Repeaters / Hub / Wireless

Repeaters
5.1.6 This page will discuss how a repeater is used on a network.


The term repeater comes from the early days of long distance communication. A repeater was a person on one hill who would repeat the signal that was just received from the person on the previous hill. The process would repeat until the message arrived at its destination. Telegraph, telephone, microwave, and optical communications use repeaters to strengthen signals sent over long distances.

A repeater receives a signal, regenerates it, and passes it on. It can regenerate and retime network signals at the bit level to allow them to travel a longer distance on the media. Ethernet and IEEE 802.3 implement a rule, known as the 5-4-3 rule, for the number of repeaters and segments on shared access Ethernet backbones in a tree topology. The 5-4-3 rule divides the network into two types of physical segments: populated (user) segments, and unpopulated (link) segments. User segments have users' systems connected to them. Link segments are used to connect the network repeaters together. The rule mandates that between any two nodes on the network, there can only be a maximum of five segments, connected through four repeaters, or concentrators, and only three of the five segments may contain user connections.

The Ethernet protocol requires that a signal sent out over the LAN reach every part of the network within a specified length of time. The 5-4-3 rule ensures this. Each repeater that a signal goes through adds a small amount of time to the process, so the rule is designed to minimize transmission times of the signals. Too much latency on the LAN increases the number of late collisions and makes the LAN less efficient.

The next page will discuss hubs.

Hub
5.1.7 This page will describe the three types of hubs.


Hubs are actually multiport repeaters. The difference between hubs and repeaters is usually the number of ports that each device provides. A typical repeater usually has two ports. A hub generally has from 4 to 24 ports. Hubs are most commonly used in Ethernet 10BASE-T or 100BASE-T networks.

The use of a hub changes the network from a linear bus with each device plugged directly into the wire to a star topology. Data that arrives over the cables to a hub port is electrically repeated on all the other ports connected to the network segment.

Hubs come in three basic types:

• Passive – A passive hub serves as a physical connection point only. It does not manipulate or view the traffic that crosses it. It does not boost or clean the signal. A passive hub is used only to share the physical media. A passive hub does not need electrical power.

• Active – An active hub must be plugged into an electrical outlet because it needs power to amplify a signal before it is sent to the other ports.

• Intelligent – Intelligent hubs are sometimes called smart hubs. They function like active hubs with microprocessor chips and diagnostic capabilities. Intelligent hubs are more expensive than active hubs. They are also more useful in troubleshooting situations.

Devices attached to a hub receive all traffic that travels through the hub. If many devices are attached to the hub, collisions are more likely to occur. A collision occurs when two or more workstations send data over the network wire at the same time. All data is corrupted when this occurs. All devices that are connected to the same network segment are members of the same collision domain.

The next page discusses wireless networks.
Wireless
5.1.8 This page will explain how a wireless network can be created with much less cabling than other networks.


Wireless signals are electromagnetic waves that travel through the air. Wireless networks use radio frequency (RF), laser, infrared (IR), satellite, or microwaves to carry signals between computers without a permanent cable connection. The only permanent cabling can be to the access points for the network. Workstations within the range of the wireless network can be moved easily without the need to connect and reconnect network cables.

A common application of wireless data communication is for mobile use. Some examples of mobile use include commuters, airplanes, satellites, remote space probes, space shuttles, and space stations.

At the core of wireless communication are devices called transmitters and receivers. The transmitter converts source data to electromagnetic waves that are sent to the receiver. The receiver then converts these electromagnetic waves back into data for the destination. For two-way communication, each device requires a transmitter and a receiver. Many networking device manufacturers build the transmitter and receiver into a single unit called a transceiver or wireless network card. All devices in a WLAN must have the correct wireless network card installed.

The two most common wireless technologies used for networking are IR and RF. IR technology has its weaknesses. Workstations and digital devices must be in the line of sight of the transmitter to work correctly. An infrared-based network can be used when all the digital devices that require network connectivity are in one room. IR networking technology can be installed quickly. However, the data signals can be weakened or obstructed by people who walk across the room or by moisture in the air. New IR technologies will be able to work out of sight.

RF technology allows devices to be in different rooms or buildings. The limited range of radio signals restricts the use of this kind of network. RF technology can be on single or multiple frequencies. A single radio frequency is subject to outside interference and geographic obstructions. It is also easily monitored by others, which makes the transmissions of data insecure. Spread spectrum uses multiple frequencies to increase the immunity to noise and to make it difficult for outsiders to intercept data transmissions.

Two approaches that are used to implement spread spectrum for WLAN transmissions are Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). The technical details of how these technologies work are beyond the scope of this course.

A large LAN can be broken into smaller segments. The next page will explain how bridges are used to accomplish this.