cpre 458/558: real-time systems

CprE 458/558: Real-Time Systems (G. Manimaran) 1

CprE 458/558: Real-Time Systems

Real-Time MAC Protocols

(Contd.)


DCR protocol: disadvantages

• Poor schedulability because it does not take message deadlines into consideration


DOD-P (Deadline-based DCR protocol)

• Uses the message class concept

• All the messages which have similar (fairly equal) deadlines are classified into one single class

• The total number of message classes is a design parameter

• A time tree is constructed with each node representing a message class

• The time tree is traversed in a pre-order manner

• Each node when traversed in turn runs the DCR to resolve collision with messages belonging to that class alone


DOD-P: Example

Time tree with three message

classes

Traversed in pre-order manner

1

2 5

3 4 6 7

1

32

Perform normal DCR with just the GREEN messages

Perform normal DCR with just the YELLOW messages

Perform normal DCR with just the PINK messages


DOD-P Protocol: State Diagram

READY

WAIT

IDLE

Channel busy

Channel idle

Successful transmission

Message arrival

TIME TREE SEARCH

TRANSMIT

End of epoch

TRANSMITDCR MODEEligible

transmission

Transmission completeNot eligible for

transmission

collision

Eligible transmission

Transmission complete

collisionDCR tree search

complete

Not eligible for transmission


Carrier Sense Medium Access (CSMA)

Channel busy ?

Have a message ?

YES

Transmit message

Collision ?

Re-transmit later

NO

YES

NO

This step depends on the exact

CSMA variation


Virtual Time CSMA (VTCSMA)

• The basic idea is to incorporate a priority mechanism into the CSMA

– various priority policies can be incorporated

• No coordination between nodes

• VT-CSMA uses the following to decide whether or not to attempt message transmission

– The state of the channel

– The priorities of the messages waiting for transmission

– The time according to the synchronized clock

– It has no knowledge about the priorities of the messages waiting at the other nodes


VTCSMA

• Maintains two clocks at each node: real clock (RC) and virtual clock (VC)

• RC gives real time

• VC gives virtual time

• RC and VC at a node are synchronized with the corresponding clocks at the other nodes.

• VC runs η ( > 1) times faster than the RC

• For example, η = 2 means that VC runs twice as fast as the RC when the channel is idle


VTCSMA

• When a channel is busy, the VC stops running

• When the channel is idle, the VC is reset according to the priority policy as follows

VC = • No change, for FCFS• RC, for EDF & LLF• 0, for SMF


Virtual time of the message

• When a message M arrives at a node, its VSM (virtual start time) is set as follows, depending on the policy:

VSM =

• AM (msg arrival time) for FCFS policy

• DM (msg deadline) for EDF policy

• LM (msg laxity) for LLF policy

• TM (msg transmission time) SMF policy

• A message is (eligible) transmitted only if its VSM <= VC

• Multiple messages may be eligible at the same time, which results in collision when they transmit simultaneously


Modifying VSM

• When a node senses the channel is idle and the node has an eligible message, it transmits the message

• If the message collides with another message, each node involved in the collision either retransmits its message immediately, with probability “p”, or modifies the VSM of its message, with probability (1 – p), to a random number drawn from an interval (A,B), where(A, B) =

• (VC, LM) for FCFS policy• (RC, DM) for EDF policy• (RC, LM) for LLF policy• (0, TM) for SMF policy


VTCSMA algorithm

New message ? Initialize VSM

Channel busy ? Initialize VC

Repeat1. Discard negative latency messages2. RC = RC + 13. VC = VC + ηUntil all messages with VSM less than VC are transmitted

OR the channel becomes busy

RC = RC + 1

Channel busy ?

Transmit message Collision ?Retransmit or modify VSM

YES

YES

NO

NO

YES

YES

NO


Impact of η

• Too large η– Too many collisions

• Too small η– Too many idle slots


VTCSMA-LLF: Example

Node RC at arrival

DM LM RC (VC) at transmission

1 0 28 12 6 (12)

2 12 38 22 22 (22)

3 16 52 36 38 (38)

4 20 68 52 45 (52)

5 20 68 52 45 (52)

Transmission status table for η = 2, transmission time = 16

cpre 458/558: real-time systems

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