1 What Is A Network? ...................................................................................................................... 2
1.2 The SubNet and Network Topologies .........................................................................................2
3 The Development of Networks ...................................................................................................... 4
3.1 Master-Slave Model....................................................................................................................4
3.2 Distributed networks and the peer-to-peer model .......................................................................4
3.3 The client-server model ..............................................................................................................5
4 Basics of message transmission ................................................................................................... 5
4.1 Circuit Switching .........................................................................................................................5
4.2 Packet Switching ........................................................................................................................6
5 Types of Network ......................................................................................................................... 6
5.1 Local Area Network (LAN) .........................................................................................................6
5.2 Wide Area Network (WAN) .........................................................................................................6
5.3 Metropolitan Area Network (MAN) ..............................................................................................6
5.4 Example LAN network configurations .........................................................................................7
5.4.1 Small Office System ................................................................................................................................................................... 7
5.4.2 Small office/home system connected to the Internet .................................................................................................................. 8
6 The Communications Problem ...................................................................................................... 9
6.1 How can we achieve reliable communication? ......................................................................... 10
6.2 What actually is a communications protocol? ........................................................................... 10
7 Network Standards ..................................................................................................................... 10
7.1 Practical Network Standards .................................................................................................... 11
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Nodes connected by a series of links called a Subnet
1.1 Nodes
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Nodes may be computers, terminals, peripherals or communications devices
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Nodes communicate by transmitting data, requesting/delivering services etc. across the subnet;
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Transmissions must arrive at right destination and be interpreted correctly by the receiver
If reliable communications is required (e.g. file transfer) errors (e.g. due to electrical noise on the communications lines, etc.) must be corrected for.
Could be……:
Communication can be:
Simplex Transmission only possible in one direction.
Half-Duplex Transmission possible in both directions but not at the same time.
Full-Duplex Transmission possible in both directions simultaneously
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no single path exists between source and destination
 the message is routed through the network from node to node until it reaches the destination
 at each node the message is switched onto a line as determined by the routing algorithm, e.g. to give the shortest path from source to destination
The long distance communication systems between cities and countries maintained by companies such as BT which are used to support WANs (Wide Area
Networks) can be considered as a mesh.
Signal transmitted by any station propagates in both directions until being absorbed by terminating resistors at each end ( are terminating resistors)
A ring network, as its name suggests, consists conceptually of a single loop of cable, usually coaxial or twisted telephone pair, along which traffic flows in one direction from node to node
Signal transmitted by a node is repeated by the intermediate node until it returns to the transmitter (which then removes it)
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Broadcast a sender transmits a message which is received by all notes
Bus and ring topologies are broadcast networks in that every node receives every transmission:
 on a bus the signal propagates both ways along the bus being received by each node in turn
 on a ring the signal is transmitted from node to node around the ring in practice only the node(s) with the appropriate destination address would read the message off the network
(others would ignore it)
Multicast is communication between a single sender and multiple receivers
Unicast is communication between a single sender and a single receiver over a network
Anycast is communication between any sender and the nearest of a group of receivers in a network
Until the late 1970’s when cheap microcomputer systems started to appear computer systems were very expensive, i.e. starting at £20000+ to 10’s of millions, and only large organisations could afford to purchase and operate them. In this environment an organisation (bank, airline, university, etc.) tended have a central computer system which was accessed by dumb terminals either locally or from remote sites via telephone lines. A dumb terminal has no processing power (it is just a display screen and keyboard) and all processing was carried out by the central computer, i.e. as users hit the keyboard the characters were transmitted to the central computer, processing carried out and results sent back to the display screen. This configuration was called a master-slave network as the central computer was in complete control.
The advent of cheap processing power in microprocessors has changed the economics of networking. It is now easy to distribute processing power throughout the network and dedicate it to individual users or particular tasks; it is also often cheaper in terms of support cost. These independent processors still need to exchange information with each other, to share information and resources and perhaps to provide backup facilities for each other. This is often called a peer-to-peer network and the connection of these independent peer machines is at the heart of distributed networks both local and wide area.
