With the publication of Telecommunications Systems Bulletin, or TSB-72,* centralized optical-fiber cabling guidelines, optical fiber has become an economically viable choice for today`s horizontal cabling applications. This TSB lets users centralize electronics--routers, bridges, hubs and switches--in one communications room within a building. This network architecture can significantly reduce installed first costs, while at the same time increasing network flexibility, simplifying network management and reducing network outages.
Until the publication of this TSB, the commercial building telecommunications cabling standard TIA/EIA-568A required that all horizontal and backbone cables be terminated in a telecommunications closet on each floor, using a full-horizontal crossconnect. For optical-fiber cable, this required the installation of four connectors and four adapters for every fiber, adding cost to the installation. With TSB-72, these patch panels are replaced with pull-through cables, a splice or an interconnect in the telecommunications closet.
The changes to the standard reflect the growing trend toward installing optical-fiber cable in horizontal network applications. The TIA/EIA-568A standard was originally drafted with copper wiring in mind. The 100-meter length limit on cable runs between electronics and the workstation is based on the technological limitations of this medium. This length limitation often required that users distribute electronics in multiple communications closets near the workstations. TSB-72 allows cable runs to 300 meters by taking advantage of the bandwidth and low attenuation of 62.5-micron multimode fiber, which can support to 2.5 gigabits per second over this distance. Therefore, you do not need to install network electronics so close together; placing passive telecommunications closets near users is sufficient.
While there are several advantages in deploying centralized cabling designs, the most immediate benefit to users is that centralized cabling typically lowers network costs by effectively reducing the number of ports and chassis throughout the network.
Consider, for example, a building with six telecommunications closets supporting 72 users per closet with 24-port hubs. In a network architecture using a distributed electronics design, on average, 70% of the electronics will be used--at a cost of $214 per user. However, using a centralized architecture to serve the same 432 users increases electronics utilization to an average of 90%--at a cost of $166 per user. The 20% difference in equipment use equates to an average savings of $48 per user, based on hub costs of $150 per port. The actual savings will vary, of course, according to the number of users per closet, the number of closets and the port size of the electronics. But the payback is immediate.
This kind of savings helps make fiber-based networks more cost-competitive with copper-based local area networks. In addition, the reliability and higher bandwidth of optical-fiber cable can help users save money on network costs by reducing maintenance costs.
According to Forrester Research (Cambridge, MA), the average corporation spends approximately $610 per user in one year to maintain a copper-based local area network:
- $280 for physical local area network support
- $110 for bridge/router support
- $220 for third-party maintenance.
And, according to a Communications Week study, 17% of network outages are caused by problems associated with copper cabling, including electromagnetic and radio-frequency interference, impedance mismatches and excessive link distances.
In comparison, a centralized optical-fiber network would have a total annual operating cost of approximately $450 to $490 per user, providing an annual savings of $125 to $165 per user. This estimate assumes that users can gain 25% to 35% efficiency in local area network support by centralizing network administration, and can also eliminate 80% of network problems typically associated with copper.
TSB-72 helps users take advantage of the benefits that optical-fiber cable can bring to horizontal cabling applications. The TIA/EIA has made it easier for users to deploy fiber-optic networks all the way to the desktop.
Tony Beam, director of fiber systems marketing at AMP Inc. (Harrisburg, PA), chairs the Fiber Optic Task Group for TR-41.8 and is a member of the TIA Fiber Optics LAN Section.