Cable is the medium through which information usually moves from one network device to another. There are several types of cable, which are commonly used with LANs. In some cases, a network will utilize only one type of cable; other networks will use a variety of cable types. The type of cable chosen for a network is related to the network's topology, protocol, and size. Understanding the characteristics of different types of cable and how they relate to other aspects of a network is necessary for the development of a successful network.
Unshielded Twisted Pair
UTP may vary from telephone-grade wire to extremely high-speed cable. This cable has four pairs inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices.
UTP can support telephone, 4 & 16 Mb/s Token Ring, Ethernet, 100 Mb/s Ethernet, Copper FDDI (CDDI), 155 Mb/s ATM. UTP cable is rated by the EIA/TIA standards into categories. Among these, the best value on pricing is Category 3 and Category 5. However, Category 3 is rated to 10 MHz, suitable for Ethernet (10 Mb/s), and Category 5 is rated to 100 MHz, suitable for Fast Ethernet (100 Mb/s) and ATM (155 Mb/s).
There is also Category 5e (Enhanced Category 5). It is the recently ratified standard designed to make the world safe for full-duplex Fast Ethernet. The main difference between Category 5 and Category 5e can be found in the specifications and the performance has been raised slightly. UTP cable is generally wired in the star topology due to the troubleshooting advantages associated with stars.
Shielded Twisted Pair
A disadvantage of the UTP is that it is susceptible to radio and electrical frequency interference. Shielded Twisted Pair is suitable for environments with electrical interference. It has a foil shielding that can block out the electrical interference, but this makes the cable bulky and often difficult to work with and it uses a Universal Data Connector. However, a new version of STP cable introduced and promoted by companies like ITT Datacomm uses RJ-45 connector. It is not bulky, and it is easy to work with. It has a much better signal carrying capability than the UTP.
Fiber Optic cabling consists of a center glass core surrounded by several layers of protective materials. Fiber optic cable offers up the possibility of very high bandwidth and perfect immunity to noise. It transmits light rather than electronic signals, eliminating the problem of electrical interference. This makes it ideal for environments with a large amount of electrical interference and it has also been made a standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.
Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pairs. It also has the capability to carry information at vastly greater speeds. This capacity broadens communication possibilities to include services such as video conferencing and interactive services. However, it costs significantly more to purchase fiber optic cable, connectors, patch panels, jumper cables, tools, and network interface cards. It is also difficult to install and modify.
There are two types of fiber optic cabling- multimode (MMF) and single-mode (SMF). Light propagates through the core (central portion) of optical fiber. Multimode fiber, with a typical core diameter of 62.5 microns or 50 microns, is designed for coupling light from low-cost LED-based transmitters. Singlemode fiber has a core diameter of 10 microns and is only suitable for laser-based transmission. Much of the installed base of optical fiber supporting LAN backbones is multi-mode because most of the current-generation 10 or 100 Mb/s LAN equipment is LED-based.
Gigabit Ethernet operating at 1.25 Gbps is too fast for LEDs and requires the use of lasers. Traditionally, laser-based data transmission has been used with singlemode fiber. The 1000Base-X standard has introduced laser-based transmission over multimode fiber and this new type of transmission has introduced new types of physical layer issues.
Evolution of UTP Categories
With the publication of TIA/EIA-568 standard in 1991, the term “Category” made its way into the jargon of cable installers and LAN managers to describe the performance characteristics of UTP cabling systems. Initially, category 3 cable was the biggest seller for use in structured cabling systems capable of running voice traffic and 10Base-T LAN traffic. Category 4 was introduced soon after to provide a higher grade of cable capable of running 16 Mb/s Token Ring networks. With the advent of 1ooBase-TX, Category 4 soon gave way to Category 5 cabling, which now constitutes the vast majority of installed data cabling runs.
Recently, it has become clear that Gigabit Ethernet (1000Base-T) will also force some changes in UTP cabling standards and installation practices. Specifically, 1000Base-T will require a more thorough specification of cabling performance than has been used in the past for Category 5 UTP. In addition, due to noise margin concerns, a new cabling category (5E) has been defined to better support new 1000Base-T installations.