Peer to peer exchange of information
Station A Station B send message  rec message rec message  send message send message  rec message rec message  send message
The stations must agree on when to send and receive information and the format of the messages, i.e. have a communications protocol (discussed later), e.g. otherwise it is possible for deadlock to occur if station A is waiting for a message from station B and station B is waiting for a message from station A.
The advantages and disadvantages of this approach are:
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Advantages improved reliability and fault tolerance, sharing of data, sharing of special hardware resources
(e.g. expensive printer, high-performance nodal processor, etc.)
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Disadvantages vulnerable to network and/or server failure, cost of interconnection, cost of networking/communications software, problems of access to shared file systems, difficult to manage, etc.
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Since the machines in a distributed environment are independent and equal (peer-to-peer), the master-slave model of providing resources is no longer appropriate, and the access to resources is based on a client-server model:
Each system resource (printing, database, file system, etc.) is managed by a server process running on a computer somewhere in the network;
A 'client' process on any machine in the network operates on a resource by sending a request (across the network) to the corresponding server (e.g. locate and send me file 'x' or print file 'y') and receiving a response (e.g. file 'x' not found or here is file
'x').
Client/Server exchange of information
Client Server
loop send message  wait for message rec message  send message
end loop
Note
 server runs continuously in a loop waiting for client requests o usually can deal with multiple clients concurrently using threads , i.e.
when a request arrives a thread
 is created which processes the request and then terminates client sends request, waits for response and when it arrives carries on
Thus the processing workload is shared between the client and server machines (rather than the master doing all the work). For example, in a database environment the client will handle interaction with the user (getting SQL requests or filling forms) and the server will handle the file manipulation on the machine where the data is actually stored.
The operation of the client-server mechanism is generally invisible to the user, i.e. the aim is a transparent network in that the user should not be aware of the network hardware and software. As far as the user is concerned they are sitting at a computer with access to large resources. The user requests a resource (edit file 'x' or print file 'y') using the appropriate command (e.g. selecting the file open command in a word processor) and the communication with the server processes is handled by the operating systems and network software. The user will have no idea if the file accessed is one the hard disk of the machine they are sitting at or on a machine in another country (assuming the network is sufficiently fast so response time is not degraded).
The server machines are usually powerful (relative to user machines) running a multi-tasking operating system or network software. They will be capable of handling multiple simultaneous requests from clients spread across the network.
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A route is set up through a number of exchanges to the destination. the communication channels used are dedicated for the duration of the call channels are unusable by others until the 
 call is terminated tariff is based on duration of call (not amount of data transferred)
Example: the old (non digital) telephone system, i.e. when A and D have set up a call B and C cannot use line.
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Technique used with digital connections to allow multiple calls to exist on same circuit.
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Data broken into small blocks ("packets")
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Packet includes extra information in Header
(serial number, destination address, etc).
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Each packet is routed to its destination individually
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Packets re-assembled into original message when destination reached.
Data tend to be in bursts and charging can be based on the amount of data transferred not duration of call.
ADVANTAGES OF PACKET SWITCHING
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Communication channels can support many calls simultaneously (interleaved).
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Short messages not delayed by long messages.
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More efficient than circuit switching.
DISADVANTAGES
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Performance drops when many users share same network
Today the majority of communication systems use packet switching.
A general purpose network serving a variety of devices which is local to a room, building or site and is usually owned by the organisation concerned.
A large organisation may have a number of LANs which may be interconnected, e.g. on a university campus
 mainly used for local information sharing, e.g. file systems, databases, etc
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Connection of machines, typically on a single site, e.g. university campus
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Well defined topology (bus, ring, star)
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Usually owned and managed by the organisation
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Does not normally use public communications facilities
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Usually quite fast
The majority of LANs only carry data but technology moving towards also supporting speech, video, etc.
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Extends beyond one site - covers a large area (national and international distances)
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Usually use facilities provided by the public telecommunications system or equivalent private systems
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No simple topology - usually pictured as a sparsely connected mesh
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Can be quite slow (depends upon cost) can carry any type of information (data, video, etc.)
