Idle RQ specification

Idle RQ specification: can be combined & used with sequence diagrams

• state transition diagrams    

• extended event-state tables

(1) State Transition Diagrams:

state variables: retain variable information as incoming events are received

• useful for showing correct protocol operations

• space limitations a impractical to show all possible events & conditions

• most state transition diagrams are incomplete specifications

• complete protocol requires many such diagrams

•states are ovals stable point, can be altered by external events

•arcs indicate possible transition between states

•events are occurrences that can cause change in state

- incoming event causing transition (source = LS user, PHYprovider, TIMprovider)

- outgoing events that result from transition (source = LSprovider, PHYuser)

• specific actions : actions internal to state

(2) Extended event-state table: more rigorous method to specify protocol

• basicevent-state table: for any incoming event - only 1 action & 1 next state

• extended: predicates allow alternate actions/next states

for each protocol state (present) and for all possible incoming events

• define outgoing events

• define specific actions

• define alternative actions using predicates

• define new state

• interpreting actions for predicate - assume they are ordered

• lends itself to implementation in SW

Explicit Retransmission

2. Explicit Retransmission

- P transmits I-frame and starts timer

- S ACKs on receipt of error free I-frame

- S NAKs on receipt of corrupt I-frame

- P can transmit another I-frame on receipt of error free ACK

- P retransmits I-frame on timeout or NAK

- corrupt Frames are discarded (I-Frames, ACKs, and NAKs)

Implicit Retransmissions

1.Implicit Retransmissions:

- P transmits I-frame and starts timer

- S ACKs on receipt of error free I-frame

- P can transmit another I-frame on receipt of error free ACK

corrupt frames are discarded (I-Frames and ACK frames)

- P retransmits I-frame on timeout (w/o ACK) 

- S discards duplicate I-frames and ACKs duplicate (ACK corrupted)

Idle RQ

Idle RQ: (also known as send & wait or stop & wait)

Idle RQ technique is a stop and go protocol, a fundamental backward correction scheme used in many protocols. The data is transfered in packets by using error detection.

The receiver checks the incoming packets and sends an acknowledgement (ACK) to the sender if the packet was valid. If the sender receives an acknowledgement in the specified time it sends the next packet to the receiver. Otherwise, the sender must resend the packet. Idle RQ is very simple but it is often too inefficient. It can only send data to one direction at a time. In addition, the delay on the data transfer may result in situation where only a small fraction of the capapacity of the communications link is used.

Elements of Idle RQ

P (primary): source DTE

S (secondary): destination DTE

I-frame: data or information frame

ACK frame: acknowledges I-frame correctly received

NAK frame: acknowledges I-frame not received (corruption detected)

Idle RQ Strategy

-         Used to reliably transfer a frame or block ofdata.

-         The strategy will allow the data to betransferred without error or replication.

-         The data frames are referred to as I-Framesor information frames.

-         The Idle RQ operates in the half duplex modesince the source and the destination do nottransmit at the same time.

Type of Idle RQ

1.     Implicit Retransmission

-         The destination will only ACK correctly receivedframes.

-         The source interprets the absence of an ACK as

an indication that the previous frames was

corrupted.

2.     Explicit Request

-         If the destination detects that a frame is corrupteda negative acknowledgement is sent to explicitlyrequest a retransmission.