Token Ring

Developed by IBM is standardized to IEEE 802.5. It uses a star topology, but it is wired so the signal will travel from hub to hub in a logical ring. These networks use a data token passed from computer to computer around the ring to allow each computer to have network access. The token comes from the nearest active upstream neighbor (NAUN). When a computer receives a token, if it has no attached data and the computer has data for transmission, it attaches its data to the token then sends it to its nearest active downstream neighbor (NADN). Each computer downstream will pass the data on since the token is being used until the data reaches its recipient.The recipient will set two bits to indicate it received the data and transmit the token and data. When the computer that sent the data receives the package, it can verify that the data was received correctly. It will remove the data from the token and pass the token to its NADN.

Characteristics

Maximum cable length is 45 meters when UTP cable is used and 101 meters when STP is used. Topology is starwired ring. It uses type 1 STP and type 3 UTP. Connectors are RJ-45 or IBM type A. Minimum length between nodes is 2.5 meters. Maximum number of hubs or segments is 33. Maximum nodes per network is 72 nodes with UTP and 260 nodes with STP. Speed is 4 or 16 Mps. Data frames may be 4,000 to 17,800 bytes long.

Hubs

A token ring network uses a multistation access unit (MAU) as a hub. It may also be known as a Smart
Multistation Access Unit (SMAU). A MAU normally has ten ports. Two ports are Ring In (RI) and Ring Out (RO) which allow multiple MAUs to be linked to each other. The other 8 ports are used to connect to computers.

Cables

UTP or STP cabling is used as a media for token ring networks. Token Ring uses an IBM cabling system based on American Wire Gauge (AWG) standards that specify wire diameters. The larger the AWG number, the small  diameter the cable has.

Token ring networks normally use type 1, type 3 or regular UTP like cable used on ethernet installations. If
electrical interference is a problem, the type 1 cable is a better choice. Cable types:

Type Description

1. Two 22 AWG solid core pair of STP cable with a braided shield. This cable is normally used between MAUs and computers.
2. Two 22 AWG solid core pair with four 26 AWG solid core of STP cable.
3. Four 22 or 24 AWG UTP cable. This is voice-grade cable and cannot transmit at a rate above 4Mbps.
4. Undefined.
5. Fiber-optic cable. Usually used to link MAUs.
6. Two 26 AWG stranded core pair of STP cable with a braided shield. The stranded-core allows more flexibility but limits the transmission distance to two-thirds that of type 1.
7. Undefined.
8. Type 6 cable with a flat casing to be used under carpets.
9. Type 6 cable with plenum-rating for safety.

Beaconing

The first computer turned on on a token ring will be the active monitor. Every seven seconds it sends a frame to its nearest active downstream neighbor. The data gives the address of the active monitor and advertised and sends it to its nearest active downstream neighbor. When the packet has traveled around the ring, all stations know the address of their upstream neighbor and the active monitor knows the state of the network. If a computer has not heard from its upstream neighbor after seven seconds, it will send a packet that announces its own address, and the NAUN that is not responding. This packet will cause all computers to check their configuration. The ring can thereby route around the problem area giving some fault tolerance to the network.

Network Cabling

This section may be skipped by those more interested on the software aspects of networking or those
learning networking, but all readers should at some time be aware of the terminology used in this section
since they are used with regard to cabling. If this section is skipped by those learning networking, it
should be read later. This section should be read by those who plan to physically install their own
network.

Types of Transmission

1. Baseband - Data bits are defined by discrete signal changes.
2. Broadband - Uses analog signals to divide the cable into several channels with each channel at its
    own frequency. Each channel can only transmit one direction.

Physical media

1. Twisted pair - Wire is twisted to minimize crosstalk interference. It may be shielded or
    unshielded.

• UTP-Unshielded Twisted Pair. Normally UTP contains 8 wires or 4 pair. 100 metermaximum length. 4-100 Mbps speed.
• STP-Shielded twisted pair. 100 meter maximum length. 16-155 Mbps speed. Lowerelectrical interference than UTP.

2. Coaxial - Two conductors separated by insulation such as TV 75 ohm cable. Maximum length of 185 to       500 meters.

1. Thinnet - Thinnet uses a British Naval Connector (BNC) on each end. Thinnet is part of the RG-58 family of cable*. Maximum cable length is 185 meters. Transmission speed is 10Mbps. Thinnet cable should       have 50 ohms impedance and its terminator has 50 ohms impedance. A T or barrel connector has no impedance.

2. Thicknet - Half inch rigid cable. Maximum cable length is 500 meters. Transmission speed is 10Mbps. Expensive and is not commonly used. (RG-11 or RG-8). A vampire tap or piercing tap is used with a transceiver attached to connect computers to the cable. 100 connections may be made. The computer has an attachment unit interface (AUI) on its network card which is a 15 pin DB-15 connector. The computer is connected to the transceiver at the cable from its AUI on its network card using a drop cable.

Coax cable types:

• RG-58 /U - 50 ohm, with a solid copper wire core.
• RG-58 A/U* - 50 ohm, with a stranded wire core.
• RG-58 C/U* - Military version of RG-58 A/U.
• RG-59 - 75 ohm, for broadband transmission such as cable TV.
• RG-62 - 93 ohm, primarily used for ArcNet.
• RG-6 - Used for satellite cable (if you want to run a cable to a satellite!).
• Only these are part of the IEEE specification for ethernet networks.

3. Fiber-optic - Data is transmitted using light rather than electrons. Usually there are two fibers, one for each direction. Cable length of 2 Kilometers. Speed from 100Mbps to 2Gbps. This is the most expensive and most difficult to install, but is not subject to interference. Two types of cables are:

1. Single mode cables for use with lasers.
2. Multimode cables for use with Light Emitting Diode (LED) drivers.

Cable Standards

The Electronic Industries Association and Telecommunications Industries Association (EIA/TIA) defined a standard called EIA/TIA 568 which is a commercial building wiring standard for UTP cable. It defines transmission speed and twists per foot.

Category Speed                                  Notes

1      None                               Used for old telephone systems

2      4Mps

3      10Mps                            The minimum category for data networks

4      16Mps

5      100Mps                          Cat 5 network cable, used by most networks today

6                                             Data patch,Two pair with foil and braided shield

7                                             Undefined

8                                             Flat cable for under carpets with two twisted pair

9                                             Plenum cable with two twisted pair. It is safe if you're having a fire.

The maximum transmission length is 100 meters. This cable is susceptible to interference.

STP

Shielded twisted pair has a maximum cable length of 100 meters (328 feet). Data rate from 16 to 155 Mbps. Cables require special connectors for grounding but this cabling method resists electrical interference and is less susceptible to eavesdropping. Costs more than UTP or Thinnet, but not as much as Thicknet or Fiber-optic.

Terms

• Attenuation - Signal loss due to impedance.
• Bandwidth - Indicates the amount of data that can be sent in a time period. Measured in Mbpswhich is one million bits per second.
• Impedance - The amount of resistance to the transmission device.
• Interference - Electromagnetic Interference (EMI). Crosstalk - When wires pick upelectromagnetic signals from nearby wires also carrying signals.
• Plenum - Space above a false ceiling in an office area where heat ducts and cables may be run.Plenum cabling is special fire resistant cabling required for use in these areas due to fire hazards.
• Shielding - Used to minimize interference.