Alternating%20Bit%20Protocol

1
Alternating Bit Protocol

• Protocol for simplex data-transfer channel
• data flows from sender to receiver
• control flows in both directions
• the transfer medium may corrupt messages
• The protocol attaches a status bit, the
  alternation bit to messages.
• Assume send from A to B
• B receives message and accepts it only if bits
  differ
• B sends verification message (an ack) to A
  (receipt of message)
• A receives ack and deduces
• message sent was received correctly
• send next message with flipped bit
• message sent was corrupted
• resend message
• nothing - because ack was corrupted
• resend message
• We do not indicate the exact corrpution of a
  message, just turn any corrupted message into
  error message.

2
Alternating Bit Protocol

• messages
• mtype data, control, error
• channels
• chan fromA N of byte, byte, bit /
  messagetype1, data, statusbit /
• chan toB N of byte, byte, bit /
  messagetype1, data, statusbit /

B
A
fromA
fromB
toB
toA
lower layer
3
Lower Layer

• transfer medium (perfect channel)
• proctype lower (chan fromA, toA, fromB, toB)
• byte d bit b
• do
• fromA?data(d,b) -gt toB!data(d,b)
• fromB?control(b) -gt toA!control(b)
• od

transfer medium (imperfect channel) proctype
lower (chan fromA, toA, fromB, toB) byte d bit
b do fromA?data(d,b) -gt if
toB!data(d,b) / correct /
toB!error(d,b) / error /
fi fromB?control(b) -gt if
toA!control(b) / correct /
toA!error(b) / error /
fi od
4
Sender

• Sender - without initialisation
• proctype A (chan in, out)
• byte mt / message type /
• bit at / status bit /
• bit vr / verification bit /
• FETCH / get actual data /
• out!data(mt,at) / send the data /
• do / wait for correct response /
• in?control(vr) -gt / got a control message
  /
• if
• (vr 1) -gt / correct ! /
• FETCH / get next
  actual data /
• at at -1 / alternate status bit
  /
• (vr 0) -gt / an error in
  send /

5
Receiver

• Receiver - without initialisation
• proctype B (chan in, out)
• byte mr / message type /
• byte last_mr / mr of last error-free msg /
• bit ar / status bit /
• bit lar / ar of last error-free msg /
• do
• in?error(mr,ar) -gt / receive corrupted msg /
• out!control(0)
• in?data(mr,ar) -gt / receive correct msg /
• out!control(1)
• if
• (ar lar) -gt /
  retransmission, do not accept /
• skip
• (ar ! lar) -gt / new message, accept /

6
Initialisation

• Send message from A to B (error-free, but not
  necessarily accepted) which forces setting of Bs
  alternation bit.

7
Sender

• Sender - with initialisation
• proctype A (chan in, out)
• byte mt0 / message type /
• bit at 0 / status bit /
• bit vr / verification bit /
• out!data(0,1) / initialise /
• do in?error(vr) -gt / an error in ack/
• out!data(0,1) / resend /
• in?control(0) -gt /an error in send
  /
• out!data(0,1) / resend /
• in?control(1) -gt / successful
  initialisation /
• break
• od
• FETCH / get actual data /
• out!data(mt,at) / send the data /
• do / wait for correct response /
• in?control(vr) -gt / got a control message
  /

8
Receiver

• Receiver - with initialisation
• proctype B (chan in, out)
• byte mr / message type /
• byte last_mr / mr of last error-free msg /
• bit ar / status bit /
• bit lar / ar of last error-free msg /
• bit ini0 / initialised /
• do
• in?error(mr,ar) -gt / receive corrupted msg /
• out!control(0)
• in?data(mr,ar) -gt / receive correct msg /
• out!control(1)
• if
• (!ini) -gt / not initialised /
• ini 1 lar ar / initialise /
  
• (ini) -gt / initialised /

9
Assertions

• define ACCEPT assert(mr (last_mr1)MAX)
• Full program
• define N 2
• define MAX 8
• define FETCH mt (mt1)MAX
• / sequence of numbers /
• define ACCEPT assert (mr (last_mr1)MAX)
• / messages received in sequence /
• init
• chan fromA N of byte, byte, bit
• chan toB N of byte, byte, bit
• chan fromB N of byte, bit
• chan toA N of byte, bit
• atomic run A(toA, fromA)

10
Message Sequence Charts

• 1. Message and ack is successful, 2 times.
• 2. Message is corrupted first time, success 2nd
  time.
• 3. Message is received, but ack is corrupted
  first time success 2nd time.
• Use global channel names show states of
  variables.

11
Using Spin for Simulation
Run spin on a linux machine by /local/fda/tools/i
nstall/i386-unkown-linux/bin/xspin-linux Run
from your own directory. Set Simulation
Parameters Display Mode
Simulation Style

MSC Panel

Random

Seed1 Timed Sequence

Guided

Steps skipped 0


Interactive Data Values

Full Queue

Blocks New Messages


Loses New Messages
x
x
x