U1. OSI 7 Layer Reference Model

Annotations:

• 1. What are the advantages of a connection-oriented service?   = It’s reliable. When information has been sent, the service makes sure that it arrives. Connectionless services don’t provide this reliability.   2. So are there any disadvantages to connection-oriented services?  There is a cost, usually in the time and network traffic required to set up the connection. If the user only has a very small amount of information to send, then the set-up time and set-up traffic might be greater than the information that needs to be sent. If a user sends information irregularly, then it may be inefficient to keep a connection open permanently, but it might also be inefficient to open a connection every time information needs to be sent. So connection-oriented services at the network layer generally seem to be useful only when the user wants to send a reasonable amount of data or a constant stream of data that must arrive without error and in order, for example the transfer of a data file.   3. Why would any want to use the unreliable connectionless service?   = Networks aren’t reliable, and many applications assume this and provide their own error detection, flow control and congestion management at a higher layer in the transport layer. If the sender and receiver are going to provide their own control mechanisms, why provide it in the network layer? So in this case, the connectionless service just provides a raw service of sending packets from sender to receiver.   4. Could we build a connection-oriented service on top of a connectionless service?   = Of course! A connectionless service could provide the basic service of sending a packet from sender to receiver with no guarantees, and a higher layer connection-oriented service could provide the reliable service. A good example to illustrate this is mobile telephone services. Speed is more important than errors since the service must be fast to keep up with the speed of human conversation. Remaining in synch is a greater priority than resending corrupted or lost voice data. So although mobile services are connection oriented at the application layer, they are actually connectionless at the network layer for speed.   5. So which service did the ISO committee choose to provide in the network layer?  Both. The debate over which service should be provided in the network layer was not resolved, and perhaps shouldn’t be as providing both allows network designers to choose which is most appropriate for their particular technology.  6. If Layer 4, the transport layer, is the last layer to provide an end-to-end service, why is there a connection-oriented service in Layer 3? Surely this provides an end-to-end service?  Layer 4 is the last true end-to-end layer. Layer 3 may or may not provide an end-to-end service, so end-to-end communication is not guaranteed in Layer 3 whereas Layer 4 will always provide a true end-to-end connection-oriented service. 

1. 2. Data Link Layer
   1. 1. Physical Layer
      1. Concerned with the details of bit transmission over a physical channel.
         1. Design issues include
            1. the definition of 0 and 1, e.g. how many volts represents a 1, and how long a bit lasts;
            2. whether the channel is simplex or duplex;
            3. how many pins a connector has, and what the function of each pin is
   2. Takes the ‘raw’ transmission facility provided by the physical layer and uses it to provide an error-free transmission service for data
      1. Bit errors not packets which are handled at level 4
      2. Breaks the data stream into frames
         1. Creates and recognises frame boundaries
         2. Deal with duplicate frames
      3. regulates traffic flow to prevent swamping a slow receiver
2. has 3 goals
   1. Enable the higher layers to be independent of the underlying network technologies
   2. Allow transport layer to be insulated from details of the number, type and topologies of the different physical networks that have to be crossed.
      1. 2 Types of Service
         1. Connection oriented
            1. reliable end-to-end connection. To communicate, user requests a connection, uses cconnection until they’re done, and then closes the connection.
               1. Comes with cost overhead in network traffic and time to setup
               2. Inefficient to keep open if not used often
               3. Best if you want to send a reasonable amount of data or a constant stream that must arrive without error
         2. Connectionless
            1. Provides a service where the user packages their information, addresses it and sends it, and receives no confirmation that it reaches the destination.
               1. useful if the application provides error detection and management, no need to use it in the NW layer
      2. Also does congestion control
   3. provide a uniform addressing scheme for network addresses.
      1. provides address conversion

1. also responsible for ensuring that all the pieces are received correctly and reassembled in the correct order.
   1. transport layer will create a distinct network layer connection for each transport connection requested by the session layer.
      1. However, depending on the data load and the capacity of a single session channel: multiple network connections might be used to support a single high-bandwidth session connection; or one high-bandwidth network connection might be used to support several session connections.

1. IE:an error-free, point-to-point channel, guaranteeing that data is delivered in the correct order
2. transport of isolated messages with no guarantee of correct ordering
3. message broadcast to multiple destinations

1. Dialogue control and token management
   1. If comms are half-duplex (communication is in one direction at a time, full duplex = both directions at the same time) then session layer manages which entity sends at which time
2. Synchronisation
   1. synchronisation service provides a means for checkpoints to be inserted into a data stream so that, after a crash, only data sent since the last checkpoint needs to be re-transmitted.

1. Is where data compression and encryption take place