, though, as it allowed users the freedom to attach unauthorized equipment, and also cause disruption to the network by

damaging their local connection. These factors make network management of multidrop networks difficult and highly dependant on a responsible user population. There are horror stories of maintenance staff taking 48 hours to locate a faulty repeater which had been tucked neatly and obscurely between office partitions by the local occupants. Ethernet as a multidrop LAN was born in the era of flower power, love and equality, nurtured by computer literate hippie gurus who empathized with the freedom it offered, hence it was very much a product of its age. However by the late 1980s it was being used by the masses of office workers who little knew or cared that they were connected over a network and needed the discipline and protection that a more rigourous network management could give.

S T R U C T U R E D CABLING - TODAYS SOLUTION

Today many companies are adopting a structured cabling strategy to overcome the shortcomings of traditional cabling systems. These are not only targeted at data communications but voice tele- phones as well. Structured wiring employs a strategy of flood wiring the office environment with data/voice outlets. This is initially a heavy extra cost to bear. A typical horizontal run to an outlet of top quality UTP can cost around $60 or $80 for IBM STP, not counting backbone cabling. However, the costs of a move can be anywhere between $60 to $300 so it can easily repay itself within five years, not counting the cost of disruption caused.

There are five major elements of a structured wiring system; the BALUN/terminal adapter, the floor/ wall socket, the floor (horizontal) cabling, the patch panel, and the backbone cabling. In a typical structured cabling installation, a telecoms outlet with a voice and data socket would be installed for every three or four square metres of usable office space. Each outlet would be connected back to the patch panel by four twisted pairs of copper cable. These copper twisted pairs would be

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punched down on one side of a a patch panel, which would normally be within 90 metres of the outlets that it serves.

On the other side of the patch panel the IT services or the connection to the backbone cabling would be punched down. A patch cord on the patch cross connects any floor socket to any backbone cable or any IT service as required. Backbone cabling will generally consist of 25 pair UTP multiway cables, but fibre is also a prime candidate.

At the terminal end, the appropriate BALUN or transceiver and cable would be used to connect the terminal to the floor socket. A range of BALUNs are available for such as 3270, IBM System 3X, AS400, WANG. They match the impedance between a BALanced (the twisted pair link) and an UNbalanced link (the coaxial link). Moves and changes are handled very quickly. Typically, a patch cord is changed at the patch panel and the user moves the terminal and the appropriate BALU N.

In many cases a single cable type, twisted pair, can support all terminal and LAN needs. This means only one cable technology has to be supported for the floor cabling. New users can be added quickly. Ideally, the IT services will be provided by modular devices such as the ISOLAN Modular Repeater.

Cabling choices

There is no universal solution for either cabling or networking. Each cable type has its strengths and weaknesses. This section discusses cable types.

There are two main types of twisted pair cabling, 150 ohm Shielded Twisted Pair (STP) and 100ohm Unshielded Twisted Pair (UTP). The shield in STP cable gives some protection from interference from other equipment, and protects other equipment from cable emissions. Additionally, it supports high data rates, such as 16 Mbit/s token ring. Its weaknesses are that it is more expensive than UTP, and the cables are bulkier. IBM is the main proponent of STP.

There are other variants of STP which are often encountered in Europe, and are essentially similar to four pair 100 ohm UTP with an overall screen. This has found application in Europe in place of 100ohm UTP because of a preference for screened cable and a suspicion of UTP. Essentially, this cable can be used in applications that are designed for UTP if due regard is paid to the potential increased attenuation of this cable.

UTP is less expensive and takes up less room than STP. Its weaknesses are that it is potentially more vulnerable to RFI and EMI than STP, and that it will require active components to support 16 Mbit/s. AT&T is the leading proponent of UTP.

Coaxial cable is a cost effective solution for certain specific applications. For example, where all PCs and terminals are connected via an Ethernet I_AN to the mainframes and minicomputers. This is very common where DEC's computers are used.

The pros and cons of fibre optic cable were discussed earlier. There are many types of cable construction used for fibre. The principal role of the cable is to protect the fibres from strain and environmental conditions. There are two main ways of achieving this, loose tube and tight buffering:

(1) In the loose tube arrangement the coated fibres are contained within a tube which is slightly overfilled allowing the fibre to take up a position of minimum stress and maintaining minimum contact with the walls of the tube. The advantage is that several fibres can be housed within the same tube, making it easy and economic to customize the number of fibres or types to the application. Several tubes can be laid around a central strain member to form a multicore cable: this arrangement is well suited to outdoor or backbone cable, but is more difficult to terminate and usually would be split out in a splice box to individual tails.

(2) With the tightly buffered arrangement an individual fibre is

usually plastic coated and tightly bound with a surrounding fibrous yarn strain member material, and covered with a plastic sheath. This form is often used with connecters as a patch cord. In multicore applications several elements are laid to form the cable, the advantage being that individual tails can easily be split and connected. The disadvantage is that it is more expensive than loose tube.

