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### After reading this you should be able to

• Describe the basic and hybrid LAN physical topologies, and their uses, advantages, and disadvantages
• Describe the backbone structures that form the foundation for most LANs
• Understand the transmission methods underlying Ethernet networks
• Compare the different types of switching used in data transmission

### Simple Physical Topologies

Physical topology is

• The physical layout or pattern of the nodes on a network
• It depicts a network in broad scope (does not specify device types, connectivity methods, etc.)

Physical topologies are divided into 3 fundamental geometric shapes

1. Bus
2. Ring
3. Star

Bus Topology

• Consists of a single cable, called the bus, that connect al;l nodes on a network without intervening connectivity devices
• Can support only one channel for communication – every node shares the bus’s total capacity
• Most buses use coaxial cable as their physical medium
• Bus networks rely on a passive topology – where each node passively listens for, then accepts data directed to it
• When one node wants to transmit data to another node, it broadcasts an alert to the entire network, other nodes ignore the message other than the node due to receive it
• Since all nodes connected to a bus network can communicated directly via broadcast transmissions, makes them part of a single broadcast domain
• Routers separate broadcast domains
• AT the end of each bus network are 50 ohm resistors known as terminators
• Terminators sop signals after they have reached the end of the wire – without them signals on the bus would travel endlessly between the two ends of the network (known as signal bounce)
• Bus networks do not scale well, as you add more nodes the network performance decreases – only suitable to small networks of maybe 12 machines
• Bus networks are also challenging identifying fault points as every  node in the network could potentially be causing the issue
• Bus networks are not very fault tolerant – a single break or a fault effects the entire network

Ring Topology

• Each node is connected to the two nearest nodes to that the entire network forms a circle
• Data is transmitted clockwise, in one direction, around the ring
• Each node accepts and responds to packets addressed to it, then forwards other packets to the next node
• Because all nodes participate in delivery makes the ring topology an active topology
• Ring topologies have no ends
• Most ring topologies twisted pair or fibre optic cabling is used as the physical medium
• Big drawback is that a single malfunctioning node can cripple the entire network
• Ring topologies are rarely used in contemporary LANS

Star Topology

• Every node on the network is connected through a central device (hub, router or switch)
• Usually built with twisted pair or fibre optic as their network medium
• Any single cable on the network connects only two devices – so a cabling problem will only affect two devices
• Star topologies require the more cabling than ring or bus topologies
• They are more fault tolerant, although reliant on the central hub working correctly
• Because of central connection point, they can be easily isolate or moved
• They are the most popular topology with modern day LAN’s
• Star networks can support a maximum of 1024 addressable nodes on a logical network

### Logical Topologies

• Logical topologies refer to the way in which data is transmitted between nodes, rather than the physical layout of the paths that data takes
• A networks logical topology will not necessarily match its physical topology
• The most common logical topologies are bus and ring

In a bus logical topology

• signals travel from one network device to all other devices on the network
• A network that uses a bus topology, also uses a bus logical topology, networks that use either the star or star-wired bus physical topologies also result in a bus logical topology

In a ring logical topology

• signals follow a circular path between sender and receiver
• Networks that use a pure ring topology use a ring logical topology
• The ring logical topology is also used by the star wired hybrid physical topology because signal follows a circular path
• Ethernet networks use the bus logical topology, token ring networks use the ring logical topology

### Hybrid Physical Topologies

Simple topologies are too restrictive, most likely you will encounter hybrid topologies

• Star-Wired Ring – uses physical layout of star in conjunction with the ring logical topology
• Star-Wired Bus – uses physical layout of a star in conjunction with the bus logical topology – this forms the basis for modern Ethernet networks

### Backbone Networks

A network backbone is the cabling that connect the hubs, switches, and routes on a network.

Serial Backbone

• Simplest kind of backbone
• Consists of two or more internetworking devices connected to each other by a single cable in a daisy chain fashion
• A daisy chain is simply a linked series of devices

Distributed Backbone

• Consists of a number of connectivity devices connected to a series of central connectivity devices such as hubs, switches, or routers in a hierarchy
• This topology allows for simple expansion and limited capital outlay for growth
• Provides network administrators with the ability to segregate workgroups

Collapsed Backbone

• Uses a router or switch as the single central connection point for multiple sub-networks
• A single router or switch is the highest layer of the backbone
• The router or switch usually contains multiprocessors to handle the heavy traffic going through it
• It is risky because failure in the central router can bring down the entire network
• A significant advantage is that arrangement allows you to interconnect different types of sub-networks
• Another advantage is that you can centrally manage maintenance and troubleshooting

Parallel Backbone

• The most robust type of network backbone
• Consist of more than one connection from the central router or switch to each network segment
• Each switch is connected to the router by two cables
• Most significant advantage is that its redundant
• Parallel backbones are more expensive than other backbones due to the additional wiring and hardware
• The offer increased performance and greater fault tolerance

### Switching

Switching is a component of a network’s logical topology that determines how connections are created between nodes.

