Day 5 - Wednesday, 25 April 2012

Hi everyone. How are you guys? I hope you guys are doing fine.

The topic for today's class is CABLING.

Cable is the medium through which information usually moves from one network device to another. Several types of cable 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. 

CABLING

GUIDED = BOUNDED

UNBOUNDED = UNGUIDED

Types of Cables

1) Twisted-Pair Cable

A twisted pair consists of two insulated copper wires, typically about 1 mm thickness. The wires are twisted together in a helical shape. The purpose of twisting the wires is to reduce electrical interference from similar pairs that are close by.

Twisted pair wires are commonly used in local telephone communication and for digital data transmission over short distances up to 1 km. When many twisted pairs run in parallel for a substantial distance, such as all the wires coming from a multistory apartment building to the telephone exchange, they are bundled together and placed in a protective sheath. The pairs in these bundles would interfere with one another if they are not twisted.

Unshielded Twisted Pair (UTP) Cable

Advantages of Twisted-Pair Wire

• Being the oldest method of data transmission, trained manpower to repair and service this media of communications are easily available. 
• In a telephone system, signals can travel several kilometers without amplification when twisted pair wires are used.
• These media can be used for both analog and digital data transmission. The bandwidth depends on the thickness of the wire and the distance traveled, but several megabits per second can be achieved for a few kilometers in may cases.
• It is the least expensive media for transmission for short distances.
• If a portion of a twisted-pair cable is damaged, the entire network is not shut down as it may be the case with coaxial cable.

Disadvantages of UTP Cable

• Easily pick up noise signals which results in higher error rates when the line length exceeds 100 meters.
• Being this in size, it is likely to break easily.
• It can support 19,200 bps up to 50 feet on RS-232 port.

Shielded Twisted Pair (STP) Cable

Shielded Twisted Pair (STP) has a grounded outer copper shield around the bundle of twisted pairs or around each pair.This provide added protection against Electromagnetic Interference (EMI). 

Unshielded Twisted Pair (UTP) does not have a grounded copper shield. UTP cables are easier to work with and are less expensive than shielded cables.

Shielded wire is used in an electricity noisy environment to limit the effects of noise absorption. Twisted-pair wiring is more commonly used for LAN media.

The unique thing about twisted pair network cabling is the fact that the wires are twisted. This is done because we run an electrical current through a piece of copper wiring, Electromagnetic Interference and cross talk around the wires.  That can be a problem when we have a second wire in close proximity because it can absorb the signal from the first wire. In that case the data on wires can get corrupted. 

2) Coaxial Cable

A coaxial cable is one that consists of two conductors that share a common axis. The inner conductor is typically a straight wire,either solid or stranded and the outer conductor is typically a shield that might be braided or a foil. Coaxial cable is a cable type used to carry radio signals, video signals, measurement signals and data signals. Most common coaxial cable impedances in use in various applications are 50 ohms and 75 ohms.

Advantages of Coaxial Cable

• It has better shield against electromagnetic interference than twisted pair cable, so it can span longer distance a t higher data bits per second (bps).
• It can be used for both analog and digital data transmissions. For analog g data transmission, 75-ohm broadband coaxial is used and for digital transmission, 50 - ohm base band cable is used.
• It is inexpensive as compared to twisted pair wires and UTP cables but easy to handle. 

3) Fiber Optic Cable

Optic Fiber is the newest form of bounded media. This media is superior in data handling and security characteristics. The fiber optic cable transmits light signals rather than electrical signals. It is far more efficient that the other network transmission media. Each fiber has an inner core of glass or plastic that conducts light. There are two types of light sources for which fiber cables are available. These sources of light are:

a. Light Emitting Diodes (LEDs)

b. Light Amplification by Stimulated Emission Radiation (Lasers) 

Below figure shows the principle of operation of the fiber optic system. The system basically consists of fiber optic cables that are made of tiny threads of glass or plastics. In a single- mode fiber, the core is 8 to 10 microns ( about the size of hair). In multimode, the core is about 50 microns on diameter. 

Advantages of Fiber Optic Cable

• It can handle much higher bandwidth than copper. Due to the low attenuation, repeaters are needed only about every 30 km on fiber lines, versus about every 5 km for copper. 
• Fiber is not affected by power surges, electromagnetic interference, or power failure. Nor is it affected by corrosive chemicals in the air, making it ideal for factory environments where electrical interference is very high. 
• Fiber is lighter than copper. One thousand twisted pairs copper cables of 1 km long weigh 800 kg. But fibers have only 100 kg.
• Fibers do not leak light and are quite difficult to tap. This gives them excellent security against potential wire-tappers.

Disadvantages of Fiber Optic Cable

• Fiber is an unfamiliar technology requiring skills which may not be easily available. 
• Since optical transmission is inherently unidirectional, two-way communication requires either two fiber cables or two frequency bands on one fiber.
• Fiber interfaces cost more than electrical interfaces.