An interesting new development is a novel form of loose tube called blown fibre. Blown fibre was invented by British Telecom, and essentially is a type of installation technique where initially cables with empty tubes are installed and fibres are then installed into the tubes by blowing them in using an air compressor. This technique uses a special low friction tube and fibre bundles coated with a high viscous drag material. Typically, this could be used to install fibres in the horizontal runs between the wiring closet patch panel and the work area outlet. Continuous lengths of up to 250 metres can be installed in this way. Advantages with this form of installation are that tubing can be pre- installed and the fibre not installed until the requirement is known. Additionally, fibres can be removed or replaced for repair or upgrading. The economics of this technique needs to be carefully considered for each case, since inevitably the direct cost of installation must be greater than installing standard cable once. A promising approach would be to install compositie cable consisting of unscreened twisted pair UTP with a blown fibre tube to each workstation. This would allow easy upgrading to fibre at a later stage.

When to consider fibre

This will depend on many factors such as current and projected future needs as well as opportunity. Naturally, if contemplating a new installation in a greenfield site, or a major refrubishment, you would be more inclined to include fibre to the desk as well as in the backbone. The following points may be useful:

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• Exclusively install fibre in the backbone between buildings. The cost of additional fibres in a multi- fibre cable is only a small percentage of the installed cost, and at least four single mode fibres should be included in inter- building cables.

• Fibre optic cabling in the backbone is an essential prerequisite for implementing a very high speed corporate backbone network such as FDDI. FDDI networks will be very common in 1994. Its therefore recommended that you do this as a minimum requirement. Install excess capacity and include at least four single mode fibres between equipment rooms.

• Installing fibre to the desk can be done selectively where there is a current or potential future need for large concentrations of high volume data computing such as computer rooms, CAD work- station, DTP or graphics-intensive activities. If you want the cabling system to last for more than five years without major refurbishment and you have a policy of using up- to-date IT equipment, then it is recommended that either you install fibre to the desk or make provision so that you can easily put fibre to the desk. By 1994, only a small percentage of users will require the very high bandwidth of FDDI; however, experience has shown that users frequently relocate within the office. A piecemeal implementation of fibre optic cable will work out to be far more expensive in the longer term than a proper structured cabling implementa- tion now. A possible way out may be to provide a limited number of fibre outputs, but use office mounted fanout equipment(such as multiport repeaters or concentrators) to serve several terminals from a single outlet. This is rather against the principles of the structured wiring approach, but can be very cost effective at the expense of more casual work area wiring and management difficulties.

Selecting the right fibre

The FDDI standard specifies a multimode fibre with a bandwidth equal to, or in excess of, 500 MHz.km (megahertz times kilometres) at 1300 nm wavelength. To meet this requirement both 50/ 125 and 632.5/I 25 graded index fibre can be used. Before deciding which fibre to use it is worth considering the following:

• The 50/125 fibre was originally designed for telecommunications applications and so has been optimized to yield high bandwidth and low attenuation. However, its small NA, has the effect of restricting the amount of light which can be launched into it from an LED. This in turn reduces the power budget for the cable plant by approximately 2.5dB depending on the source characteristics.

• Conversely, 62.5/125 fibre has a larger NA, and therefore provides a more efficient coupling to the optical emitter. The down side of the 62.5/125 fibre is that its bandwidth is limited to a maximum of 500MHz.km at 1300 nm. It used to be less easily available and more expensive than 50/125, but that situation is rapidly changing.

There is no point considering wiring single mode fibre to the desk, since multimode fibre is more than adequate to meet the bandwidth demands of short runs for all foreseeable requirements well into the next century. Single mode should primarily be regarded as long haul 'trunk' media.

How many fibres? For backbone networks between buildings and in risers between floors, least eight fibres should be installed in each cable for resilience and future expansion. If fibre is installed to the desk now, least four fibres should be installed to allow for connecting an FDDI dual attached workstation at the desk, or a single attached station plus two spare fibres to be used if one pair breaks or an additional workstation is required.

What about the cost? The first

point to consider is the relative cost of fibre optic cable compared to the other costs. As a guide, on a large installation, 55% of the cost is installation and management, 30% in network equipment, and only 15% on cable. The proportional cost of redundant fibres is therefore small in comparison to the overall cost of the system installation, and pales into insignificance when compared to the true cost of additional cable installation at a later date. The cable strategy should reflect the long-term needs, even if this means higher short-term costs.

Cable management It is critically important that all cable runs are identified at each termination point with a systematic reference which is documented by the installer and subsequently maintained by the occupier. There are several computerized cable management systems available, ranging from high-end offerings which can be used to design the cabling plant (as well as other building facilities) such as ISICAD, to simpler database record systems. Depending on the size and future growth requirements you should consider using a computer-based system which will be easy to keep up to date. It is not uncommon for businesses to allow their wiring to get out of control so that they can no longer maintain it through lack of documentation.

CONCLUSION

Structured cabling is not solely an economic or performance issue, but is one step in a 'Structured networking' philosophy which encompasses management and control built into a tAN. At first glance these may seem unrelated topics, however selecting the right cabling system is an important step to implementing an easily controlled and managed tAN. Fibre within the cabling system can be selectively applied in a cost effective way, and should be carefully considered for the future growth of one of your most valuable assets.

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