There are 3 methods for switching

1. Circuit Switching
2. Message Switching
3. Packet Switching

Circuit Switching

• Connection is established between two nodes before they begin transmitting data
• Bandwidth is dedicated to this connection and remains available until the users terminate communication between the two nodes
• While the nodes remain connected, all data follows the same path initially selected by the switch
• Traditional telephone calls use a circuit switched connection
• Can result in a waste of resources
• Some application benefit from this type of connection such as live video or audio conferencing

Message Switching

• Establishes a connection between two devices, transfers the information to the second device, then breaks the connection
• The information is stored and forwarded from the second device after a connection between that device and a third device on the path is established
• The store and forward routine continue until the message reaches its destination
• All information follows the same physical path, however the connection is not continuously maintained
• Message switching requires that each device in the data’s path has sufficient memory and processing power to accept and store the information before passing it to the next node
• None of the network transmission technologies discussed in this chapter use message switching

Packet Switching

• The most popular method for connecting nodes on a network is packet switching
• Packet switching breaks data into packets before they are transported
• Packets can travel any path on the network to their destination
• Packets can attempt to find the fastest circuit available at any instant – they need not follow each other along the same path, nor must they arrive at their destination in sequence
• When packets reach their destination, the node reassembles them in sequence based on their control information
• The greatest advantage of packet switching lies in the fact that it does not waste bandwidth by holding a connection open until a message reaches its destination as circuit switching does
• Ethernet networks and the Internet are the most common examples of packet switching networks

MPLS (Multiprotocol Label Switching)

• MPLS is a new type of switching that was introduced in 1999 by the IETF
• It is commonly used with protocols designed for LAN’s
• It addresses some of the limitations of packet switching (i.e. faster routing, prioritization)
• It is often called a shim because it adds additional information onto the packet being transported

### Ethernet

• Originally developed by Xerox in the 1970’s
• By far the most popular network technology used on modern LAN’s
• There are many different versions of Ethernet, but they all have one common property – their access method, which is known as CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
##### CSMA/CD

A networks access method is its method of controlling how network nodes access the communications channel.

To understand Ethernet, you must first understand CSMA/CD.

Carrier Sense – refers to the fact that Ethernet NIC’s listen on the network and wait until they detect that no other nodes are transmitting data over the signal (or carrier) on the communications channel before they begin to transmit

Multiple Access – refers to the fact that several Ethernet nodes can be connected to a network and can monitor traffic, or access the media, simultaneously

When a node wants to transmit data it must first access the transmission media and determine whether the channel is free. If the channel is not free, it waits and check again after a brief amount of time. If the channel is free, the node transmits its data. Any node can transmit data after it determines that the channel is free. If two nodes simultaneously check the channel and determine it is free, the transmissions interfere with each other (a collision).

Collision Detection – refers to the way nodes respond to a collision. In the event of a collision the network performs a series of steps known as the collision detection routine. If a node’s NIC determines that its data has been involved in a collision, it immediately stops transmitting. A process called jamming happens where the NIC issues a special 32 bit sequence that indicates to the rest of the network nodes that its previous transmission was faulty and that those data frames are invalid. After waiting, the NIC determines if the line is free again and resumes the same process of transmission as before.

On heavy traffic networks, collision detection is common, the more nodes that are transmitting on a segment, the more collisions that will occur. A collision rate of greater than 5% of network traffic is usually an indication that something is wrong with the network.

Collision Domain

• The portion of a network in which collisions occur if two nodes transmit data at the same time. Repeaters regenerate any signal they receiver – so they will just regenerate collisions. Switches and routers however separate collision domains.
• Collision domains differ from broadcast domains in that they define a logically shared space for layer 2 communications
• Collision domains are also affected by the length of the network cable – if the cable is to long, CSMA/Cd becomes ineffective because of the time it takes for a collision to traverse the network
##### Ethernet Standards for Copper Cable
• 10Base-T – old standard, not so common now days. Max throughput is 10 Mbps. Follows the 5-4-3 rule
• 100Base-TX – also known as Fast Ethernet, common on networks today. Does not follow the 5-4-3 rule. Uses only two wires to transmit and receive data
• 1000Base-T – becoming more popular, also known as Gigabit Ethernet. Uses all four pairs of wires to transmit and receive data
• 10GBase-T – requires Cat 6 or Cat 7 cable. Main advantage is it makes fast data transmission at low rates compared to Fibre Optic technologies.

5-4-3 rule - This rule says that between two communicating nodes, the network cannot contain more than five network segments, connected by four repeating devices and no more than three of the segments may be populated.

The “T” part of the code stands for twisted pair.

##### Ethernet Standards for Fibre-Optic Cable
• 100Base-FX – Has a maximum segment length of 412 meters.
• 1000Base-LX - The most common 1-Gigabit Ethernet standard in use today – LX represents its reliance on  long wavelengths of 1300 nanometres
• 1000Base-SX – Similar to 1000Base-LX however relies on multimode fibre-optic cable as its medium which makes its less expensive to install. Best suited for shorter network runs as only one repeater may be used.
##### 10-Gigabit Fibre Optic Standards
• 10GBase-SR
• 10GBase-LR and 10GBase-LW
• 10GVase-ER and 10GBase-EW

### Summary of Common Ethernet Standards

Ethernet Frames

• The use of larger frame sizes on a network generally results in faster throughput – because CSMA/CD is expensive
• Ethernet_II is the standard frame type being used in modern networks

PoE (Power over Ethernet)

• A relatively new standard
• PoE standard specifies two types of device: PSE (power sourcing equipment) and PD’s (powered devices)
• PoE requires CAT 5 cable or higher
Posted on Saturday, January 14, 2012 12:01 PM UNISA COS 2626 Networks | Back to top

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