Has similar characteristics to a LAN but covers larger distances and operates are higher data rates
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Between LAN and WAN
 can be private or public
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Covers metropolitan area (town/city)
 typically ring or bus topology
 typically faster than LANs some MANs can any type of information
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Consider the network shown below where each office of a small company has its own Ethernet network (with fileserver and user stations) interconnected via bridges and an Ethernet backbone. Local traffic is contained within the subnetworks with only inter-office traffic crossing onto the backbone. Consider if station A in the sales office wishes to communicate with station R in the research and development office. Bridges B1, B3 and B4 act as routers to transfer the information (note that it is important that there is only one path between stations!).
Ethernet backbone with bridges to subnetworks
The above diagram shows the logical structure of the network. Today the network segments would probably be connected in a star arrangement via a network hub (discussed in LAN notes).
If an internetwork is configured correctly the majority of traffic on the network is local to the semi-independent sections with the bridges and gateways handling a (relatively) small amount of long distance traffic, e.g. data transmitted to a remote minicomputer or mainframe for analysis. In extreme cases (e.g. when security is paramount) the network sections can be physically independent and linked via a bridge only when necessary (using a physical switch).
In practice the bridges, gateways and routers may be 'off-the-shelf' devices or a fileserver equipped with two or more network cards and suitable software (e.g. Novell NetWare).
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ADSL converts existing telephone lines into high-speed connections between a house and an ISP by using Discrete
Multitone (DMT) modulation which uses frequencies above the voice band to carry data.
It's called Asymmetric because it allows the downstream connection (from ISP) to be much faster than the upstream, e.g. for a 500kbit/s downstream rate, the upstream rate can be anything from 64kbit/s to 256kbit/s depending on distance to the telephone exchange and the quality of the telephone line.
The diagram above (taken from an D-Link DSL-604G+ Wireless ADSL router – see http://www.dlink.co.uk/dsl-
604plus.htm) shows a practical example of the frequency spectrum of the upstream and downstream channels.
Filters are required to separate the telephone and modem signals.
There are various ways of connecting to the internet. A computer can act as a gateway being connected to the internet
(e.g. via an ADSL modem). This computer would have a firewall and control access. Other computers connect via this computer either directly using extra network cards or through a network hub or router.
Alternatively, as shown in the diagram alongside, a network hub or router connects to the internet with the computers connected to the router. Each computer would need its own firewall. picture from http://www.homenethelp.com/web/diagram/sharerouter-wireless.asp
IP (Internet Protocol) addresses, Domain names and ports
A computer on a TCP/IP network has to have a unique address or number, called its IP address, so packets can be sent to it properly, e.g. DMUs main email server has IP address 146.227.1.2
Numbers are hard for humans to remember, so we have domain names as well, e.g. DMUs email server is called helios.dmu.ac.uk (which has IP address 146.227.1.2)
DHCP (Dynamic Host Configuration Protocol) A machine can have a static IP address which is the same each time it connects or a dynamic address which is assigned when it connects to the Internet. (and can be different each time). DHCP is the protocol for assigning dynamic IP addresses – the ISP has a range of IP addresses available which are assigned when devices connect and become free on disconnection.
NAT (Network Address Translation Protocol) An organisation may be assigned one IP address (or a small number) yet have many machines, e.g. an domestic ADSL line is assigned one IP address yet a house may have four or five PCs. NAT is an Internet standard that enables a LAN to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. A NAT box located where the LAN meets the Internet makes all necessary
IP address translations from internal to external.
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Communication must be reliable and error-free, across a potentially unreliable subnet, i.e.
 data signals travelling along a cable being corrupted by electro/magnetic noise induced in cable by external equipment or radio signals
Achieving this is a complex problem, e.g.
1 The simplest subnet – single point-to-point line
` What issues need to be considered to ensure reliable communication?
2 Any additional issues for a multipoint network, e.g. a bus topology? i.e. addressing – who is the message for?
Which station can use the network at any instant?
3. And for a switched system, e.g. a mesh topology? i.e. routing – how is a packet routed across the network
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Through the use of Communication Protocols
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Systems adopted by both sides for the purpose of managing the communication
The protocols will define sets of rules dictating how each side should behave in any given situation.
