Feature

Cabling for the future

The structured cabling topology that a business chooses is a decision that will affect network performance for years to come

The convergence of data onto a single network - or more explicitly, a single, company-wide backbone that conveys data, voice and video at gigabit speed - is a hot topic. The technology is here, but what about the actual infrastructure to move all this data around? Some network managers might question whether the CAT-5 links in their cable plant will support 1000BaseT. Even if the cabling will run at high data rates, recurring network problems may not be the fault of hubs, routers or software.

In 1976, 10Mbps Ethernet was unveiled, offering (at the time) huge amounts of bandwidth. More than two decades later, 90 per cent of desktop PC connections still communicate at this speed. The increase of PCs on every desktop and the rise of data intensive application has seen exponential increases in connection speed throughout the system. In 1995, the International Standardisation Organisation (ISO) ratified CAT-5 interconnection hardware specifications, which extended bandwidth to 100MHz and supports 100Mbps 100BaseTX Fast Ethernet systems. CAT-5 hardware will also run 155Mbps Asynchronous Transfer Mode (ATM-155) traffic.

Today's bandwidth expectations mean that CAT-5 is coming to the end of its upgrade life. The Category 5 Enhanced (5e) standards specify more headroom for 100BaseTX and ATM-155 traffic. Critically, 5e standards make reliable gigabit Ethernet connections possible. Nonetheless, many structured cabling suppliers argue that 5e is only an interim solution before the arrival of CAT-6, which will support at least 200MHz; in the interests of sufficient operating margin, the IEEE is requesting a 250MHz CAT-6 specification. Despite the fact that CAT-6 standards are only at draft stage, manufacturers are offering a host of products and claiming that they comply with the draft proposals.

Delivered over copper cabling, gigabit Ethernet challenges fibre and ATM for high-end enterprise LAN infrastructures. With the rise of CAT-5, 10BaseT dispensed with Ethernet's dependence on coaxial cabling and substituted a low-cost cable that comprises two active 16MHz bandwidth CAT-3 unshielded twisted pairs (UTPs) terminated with RJ-45 modular connectors.

Structured cabling systems provide architects with a uniform method for incorporating all the telecommunications systems that an organisation needs within a building's fabric. Two four-pair cables, or one four-pair cable, plus a fibre cable, connects each workspace to the wiring closet. In 1995, the International Standardisation Organisation (ISO) ratified CAT-5 interconnection hardware specifications, which extended link bandwidth to 100MHz and mainly support 100Mbps 100BaseTX Fast Ethernet systems. Critically, 5e standards make reliable gigabit Ethernet connections possible.

In applications such as video editing and data analyses, gigabit Ethernet offers the opportunity to work more efficiently. Instead of downloading a sequence from a server and running it locally in real-time, multiple editors can view 27Mbps video streams concurrently at their workstations. Other early adopters in the United Kingdom include the Meteorological office, which moves gigabytes of data to model weather patterns. For most users, the best reason for installing the latest CAT-6 copper technology lies with providing the greatest degree of "future-proofing".

CAT-5 cabling is adequate for most installations today, whereas 5e supports any of today's protocols and provides more margin. Compared with CAT-5 products, you can currently expect a 15 per cent premium for 5e and as much as 50 per cent for CAT-6. Important design issues include limiting the clock rate to ease cabling requirements and limiting EMC emissions without resorting to shielded cabling.

In March 1997, the IEEE Standards Board approved the 1000BaseT project under the number P802.ab. The committee's objective is to design a gigabit Ethernet system that runs over CAT-5 level cabling. The shielded-versus-unshielded cable debate has raged since 10BaseT's introduction. The Fast Ethernet-compatible symbol rate of 125MHz with two bits per symbol yields 250Mbps per twisted pair. Cable-test recommendations specify three link models. Cable management is important for CAT-5 systems and becomes critical for high-bandwidth systems - CAT-6 GigaBand system.

Bandwidth intensive data and video applications continue to push the need for high performance structured cabling systems that can support increased transmission. The obvious decision is to purchase the highest performing cabling system available to guard against obsolescence. CAT-6/Class E cabling provides 2.5 times the bandwidth and can deliver as much as 300 per cent better power sum attenuation to crosstalk (PSACR) performance at 100MHz than 5e systems for approximately 25 per cent greater installed cost. Even so, many customers question whether they should purchase CAT-6/class E cabling systems prior to TIA/EIA ratification of the standard. Many fear that they will be left with a telecommunications infrastructure that will not meet the final standard specifications or will not support all intended applications. Fortunately, by doing a little research and asking the right questions, customers can purchase a CAT-6 system that will support 250MHz applications well into the next generation of networking technologies.

Development of the TIA draft CAT-6 standard is nearing completion and is expected to be forwarded for industry ballot review in December 1999. Although it may be months before a consensus is reached and the standard is approved, it is unlikely that there will be significant changes between now and then. Since project initiation in September 1997, the key performance goals for CAT-6 and class E have remained firm. In fact, ISO/IEC had communicated these goals to the industry in 1997 specifically to "allow manufacturers to start development of the required cabling components". Standards groups are actively working to finalise the development of field and laboratory test measurement criteria at this time. Evidence of the progressing standard is the fact that a variety of manufacturers are now actively promoting CAT-6 solutions.

A company's investment in cabling is a long-term return. Although CAT-5 cabling installed for use on 10/100 networks may be fine at gigabit and beyond speeds, even small problems can become major headaches. When the standards are ratified for CAT-6, the marginal 25-30 per cent increase in installation costs will be overshadowed by reliable gigabit speeds.

Will Garside


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This was first published in March 2000

 

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