The Communication Framework (pg 232 - 237)

the functions performed within the communication systems in passing messages between source and destination, including:

– message creation:

• compiled at the source in preperation for sending that takes place using some type of software app adn involves the collection of message data from on teh system's user or participants. e.g.  user writting an email using an email client such as outlook. web server retrieving requested HTML files from secondary storage in preparation for transmission to a web browser.

– organisation of packets at the interface between

source and transmitter:

• when a messagr is being prepared for transmission it descends the stack of protocols from the application level down to where it is ready for physical transmission by the hardware operating at the transmission level. Each protocol wraps teh data packet from the layer above with its header and trailer.  Protocols withins each layer are applied independently of the protocols operatin in other layers.
• the message is then passed on to the communication control and addressing level.
• at the receiving end the process described aboce the essentially reversed - each protocol strips off tis header adn trailer, performs any error checks and passes the data packet to the transmission level protocol(s).

– signal generation by the transmitter:

• the transmitter is the physical hardware that generates or encodes the data onto the medium creatnig a signal.  in most cases transmitters and recievers are contained within the same hardware device - receiver decodes the signal on the meduim.
• Hardware is controlled by protocols operation at the transmissionlevel.
• the main task of the transmitter is to represent individual bits or pattersn of bits as a wave - this wave is the signal that is actually transmitted through the medium.
• E.g. that include transmitters are: NIC's, switches, routers, ADSL, and cable moderns, mobile phones and bluetooth devices.

– transmission:

• occurs as the signal travels or propagates through the medium. each bit of often pattern of bits moves from transmitter to receiver as a particular waveform. Transmitter creates  each waveform and maintains it on the medium fro a small period of time.
• 1/5 of a second is the time requied for the physcail transmission of one megabyte  of data or binary data uf the transmission occurs as a continuous stream of symbols and the transmitter and receiver are physically together.
• in reality - data is split into packets, which are nto sent continuously, errors occur that need to be corrected and some mediums exist over enormouse distances - e.g. satelites or across oceans.
• some protocols wait for teh acknowledgment from the receiver before they send the next data packet.

– synchronising the exchange:

• to accurately decode the signal requires the receiver to sample the incoming signal using precisely the same timing used by the transmitter during encoding. this ensures each symbol or waveform is detected by the receiver. if both transmitter and receiver use a common clock then transmission can take place in the knowledge that sampling is almost perfectly synchronised with transmitting.
• e.g. system clock is used during synchronous
• synchronous transmission systems have almost completely replaced older asynchronous links which transfered individual bytes seperately using start and stop bits. Synchronous communication does not transfer bytes individually; rather it transfers larger data packets usually called frames.
• 2 elements commonly used to assist he synchronising process:
  -Preamble - can be included at the start of each fram whose purpose is iniial syncronisation of the receiver and transmit clocks.
  -second element is inluded or embeded within the data and is used to ensure synchronisation is maintained thourgh tranmission of each frame.
• manchester encoding: transition repersented through 10baseT.

– addressing and routing:

• each new communication link will have its own protovol or set of protocols and hence each packet must ascent the protocol stack untill it reaches the addressing or routing protocol adn then desend the protocol stack as it is prepared for transmission down the next path.
• Ethernet and other transmission level protocols use the receiver's MAC address to determine the path leading to the reciever.
• e.g. ethernet switch maintains a table of all the MAC addresses od attached devices. Frames can therefoer be directed down the precise connection that leads to the receiver. most routers use the IP address withing the IP datagrams together with their own routing table to determin the next hop in a datagrams travels.

– error detection and correction:

• As messages descend the stack prior to tranmission many protocols calculate checksums or CRC (Cyclic Redundancy Check) calues and include them within their headers or footers. Once the message has been received it ascends the protocol stock where each protocol examines its own received headers and trailers.
• If error detection is used by the protocol then the error check calculation is again performed to ensure the result matches the received checksum or CRC value. Whenever as error is detected virtually all protocols discard the entire packet and the sender will need to resend the packet to correct the problem.

– security and management:

• Many protocols restrict messages based on user names and passwords, and other go a step further by encrypting messages durign transmission.
• e.g. POP (Post Office Protocol) operates on most mail servers. Top retreive email messages from a POP server the user must first be authenticated - meaning a correct user name and password combination must be included.

For each example below, identify the source, destination and medium over which messages are sent. Describe suitable communication rules (protocols).

1.       A conversation with a young child

2.       Sending a birthday card to your Grandmother

3.       Watching television

4.       Ordering a meal in a restauant