WEEK 6

WEEK 6

WEEK 6- NETWORK TOPOLOGY

What is a Topology?

The physical topology of a network refers to the configuration of cables, computers, and other peripherals. Physical topology should not be confused with logical topology which is the method used to pass information between workstations. Logical topology was discussed in the Protocol chapter.

linear Bus

Now that you know how topologies can affect networks, it's time to learn more about the different types of typologies. The term "linear" refers to a line, and the linear bus is a network configuration in which each computer is connected to the next in a (more or less) straight line. A small bus network is shown in the graphic below

One important characteristic of the linear bus is that it has a beginning and an end. When computers are cabled in a line, the signal travels down the cable. When it reaches the end, it can "bounce" back (this is called signal bounce), interfering with network communications. To prevent this, a bus network must be terminated; that is, small devices called terminators are placed at both ends of the bus.

Function of  terminator:

• To prevent signals from being echoed back through the network
•  small devices called terminators are placed at both ends of the bus

 advantages

• Simple set-up
• Least expensive layout
• Uses inexpensive thin coax cable
• Uses smallest amount of cable
• Doesn't require a hub
• Appropriate for small networks
• For instance, a temporary training room that is set up and torn down quickly

disadvantages

• Slow
• Vulnerable to attenuation, or the loss of signal strength over distance
• Inappropriate for large networks
• Less fault tolerant

STAR

A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub, switch, or concentrator.

Data on a star network passes through the hub, switch, or concentrator before continuing to its destination. The hub, switch, or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.

 







advantages

• Easy to install and wire.
• No disruptions to the network when connecting or removing devices.
• Easy to detect faults and to remove parts.

disadvantages

• Requires more cable length than a linear topology.
• If the hub, switch, or concentrator fails, nodes attached are disabled.
• More expensive than linear bus topologies because of the cost of the hubs, etc.

STAR-WIRED

The standard for the Token Ring protocol is Institute of Electrical and Electronics Engineers (IEEE) 802.5. The Fiber Distributed-Data Interface (FDDI) also uses a Token Ring protocol.

A star-wired ring topology may appear (externally) to be the same as a star topology. Internally, the MAU (multistation access unit) of a star-wired ring contains wiring that allows information to pass from one device to another in a circle or ring (See fig. 3). The Token Ring protocol uses a star-wired ring topology.

• Uses physical layout of a star topology with ring topology data transmission method
• Data is sent around the star in a circular fashion (dotted orange in figure)
• This hybrid technology has the fault tolerance of star topology and, ...
• in Token Ring networks, reliability of token passing
• Groups of nodes are connected to hubs wich are then networked on a single bus
• Used to cover longer distances
• Also used to interconnect, or isolate, different network segments
• Expensive, requires more cable and more hubs
• Modern Ethernet, and its faster cousins, are set up using this topology

TOKAN RING 

A Token Ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a bit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time. The Token Ring protocol is the second most widely-used protocol on local area networks afterEthernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5 Token Ring technology provides for data transfer rates of either 4 or 16 megabits

per second. Very briefly, here is how it works:

1. Empty information frames are continuously circulated on the ring.
2. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame.
3. The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message from the frame and changes the token back to 0.
4. When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame.
5. The frame continues to circulate as an "empty" frame, ready to be taken by a workstation when it has a message to send.

The token scheme can also be used with bus topology LANs.

The standard for the Token Ring protocol is Institute of Electrical and Electronics Engineers (IEEE) 802.5. The Fiber Distributed-Data Interface (FDDI) also uses a Token Ring protocol.

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TREE

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable (See fig. 3). Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.

advantages

• Point-to-point wiring for individual segments.
• Supported by several hardware and software venders.

disadvantages

• Overall length of each segment is limited by the type of cabling used.
• If the backbone line breaks, the entire segment goes down.
• More difficult to configure and wire than other topologies.

Physical Topology	Common Cable	Common Protocol
Linear Bus	Twisted Pair Coaxial Fiber	Ethernet
Star	Twisted Pair Fiber	Ethernet
Tree	Twisted Pair Coaxial Fiber	Ethernet