Data Link Control Protocols

Question 1

High Level Data Link Control

basic characteristics - station types

Answer 1

Station Types:
1. primary station - controls operation of link; frames issued are called commands; maintains separate logical link to each secondary station
2. secondary station - under control of primary station; frames issued are called responses
3. combined station - may issue commands and responses

Question 2

High Level Data Link Control

basic characteristics - link configuration

Answer 2

Unbalanced:
* one primary and one or more secondary stations
* supports full duplex and 1/2 duplex

Balanced:
* two combined stations
* supports full duplex and 1/2 duplex

Question 3

High Level Data Link Control

transfer modes

Answer 3

Normal Responses (NRM)
* for unbalanced configuration
* primary station initiates transfer to secondary station(s)
* secondary may only transmit data in response to command from primary
* primary polls secondary for transmitting
* used on multi-drop lines and daisy-chain polling; host computer as primary, terminals as secondary

Asynchronous Balanced Mode (ABM)
* most widely used, no polling overhead
* balanced configuration (combined stations)
* either station may initiate transmission without receiving permission

Asynchronous Response Mode (ARM)
* unbalanced configuration
* secondary may initiate transmission without permission from primary
* primary responsible for line management, rarely used method

Question 4

High Level Data Link Control

frame structure diagram

Answer 4

• flag field - frame sequence synchronization
• address field (8 bits) - indicate the secondary station having transmission or to receive frame; not useful for point-to-point links
• control field - I/S/U
• info field - user data
• FCS - error detection field

Question 5

High Level Data Control

operation

Answer 5

• exchange of info, supervisory and unnumbered frames between the data link stations
• each op has three phases:
  1. initialization
  2. data transfer
  3. disconnect