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Network Hardware

Sub-topic Outline:

• Different ways of classifying networks
• By Network Topologies

• By Transmission Technologies

  1. Broadcast networks
  2. Point-to-point networks
• By Scale
  1. Local Area Networks (LANs)
  2. Metropolitan Area Networks (MANs)
  3. Wide Area Networks (WANs)
• Wireless Networks
• Internetworks

Network Topologies

Network topology refer to how a network is connected up. It may or may not be how they physically look, but it refers to how they are conceptually (ie. it defines which machine is allowed to communicate with which other machine).

Some example topologies:

• Star
• Ring
• Bus
• Tree
• Complete
• ...and many other possibilities

Ref: Fig 1-3 Tanenbaum textbook p9
Ref: Fig 1-6 Tanenbaum textbook p13

We will discuss the details of different topologies when we study some of the protocols for them in later topics. For now, keep in mind that different topologies have different benefits/drawbacks. For example,

• In a star topology, it is easy to add and remove machines, since we only need to hook them up to a central hub (see below for a description of a hub). It is robust since no machine depends on other machines. But it requires relatively more cabling than the Bus or Ring.
• A ring topology is easy and low cost (in cabling) to install. It is easy to add a new machine as well. The drawback is in the fact that if any single machine goes down, the network is disabled.
• A bus topology is also easy and low cost to install. We also have the option of laying the main backbone line anywhere which is convenient. The drawback is in isolating where networks problems originate when they occur. In the start or ring, it is very easy to isolate problems of individual lines/machines. Also, if the central backbone goes down, everything stops.
• A tree topology is a variation of the star (think of each branch as a secondary hub). It has all the advantages/disadvantages of a star, with the added advantage of being able to add more machines to a single central hub.
• A complete topology is very robust, since every machine has connection to every other machine, so doesn't depend on other machines. It is more secure since we don't have send data through intermediate machines. But the drawback is the amount of cabling that it needs. Also, adding and configuring a new machine is very cumbersome.

Note: A hub is a device in most local area networks which connects up multiple machines. It takes a signal coming in from any line, and broadcast it out to ALL other lines. Some hubs does switching (ie. has the ability to only send out on one particular lines), but most hubs do not.

Transmission Technologies

• Broadcast
• Point-to-point

The types defines how short messages (or packets) with addresses, gets sent from sender to receiver (or receipient).

Broadcast Networks

• Packet sent to all possible destinations - only receivers accepts - others discard
• Packets can be addressed to all destinations - broadcasting
• Packets can also be addressed to only a few of the possible destinations - multicasting
• Normal for smaller networks - LANs

Normally, in broadcast networks, we can send to a particular machine (destination) by using the machine's address. We also send to ALL machines by using a special agreed-on address (in bradcasting), or a set of machines, again using special addresses (in multicasting).

The reason why this type is usually only used in smaller networks is because it can generate a huge amount of unecessary network traffic.

Point-to-Point Networks

• Network considered to be a set of individually connected machines
• Individuals machines only think about their immediate neighbourhood
• Machine receives packet - looks at the address - then forward it to next machine based on it routing algorithm
• Common for larger networks

Network Classification based on Scale

Fig 1-2 Tanenbaum textbook p8

This figure is only a guide. There is no hard and fast rule on how big a network is before it becomes a LAN, MAN or WAN. Eg. a LAN is usually identified by either all machines using the same LAN protocol (eg. IEEE 802.3 Ethernet - more on this in future topics), or they are machines communicating within the same organisation. It is possible for us to have machines on opposite parts of the world communicating over the same LAN protocol, which make them belong to a LAN. This is of course not technically good, since LAN protocols were designed for small networks.

Note: Also in the figure, "data flow machine" and "multicomputer" are terms used by Tanenbaum to refer to single and multiple processor machines. The terms are not common. Don't worry about them.

Local Area Networks (LANs)

Normally:

• privately owned
• contained in buildings or small areas (eg. campus)
• 3 characteristics of a LAN
  1. size
     • restricted
     • worst case transmission time known
     • simpler network management
  2. transmission technology
  • single cable
  • 10Mbps - 100Mbps
  • low delays, low error
  3. topologies
     • bus
     • ring
     • star

Metropolitan Area Networks (MANs)

• Similar technology to a LAN but over a wider area (eg. city)
• can include both data and voice (telephone network), and even video (eg. local cable TV)

• no switching elements

  Switching elements are devices which receives a signal from an incoming line, and can determine which output lines the signal is suppose to go, and thus only sends out on those lines.

• new standard Distributed Queue Dual Bus (DQDB)

  Fig 1-4 Tanenbaum textbook p11

  The protocol DQDB is also called IEEE 802.6. It defines a network consisting of two lines. All machines are connected to these two lines. On one line, data moves in one directioon, and on the other line, data moves in the opposite direction. Machines decide where their intended destination are, and sends their data out on the line moving data in the appropriate direction. The protocol is simple for design and implementation.

Wide Area Networks (WANs)

• Over large geographic area (eg. Australia)

Ref: Fig 1-5 Tanenbaum textbook p12

• Hosts are the individual machines with the applications, connected up in a LAN

  Hosts are also sometimes called "end systems".

• Routers acts a switch between incoming and outgoing transmission lines - also as the end point of a LAN

  In the figure, instead of using the generic word "router", we can also use the more general "packet switching nodes", "intermediate systems", or "data switching exchanges". What they do is take an incoming message, decide where it is going to go, and send it out on the appropriate line. They act as end points of LANs because they only send message into the LAN if the messages are intended for hosts in the LAN.

• The collection of lines and routers forms a subnet

  In our consideration of a WAN, we separate the network communication part (the subnet) and the user applications part (the hosts). It is to simplify design.

  In later topics, when we discuss the Internet and TCP/IP, the word "subnet" will have a specialised meaning that is slightly different from here. Keep that in mind.

• often referred to as a packet-switching network (store-and-forward)
  • packet arrives
  • stored until the route is clear
  • forwarded toward destination hosts

• point-to-point across subnet then broadcast on hosts' LAN

  Since the subnets can get quite large, it is unfeasible to broadcast over it.

Wireless Networks

Developed mainly for mobile users, or users in areas without conventional bcabling support. Their uses include having a mobile office (sending receiving email, accessing remote files, using remote machines, etc), for fleets of trucks/buses/taxis to communicate with central control, for rescue teams and military in unconventional locations.

Internetworks

• a inter-connection of different networks
• different networks not always compatible
• gateways translate packets moving between the differing types of networks

• not the same as The Internet.

  The term the Internet refers to the global network connected by the TCP/IP set of protocols. Don't confuse it with the general term internetworks. Sometimes internetworks are also called "internets" (with a small "i"), which can add to the confusion.

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