Lecture 2 (Introduction)

Slide 1 [Introduction]
So we've covered some of the basic applications of the Internet, so let's go on to look at a model which will allow us to abstract the Internet in many different ways. With this model we will be able to abstract the Internet as light pulses travelling along a cable, or a 1's and 0's in a data frame, or as data packets travelling through the Internet, or as data segments passed between application programs. This model is the OSI model, and we shall cover its structure later in this lecture.

Slide 2 [Network Types]
The first basic definition that we require is one which differentiates network sizes. Normally a LAN is defined as a network within a single building. These are normally owned by organisations. Above this we have MAN, which connect around a town or city. A WAN network covers large areas, possibly across man regions, over even countries. Examples of MANs in the UK are LonMAN (which connect the universities around London), EaStMAN (which connects the universities around Edinburgh), and NetNW (which connects the universities around Newcastle and Sunderland). We will study the architecture of some of these networks, and how the connect onto SuperJANET, and then out onto the main Internet.

Slide 3 [Examples of a WAN/MAN]
A good example of a MAN is the EaStMAN (Edinburgh and Stirling MAN) which connects the universities around Edinburgh, and also has a link off to the University of Stirling. This network is based on ATM, which is one of the main networking techniques that we will study. ATM has many advantages over traditional network techniques, such as Ethernet, as it supports the transmission of data to a certain quality of service (QoS). This QoS could relate to the minimum delay, or the maximum error rate, and so on.

We can also see that the EaStMAN network has the shape of a ring, which means that a break in any part of it will not stop nodes from communicating with each other (it will just take a little longer for two nodes which connect to the faulty connection to talk to each other). ATM is a key technology of the future, and provides much of the infrastructure of the Internet.

Slide 4 [Advantages of Networks]
So what are the main advantages of networks? Well there's possibly too many to list, but one of the main advantages is that they support the transmission of electronic mail. They also allow users to share resources.

Slide 5 [OSI layers]
The OSI model is one which will give us our foundation for the rest of this module. It will allow us to abstract the Internet in many different levels. At one time computer manufacturers defined their own standards, and it was extremely difficult to integrate equipment from different manufacturers. Each layer of the model has a specific function. This makes it easier to understand the complete system.

The OSI model allows the system to be build in layers, where the equipment and/or software can be easily integrated, as long it can interface properly o the layer above and below. Normally the bottom two layers covers the network type (such as Ethernet, ATM or ADSL), and the layer approach allows us to use different network connections, connect to the same software. This isolates the application from the network type. Thus a WWW browser application does not care if it is communicating with a modem or Ethernet. We can also use different network protocols, such as TCP/IP (for UNIX or Internet connections) or IPX/SPX (for Novell NetWare connections).

An important concept is data encapsulation. With this the original data is taken from the application, such as screen shot from this window. This data is then passed down to the presentation layer. This then adds its own little bit of data on the original data. This might be the codes which defines the language contents, or the character codes, or might even encrypt the data. The data and the added part from the presentation layer are then passed to the session layer, which adds its own data, and so on, down to the lowest layers. One of the functions of the data link layer to frame the data. Thus it adds a special bit sequence which defines the start and end of the data transmission. The physical is then responsible for transmitting the bits over the cable. This is then received by the other system, which reads the layers from the bottom up. Each layer only reads the data part that is relevant to them. Thus the network layer on the recipient only reads the information added by the network layer. When it is read, the relevant data is then stripped-off, and the rest of the encapsulated data is passed to the layer above.

The actual data flow goes from Application to Physical, and then back up from Physical to Application, but there is a virtual flow between the layers on each system.

If you forget the names of each layer, you can use All People Seem To Need Data Processing or Please Do Not Throw Sausage Pizza Away. Otherwise you could use a Scottish one, such as All People in Scotland Talk about is Neeps, Dumplins and Potatoes.