The interchange of information must be in an agreed format (an agreed ‘language’) usually implemented by sending additional management information along with the message data
The extra information imposes a Protocol Overhead in that it makes the messages larger.
Protocol: a set of rules defining how computers are allowed to talk to each other.
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What sort of cable is needed?
What shape plug is required at each end?
How will we differentiate between 0 and 1.
How will receiver know that transmission has started? how will the receiving device interpret the bit stream 01011011? how can the receiving device be sure that 01011011 was what the sender transmitted?
How data is collected into blocks for transmission;
How errors are detected and what remedial action is taken;
How the dialogue between sender and receiver is managed;
How information is presented to the user at both ends.
How the receiver knows that the transmission has finished?
In addition, if there are more than 2 devices connected together
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How can we arrange for the message to be sent to the correct destination?
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How should the receiving device be addressed? and for a broadcast network such as a bus
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What if several devices try to all talk at once!
Once physical communication is established and devices can exchange bits
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Syntax – can they understand each other’s ‘language’? i.e. character codes, number representation, message
 format, etc.
Semantics – can they understand the content of the message? i.e. if an email client sends a message requesting new mail to a web browser the browser would not understand what it was about and reject it.
The key to reliable communication is adherence to protocol STANDARDS
Transmitters and Receivers must be operating the same protocol
Protocols must be implemented and interpreted in a strictly defined order
Protocols must be standardised to ensure compatibility between different hardware, software, manufacturers, countries etc.
Aim is to produce Open Systems: An open system exists if 2 or more nodes can communicate irrespective of the platform, manufacturer, country etc.
This is oposite to closed systems which are manufacturer dependent, e.g. In the area of operating systems Microsoft
Windows is a closed system whereas Linux is an open system (full specifications and source code freely available)
International Standards Organisation (ISO) is attempting to produce
Open Systems Interconnection (OSI) by suggesting a standard architecture based on a number of layers
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Many computer system vendors, in particular in the microcomputer area, have developed their own network protocols
(hardware and software). This was in the main due to:
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The long delays in the appearance of standards: it can take five to ten years between the development of a technology and a standard being defined (the users want it now not in ten years time!)
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Standard networks (hardware and software) are very expensive when they first emerge. In the early 1980's in the area of micro computing they were so expensive that many vendors developed their own cheaper (and more unreliable) alternatives.
The advantages of using network standards are the same as for any other hardware or software standard:
Vendors a standard assures a large market for the vendor selling too many clients which in turn encourages the use of the latest hardware VLSI technology (reducing size and cost).
Users a standard allows products to be purchased from many vendors which can then communicate, i.e. gives the user greater flexibility in equipment selection and use.
There are disadvantages:
1. A standard takes a long time to emerge and tends to freeze the technology; by the time a standard emerges a more efficient alternative is usually available, e.g. the IEEE 802.3 contention bus is based on Ethernet developed by Xerox in the 1970's is still widely used.
2. There are multiple conflicting standards for the same thing.
Thus if one wishes to network high performance workstations from a particular vendor one many have to purchase a vendor proprietary network to gain the best performance. The problem is that stations from other vendors will not
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 attach to the network and the vender may drop the technology when a proper standard appears.
The organisations involved with network standards are:
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CCITT the Consultative Committee on International Telegraphy and Telephony mainly concerned with defining standards for public telecommunications systems
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ISO the International Standards Organisation defined the 7 layer OSI reference model and is mainly interested in layers 3 and above
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IEEE the Institute of Electronic and Electrical Engineers, USA through its 802 committee developed standards for LANs
DOD the US Department of Defense was involved with the development of ARPANET and the TCP/IP suite
Also involved are ANSI (American National Standards Institute) and NBS (US National Bureau of Standards).
ISO specified the 7 layer OSI (Open Systems Interconnection) Reference Model which provides a framework for networks. Some standards implemented while the OSI model was being developed do not conform exactly.
In practice there is wide agreement on layers 1-3; these are usually implemented in hardware or a combination of hardware and software. The higher layers, which represent operating system functions and routines, are much less well defined; there is also some dispute as to whether the OSI approach is the best way of supporting the functions required.
Protocol standards common to LANs and WANS.
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