The stimulus for new cabling requirements.
The prospect of Gigabit Ethernet has generated much excitement and discussion in the networking industry. The draft IEEE 802.3 standards that define Gigabit Ethernet have been under development for two years. The 802.3z specification for Gigabit Ethernet over fiber optic and twinax cabling (1000Base-SX,LX and CX) was ratified in June 1998.
The IEEE 802.3z (1000Base-SX & 1000Base-LX) standard defines the requirements for Gigabit Ethernet operation over multimode and singlemode fiber optic cabling. This standard was ratified in June of 1998. Initially, most end-users will deploy Gigabit Ethernet in their network backbones, where fiber typically is the medium of choice. The IEEE 802.3ab (1000base-T) standard will be ratified in 1999, paving the way for the eventual deployment of Gigabit Ethernet to the desktop over the installed base of Category 5 or Enhanced Category 5 twisted pair cabling.
For Video Conferencing and Tele-medicine the Gigabit Ethernet is the appropriate technology and for other critical applications. The hardware required to be installed for gigabit Ethernet are gigabit hub/switch, UTP cat E5, and above.
The IEE 802.3 working group was formed in July 1996 and standards work has been in full progress ever since. The initial goal was to ratify and publish the IEEE 802.3z standard by January 1998. The actual date was pushed out to June 1998. The reason for the delay has been the complexity of running Gigabit speeds over multimode fiber. The original goal was to support multimode fiber drive distances up to 500 meters to support campus backbone architectures. While that distance is still achievable for some types of multimode fiber, maximum distance limits have been revised downwards for other types of multimode fiber.
A new cabling Category 5E (E=Enhance) is being specified explicitly to handle the challenges of gigabit traffic. The specifications for Category 5E cabling and testing procedures are covered under TIA documents SP4194 and SP4195.Requirements for Return Loss and ELFEXT will be added in SP4195 which is expected to be published as addendum 4 to the TIA/EIA-568-A. SP4194 is expected to be published as a Technical Systems Bulletin (TSB-95) which will modestly tighten the limits for installed category 5 cabling parameters like NEXT,FEXT, and Return Loss in order to provide improved noise margins for 1000Base-T equipment. TIA is presently working on completing SP4194 and SP4195, in an effort to define the new cabling parameters (ELFEXT, return loss, and skew)before the 1000Base-T standard is published.
Category 6 & 7
Recently, there has been much speculation about possible future Category 6 & 7 cabling standards. So how real are Category 6 & 7 standards at this point? No LAN applications have yet emerged which require cabling performance beyond Category 5E. Initial conceptual discussions of Category 6 & 7 have occurred at TIA, but specific characteristics have only been defined in very early draft form. At this time, the frequency limit and the cabling specifications are still under discussion. Final ratification of any potential Category 6 & 7 standards is probably at least 1-2 years away. Since no definitions for Category 6 & 7 have been published, it is not possible to currently field test for a “Category 6 & 7” cabling system.
Comparison of Cable Media.
When comparing cabling types, remember that the characteristics you observe are highly dependent on the implementations, such as the network cards, hubs, and other devices used. Engineers once thought that UTP cable would never reliably support data rates above 4Mbps, but 100Mbps data rates are now common. Some comparisons between cable types are fairly involved. For example, although the fiberoptic cable is costly on a per-foot basis, it may be the most cost-effective alternative when you need to run a cable for many kilometers. To build a copper cable many kilometers in length, you need to install repeaters at several points along the cable to amplify the signal. These repeaters could easily exceed the cost of a fiber-optic cable run.
EIA/TIA category specifications provide for the following cable transmission speeds with specifications: -
Category 1 = No performance criteria
Category 2 = Rated to 1 Mhz ( used for telephone wiring)
Category 3 = Rated to 16 Mhz ( used for Ethernet 10 Base-T)
Category 4 = Rated to 20 Mhz ( used for Token ring, 10 Base-T)
Category 5 = Rated to 100 Mhz ( used for 100 Base-T, 10 Base-T)