Slide 6 [OSI model]
The functions of the layers:

Application. Provides application programs, such as file transfer, print access and electronic mail.
Presentation. Transforms the data into a form which the session layer and the application layer expect. It can perform encryption, translating character sets (such as converting binary values into text for transmitting a binary program over a text-based system), data compression and network redirections.
Session. Setting up, maintaining and closing down of a session. It should not depend on any specific transport or network layer, and should be able to communicate as if the session was created on a stand-alone computer (that is, the network is transparent to the session layer).
Transport. Provides for reliable end-to-end error and flow control. The network layer does not validate that the data packet has been successfully received, thus it is up to the transport layer to provide for error and flow control. A typical transport layer protocol is TCP. This will be covered in Unit 6.
Network. Defines the protocols that are re-sponsible for delivering the data to the required destination. A typical network layer protocol is IP. This will be covered in Unit 5.
Data link. Provides for the access to the net-work media and thus builds on the physical layer. It takes data packets from the upper level and frames it so that it can be transmitted from one node to another. Typical data link layer systems are Ethernet and ATM. These will be covered in Unit 3 and Unit 4.
Physical. Provides for the actual transmission of the binary digits.

Slide 7 [OSI layers]
We will go into most of the lower layers of the model in some detail in the following units, but let's look at a simple analogy of the OSI model. Let's say that you've just written a book, and you've got to send the book to your publisher (PUB_ADDRESS). Also you've been told that your postal service can only take 20 pages at a time. The layers would be:

APPLICATION. The process of actually sending your manuscript to the publisher.
PRESENTATION. Before you sent it you would format it into the required format for the font, layout, graphics format, and so on.
SESSION. You would contact the publisher to say that you were about to send your manuscript, and arrange for a README document which explained the format of the manuscript. You would also define the TITLE of the book, and ask who has dealing with the manuscript (PERSON).
TRANSPORT. The manuscript is too large to be sent in a single unit, thus you would thus split the manuscript into bundles of 20 pages each. Next you would write a sequence number on each of the bundles, and also put the TITLE of the book on each bundle (just in case that the publisher is receiving other manuscripts of the same format).
NETWORK. At this layer you would look up the address of the person to which the manuscript will be delivered too. Each bundle would then be put in an envelope and addressed with the destination address (PERSON, PUB_ADDR).
DATA LINK. The postal service will now pass the envelopes from sorting office to sorting office, with their own mechanism for finding the next sorting office. The actual address of the envelope is always used to determine the next sorting office.
PHYSICAL. The envelopes are passed from place to place using automobiles, planes, bicycles, and, of course, by foot.

Slide 8 [OSI layers]
We will study each of the layers independently, but basically the physical layer covers the light pulses and electrical signals on the line, the data link layer gets the data reliably over a network connection. This uses the actual physical address of the system. The network is responsible for getting the data across interconnected network. At this level devices called routers are used to route the data from a source to a destination, and we also have the concept of a network address. An example IP address is 146.176.120.10, which in this case defines a computer which is on the Napier network (146.176.). The next part defines the network segment (120.), and the last part defines the actual computer (.10). The transport layer is then responsible for streaming the data to different destinations, and possibly, different destinations. It is also responsible for breaking the data up into chunks (known as data segments), as the original data may be too large to be transmitted onto the network in its complete form. It must thus number each of the data chunk, so that the data can be build back, in the correct order (or missing ones can be identified). It is at this layer that the reliability is built into the system, as the transport layer can identify if data is missing, and request it to be resent.

Slide 9 [OSI Layers]
Network devices have both a physical address and a network address. The physical address is permanently assigned to the network adaptor. For example in Ethernet this address is known as a MAC (Media Access Control) address, or with a modem this would be the telephone number. This type of address is normally randomly assigned. The network address is assigned to gives some information on the actual location of the device on the network. On the Internet the network address is in the form of W.X.Y.Z, where some of the field give the actual location of the network, and the rest define the actual device on that network. The physical address is used at the data link layer, and the network address is used at the network layer.

Slide 10 [Routers, bridges and repeaters]
All the seven layers are normally implemented on the source and the destination, but some devices on the way may only implement one, two or three layers, before the data is send on. A repeater is responsible for boosting the signal, and cleaning it up. This is only implements one layer. A bridge is responsible for forwarding data frames from one network segment to another, based on the destination physical address. Thus it only implements two of the layers. A router uses the network address to make a decision on whether to forward the data. Thus it implements three layers of the OSI model.

Slide 11 [Conclusion]
Thank you for attending this lecture. You should all receive your Teaching Pack at the next practical session. Remember to register for the module on-line at:

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Please read Unit 1 before next week, and ask any questions that you have on it using the form below.