Wire Strippers

What do you want to strip today? The variety of cable strippers represented in this section is a function of the many different types of cable you can work with, different costs of the cable strippers, and versatility of the tools. Strippers for UTP, ScTP, and STP cables are used to remove the outer jacket and have to accommodate the wide variation in the geometry of UTP cables. Unlike coax, which is usually

consistently smooth and round, twisted-pair cables can have irregular surfaces due to the jacket shrinking down around the pairs. Additionally, the jacket thickness can differ greatly depending on brand and flame rating. The trick is to aid removal of the jacket without nicking or otherwise damaging the insulation on the conductors underneath. The wire stripper in Figure 6.1 uses an adjustable blade so that you can fix the depth, matching it to the brand of cable you are working with. Some types use spring tension to help keep the blade at the proper cutting depth.

In both cases, the goal is to score (lightly cut) the jacket without penetrating it completely. Then, you flex the cable to break the jacket along the scored line. This ensures that the wire insulation is nick-free. In some models, the tool can also be used to score or slit the jacket lengthwise in the event you need to expose a significant length of conductors.

Modular Patch Cables

Modular patch cables (patch cords) are used to provide the connection between field-terminated horizontal cables and network-connectivity devices such as switches and hubs and connections between the wall-plate jack and network devices such as computers. They are the part of the network wiring you can actually see. As the saying goes, a chain is only as strong as its weakest  link. Because of their exposed position in structured cable infrastructures, modular patch cords are almost always the weakest link. Whereas horizontal UTP cables contain solid conductors, patch cords are made with

stranded conductors because they are more flexible. The flexibility allows them to withstand the abuse of frequent flexing and reconnecting. Although you could build your own field terminated patch cords, we strongly recommend against it. The manufacture of patch cords is very exacting, and even under controlled factory conditions it is difficult to achieve and guarantee consistent transmission performance. The first challenge lies within the modular plugs themselves. The parallel alignment of the contact blades forms a capacitive plate, which becomes a source of signal coupling or crosstalk. Further, the untwisting and splitting of the pairs as a result of the termination process increases the cable's susceptibility to crosstalk interference. If that weren't enough, the mechanical crimping process that secures the plug to the cable could potentially disturb the cable's normal geometry by crushing the conductor pairs. This is yet another source of crosstalk interference and a source of attenuation.

Routers

Routers are packet-forwarding devices just like switches and bridges; however, routers allow transmission of data between network segments. Unlike switches, which forward packets based on physical node addresses, routers operate at the network layer of the OSI reference model, forwarding packets based on a network ID.If you recall from our communication digression in the discussion on bridging, we defined a network as a logical grouping of computers and network devices. A collection of interconnected networks is referred to as an internetwork. Routers provide the connectivity within an internetwork. So how do routers work? In the case of the IP protocol, an IP address is 32 bits long. Those 32 bits contain both the network ID and the host ID of a network device. IP distinguishes betweennetwork and host bits by using a subnet mask. The subnet mask is a set of contiguous bits with values of one from left to right, which IP considers to be the address of a network. Bits used to describe a host are masked out by a value of 0, through a binary calculation process called AND ing. Figure shows two examples of network IDs calculated from an ANDing process. We use IP as the basis of our examples because it is the industry standard for enterprise networking; however, TCP/IP is not the only routable protocol suite. Novell's IPX/SPX and Apple Computer's AppleTalk protocols are also routable.



Switches

A switch is the next rung up the evolutionary ladder from bridges. In modern star-topology networking, when you need bridging functionality you often buy a switch. But bridging is not the only benefit of switch implementation. Switches also provide the benefit of micro-LAN segmentation, which means that every node connected to a switched port receives its own dedicated bandwidth. And with switching, you can further segment the network into virtual LANs. Like bridges, switches also operate at the link layers of the OSI reference model and, in the case of Layer-3 switches, extend into the network layer. The same mechanisms are used to build dynamic tables that associate MAC addresses with switched ports. However, whereas bridges implement store-and-forward bridging via software, switches implement either store and-forward or cut-through switching via hardware, with a marked improvement of speed. Micro-LAN segmentation is the key benefit of switches, and most organizations have either completely phased out hubs or are in the process of doing so to accommodate the throughput requirements for multimedia applications. Although switches are becoming more affordable, ranging in price from $10 to slightly over $20 per port, their price may still prevent organizations from migrating to completely switched infrastructures. At a minimum, however, servers and workgroups should be linked through switched ports.

Bridges

When we use the terms bridge and bridging, we are generally describing functionality provided by modern switches. Just like a repeater, a bridge is a network device used to connect two network segments. The main difference between them is that bridges operate at the link layer of the OSI reference model and can therefore provide translation services required to connect dissimilar media access architectures such as Ethernet and Token Ring. Therefore, bridging is an important internetworking technology.

In general, there are four types of bridging:

Transparent bridging Typically found in Ethernet environments, the transparent bridge

analyzes the incoming frames and forwards them to the appropriate segments one hop at a time

Source-route bridging Typically found in Token Ring environments, source-route

bridging provides an alternative to transparent bridging for NetBIOS and SNA protocols. In source route bridging, each ring is assigned a unique number on a source-route bridge port. Token Ring frames contain address information, including a ring and bridge numbers, which each bridge analyzes to forward the frame to the appropriate ring

Hubs

Because repetition of signals is a function of repeating hubs, hub and repeater are used interchangeablywhen referring to twisted-pair networking. The semantic distinction between the two terms is that a repeater joins two backbone coaxial cables, whereas a hub joins two or more twisted-pair cables. In twisted-pair networking, each network device is connected to an individual network cable. In coaxial networking, all network devices are connected to the same coaxial backbone. A hub eliminates the need for BNC connectors and vampire taps. Figure illustrates how network devices connect to a hub versus to coaxial backbones. Hubs work the same way as repeaters in that incoming signals are regenerated before they are retransmitted across its ports. Like repeaters, hubs operate at the OSI physical layer, which means they do not alter or look at the contents of a frame traveling across the wire. When a hub receives an incoming signal, it regenerates it and sends it out over all its ports. Figure shows a hub at work.

Repeaters

Nowadays, the terms repeater and hub are used synonymously, but they are actually not the same. Prior to the days of twisted-pair networking, network backbones carried data across coaxial cable, similar to what is used for cable television. Computers would connect into these either by BNC connectors, in the case of thinnet, or by vampire taps, in the case of thicknet. Everyone would be connected to the same coaxial back-bone. Unfortunately, when it comes to electrical current flowing through a solid medium, you have to contend with the laws of physics. A finite distance exists in which electrical signals can travel across a wire before they become too distorted. Repeaters were used with coaxial cable to overcome this challenge.

Repeaters work at the physical layer of the OSI reference model. Digital signals decay due to attenuation and noise. A repeater's job is to regenerate the digital signal and send it along in its original state so that it can travel farther across a wire. Figure illustrates a repeater in action.