Protocol functions are divided into some layers.  The layered approach allows for better separation of protocol functions as well as software modularity.  For communications networks, layers are valuable because they allow for software upgrades to be deployed without affecting every node in the network.

 A protocol is necessary for any function that requires cooperation between peers. We will take a look at some examples of layered protocols. 

a) Typical telephone call communication:

ASSUMPTION:

SENDER (S) AND RECEIVER (R) SPEAK ENGLISH

– THIS PRESENTS A PROTOCOL ISSUE

ESTABLISHING CONNECTON:

 S: “IS JOHN THERE?”

 R: “YES, THIS IS JOHN.”

ERROR CHECKING:

 S: “DID YOU HEARD WHAT I JUST SAID?”

 R: “YES.”

FLOW CONTROL (E.G., DICTATING LETTER OVER PHONE):

 S: “ARE YOU READY FOR NEXT SENTENCE?”

 R: … pause … “YES.”

BOTH SENDER AND RECEIVER MUST FOLLOW SAME RULES.  

b) Fax

USER

                The top layer, the user comes up with concepts to send to the receiving party.

PAPER

                A layer down; The user puts down the concepts in a form of symbols on a    piece of paper

FAX MACHINE

                Another layer: fax machine converts the info into ‘dot patterns’

MODEM

                Modem changes the patterns into electrical signals

TELEPHONE LINE

                And finally the electrical signals are transmitted through the telephone; physical media

When the electrical signals reach the other end (recipient), the way up to the reader is through the layers from the telephone line up to the user

 c) Email

7. APPLICATION:

SPECIFIC HANDLING (e.g., PAY BILL, ENQUIRY)

6. PRESENTATION:

LANGUAGE TRANSLATION SERVICE

5. SESSION:

GROUP IN COMPANY (e.g., A/P) or PERSON (e.g., E53-321)

4. TRANSPORT:

SOURCE COMPANY TO RECEIVER COMPANY

3. NETWORK:

ROUTING FROM POST OFFICE TO POST OFFICE

2. DATA:

FLOW CONTROL AND TRAFFIC HANDLING ON HIGHWAY

1. PHYSICAL:

TRUCKS AND PLANES USED

The ISO OSI seven layer model will be explained in detail within module 8