Cabling System Fundamentals

A structured cabling architecture design is intended to accommodate telecommunications technology changes with minimal impact on any of the other cabling subsystems, such as, electrical cabling. The target life cycle of an average cabling installation is up to 20 years. It is expected that a few generations of telecommunications systems will be installed and replaced or upgraded. Another planning assumption is that networking and bandwidth requirements will certainly increase during the life cycle of the cabling system. The following are key factors used to specify networks and cabling, as identified by Avaya in its SYSTIMAX CSC guidebook:

• Usage patterns, including combined size and duration of peak loads for all applications

• Expected increase in bandwidth demands

• The number of users and anticipated changes in that number

• Location of users and maximum distances between them

• The likely rate of change in users’ locations (churn)

• Connectivity with current and future devices and software

• Space available for cable runs

• Total cost of ownership

• Regulations and safety requirements

• Importance of protection against loss of service and data theft

PBX systems traditionally have been based on a star network topology. A star network topology includes many point-to-point links radiating from central equipment. The early LAN topologies were based on ring-and-bus network designs. A ring network topology has a continuous transmission loop that interconnects every device. The most familiar example of a ring network topology is the IBM token ring LAN. A bus network topology is a communications link that connects devices along the length of a cable. The original Ethernet LAN was based on a bus network topology.

Today’s dominant LAN technology is based on Ethernet standards. The logical topology of an Ethernet LAN is a bus topology, but the physical topology of the network is a star. Ethernet workstations that connect to an Ethernet hub or switch communicate over a high-speed bus housed in a hub or switch, but these network nodes are connected in a clustered star network topology. The star topology favored by PBX systems and adapted by Ethernet LANs is now the accepted communications system network topology.

The first Ethernet LAN installations were based on coaxial cable used for the transmission medium. During the mid-1980s the cabling used by PBX systems, known as unshielded twisted pair (UTP), was adapted for Ethernet LANs. Telephony UTP cabling was classified by IBM’s cabling system specifications as Category 3 and was used for 10Base-T Ethernet LANs operating at 10 Mbps. A 10Base-T Ethernet LAN used two pairs of Category 3 UTP cabling. A 100Base-T4 Ethernet LAN used four-pair Category 3 UTP cabling. A 100-Mbps Fast Ethernet, also known as 100Base-TX, used two-pair Category 5 UTP cabling. The 1000-Mbps (1 Gbps) Ethernet, 1000BASE-T, uses four-pair Category 5 UTP cabling. The 1000Base-TX, a lower cost alternative to 1000Base-T, uses the recently introduced Category 6 UTP cabling. PBX system telephony requirements can be satisfied with any of these UTP cabling types, making possible a single network cabling system infrastructure for voice and data communications applications.

In the SYSTIMAX SCS guidebook, Avaya lists the following considerations for choosing the type of customer network cabling:

• Maximum distance between network hubs and nodes

• Space available in ducting and floor/ceiling cavities

• The levels of electromagnetic interference (EMI)

• Likely changes in equipment served by the system and the way it is used

• Level of resilience required

• The required life span of the network

• Restrictions on cable routing that dictate cable bend radius

• Existing cable installations with potential for reuse

For the past two decades, most customers have used or installed two different cabling systems for telephony and data LAN applications. The evolution of the PBX system to an IP telephony platform will allow the large installed base of customers with installed circuit switched PBX systems to slowly phase out an infrastructure with two cabling systems and allow customers who are designing an entirely new converged voice/data network the opportunity to install a single cabling system. PBX systems installed before 1990 were implemented with Category 3 UTP, but more recent installations may have been based on Category 5 UTP, the same wiring used for data LANs. A newly installed communications system installation likely would be based on a generic cabling infrastructure using Category 5 UTP to provide for future needs.

A generic cabling system is a structured telecommunications cabling system capable of supporting a wide range of customer applications. Generic cabling can be installed before the definition of required applications because application-specific hardware (telephones, computers, etc.) is not part of the structured cabling design. Generic cabling can be enhanced through the use of flood wiring, which is the installation of sufficient cabling and telecommunications outlets in a work area to maximize the flexibility of the location for devices connected to the network. Many customers are currently installing four or six telecommunications outlets per work area, although the recommended minimum is two.