4) Wireless LANs

 A wireless LAN (or WLAN, for wireless local area network, sometimes referred to as LAWN, for local area wireless network) is one in which a mobile user can connect to a local area network (LAN) through a wireless (radio) connection. The IEEE 802.11 standards use the Ethernet Protocol and CSMA/CA (carrier sense multiple access with collision avoidance) for path sharing and include an encryption method, the Wired Equivalent Privacy Algorithm. 

That is all for this week. Thank you for reading my entry.

Day 4 - Wednesday, 4 April 2012

Hi everyone. The topic for today's class is TOPOLOGY.

Network Topology is the study of the arrangement or mapping of the elements (links,nodes) of a network. It is divided into two which are the physical (real) andlogical (virtual) interconnections between nodes.

PHYSICAL TOPOLOGY

The physical layout of devices on a network.

The way that the devices on a network are arranged and how they communicate with each other.

The workstations are connected to the network through the actual cables that transmit data.

The physical structure of the network.

LOGICAL TOPOLOGY

The mapping of the flow of data between the nodes in the network determines the logical topology of the network.

The way that the signals act on the network media.

The way that the data passes through the network from one device to the next without regard to the physical in interconnection of the devices.

CLASSIFICATION OF PHYSICAL TOPOLOGY

Linear bus

Star

Star- Wired Ring

Tree

FDDI

Mesh

LINEAR BUS

consists of a main run of cable with a terminator at each end.

all nodes ( file server, workstations and peripherals) are connected to the linear cable.

Ethernet and Local Talk networks use a linear bus topology.

Advantages of a Linear Bus Topology:

Easy to connect a computer or peripheral to a linear bus.

Requires less cable length than a star topology.

Disadvantages:

Entire network shuts down if there is a break in the main cable.

A faulty cable or workstation will take the entire LAN down.

Terminators are required at both ends of the backbone cable.

Difficult to identify the problem if the entire network shuts down.

Not meant to be used as a stand- alone solution in a large building.

Summary:

Advantages of Linear Bus

Easy to install

Costs are usually low

Easy to add systems to network

Great for small networks

Disadvantages of Linear Bus

Out-of-date technology

If cable breaks, whole network is down

Can be difficult to troubleshoot

Unmanageable in a large network

RING

each of the systems is connected to its respective neighbor forming a ring.

the main difference between the bus and the ring is that the ring topology does not require termination. Because the systems are connected all together in a loop.

there is no beginning and end point as there is with the bus topology.

this configuration is seen in Fiber Distributes Data Interface (FDDI) networks.

Advantages of RING:

easy to install

costs are usually low

easy to add systems to network

great for small networks

Disadvantages of RING:

out-of-date technology

if cable breaks, whole network is down

can be difficult to troubleshoot

unmanageable in a large network

STAR

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

Data on a star network passes through the hub manages and controls all function 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.

The protocols used with star configurations are usually Ethernet or Local Talk.

Advantages of Star:

easy to install and wire

easy to add new workstations

no disruptions to the network when connecting or removing devices

any non-centralized failure will have very little effect on the network

easy to detect faults and to remove parts.

centralized control

centralized network/hub monitoring

Disadvantages of Star:

Requires more cable length than a linear topology.

If the hub or connector fails, nodes attached are disabled.

More expensive than linear bus topology because of the cost of the hub.

STAR-WIRED

A star wired topology may appear (externally) to be the same as a star topology.

Internally, the Multistation Access Unit (MAU) of a star wired ring contains wiring that allows information to pass from one device to another in a circle or ring.

The Token Ring protocol uses a star-wired topology.

TREE

A Tree (hybrid) topology combines characteristics of linear bus and star topology.

It consists of groups of star-configured workstations connected to a linear bus backbones cable.

Tree topology allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.

Advantages of Tree:

Point-to-point writing for individual segments.

Disadvantages of Tree:

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 the other topology.

FDDI

100 mbps

Normally implemented over fiber optic (fast-Ethernet, UTP)

Dual redundancy built in by use of primary and secondary ring.

Automatic by passing and isolation of faulty nodes.

LAN PROTOCOLS

ETHERNET

TOKEN RING

FDDI

GIGABIT ETHERNET

PROTOCOL

A protocol is a set of rules that governs the communications between computers on a network.

These rules include guidelines that regulate the following characteristics of a network:

- access method

- allowed physical topology

- cables of cabling

- speed of data transfer

Logical topology are bound to network protocols and describe how data is moved across the network.

The Ethernet protocol is a common logical bus topology protocol.

Local Talk is a common logical bus or star topology protocol.

Token Ring is a common logical ring topology protocol.

CABLING

Cable is the medium through which information usually moves from one network device to another.

Several types of cable 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.

TYPES OF CABLES

Non-shielded Twisted Pair (UTP) Cable

Shielded Twisted Pair (UTP) Cable

Coaxial Cable

Fiber Optic Cable

Wireless LANs

That is all for this week lesson. Thank you.