1. 3.2 Data Link Layer : Error & Flow Control
data link configurations
connectionless & connection oriented services
flow control - avoiding buffer overflow
error control - backward error control via re-transmission
data link protocols - stop & wait, sliding window
protocol performance issues - link utilization
the high level data link control (HDLC) protocol

2. Line Configurations
Topology
point to point
multipoint
Duplicity
simplex
half duplex
full duplex .

3. Point to Point - example
Establishment - ENQ (enquiry), ACK, NAC, ERP (error procedure)
Data Transfer
Termination - EOT
ENQ A
ACK B
frame
ACK

4. Multipoint Links - Primary & Secondary Nodes
Polling
P : POLL - X (note: need address)
X : NAK or X : frame to P
P : ACK
Select
P : SEL - X
X : ACK
P : frame to X

5. Flow Control - Stop & Wait
Receiver (B) needs one buffer
A sends Frame to B
B sends ACK to A
A sends Frame to B
...
Assumes no errors, in frames or ACKs
Small frames, or blocks, are:
fairer, fewer re-xmit, smaller buffers required

6. Performance - Link Utilization = U
TF = total time to send frame
= one way propagation delay
TF = Tf +2  +Tack ~= Tf +2 
U ~= n Tf / (2n  +nTf) => U = 1/ (1+2a)
where a =  / Tf Tf B A 

7. Definition of Parameter “a”
a = propagation time / transmission time
d = distance
V = velocity of light = 2 or 3 * 8 meters/sec
R = data rate in bits/sec
L = length of frame in bits
a =  / Tf
a = (d/ V)/ (L/ R)
a = Rd/ VL

8. Impact of a on Utilization
a =  / Tf < 1 Tf  B A Tf
a > 1  B A

9. Flow Control - Sliding Window
Consider A to B on full duplex link
B allocates N frame buffers
each frame given sequence number
ack includes 1+ sequence# of last frame rec’d
A can send N frames before receiving an ACK
frames that can be sent
sender
received
receiver

10. Flow Control - Sliding Window (N=4)
A’s window: & B’s window:
A sends B F0, F1, F2 & B receives all
A’s window: & B’s window: 3
B sends ACK 3
A’s window: & B’s window:
A receives ACK 3
A’s window: & B’s window:
U = 1 if N > 2a+1 else U = N/ (1+2a)

11. Error Control-ARQ (automatic repeat request)
Two basic error types:
lost frame (or ack/ control frame)
damaged frame (or ack/ control frame)
Control Mechanisms
error detection via CRC
retransmission after timeout
negative acks & retransmission
Three Common Versions:
Stop & Wait ARQ
Go-back-N ARQ
Selective-reject ARQ

12. Automatic Repeat Request protocols (ARQ)
sender required to retransmit erroneous frames
receiver detects errors
fewer bits needed to reliably detect errors
sender notified to re-send frame
positive ack of frames received
sender times out on frames that are not acknowledged and re-transmits
negative acknowledgement for frames received with errors - more efficient

13. Error Control
Stop & Wait ARQ (automatic repeat request) F0 A1 F1
alternate 0/ 1 labels
to avoid receiving
duplicates A0
F0 lost
F0 re-sent after timeout
A1 lost
F0 re-sent
after timeout

14. Sliding Window Based- Go-back-N ARQ
Damaged Frame
B has correctly received frames up to I-1, A sends I, B detects error, sends NAK I
frame I lost in transit, I+1 sent, B receives I+1 out of order, sends NAK I, else
A sends nothing after I, hears nothing, resends after timeout
Damaged ACK
B receives I, sends ACK I+1 which is lost, A may receive subsequent ACK or timeout
Damaged NAK
A will eventually timeout and resend all

15. Go-back-N ARQ
frames
ACK1
ACK3
ACK5
ACK2
ACK4
NAK2 X X X X
error
discarded

16. Performance of Stop & Wait ARQ
Without frame xmission errors & Tack = 0
U = Tf / ( Tf + 2 )
U = 1/ (1 + 2a) where a =  / Tf
With frame probability error = P & Tack = 0
U = Tf / Nretrans(Tf + 2 )
U = 1/ Nretrans(1 + 2a)
Nretrans =  i P i-1(1-P) = 1/ (1 - P)
U = (1 - P)/ (1 + 2a)

17. Performance of Sliding Window: no errors
N = window size, Tack = 0 & Tf = 1
if N > 1 + 2a then receive an ACK before window exhausted, can xmit continuously
=> U = 1
if N< 1 + 2a window exhausted at t0 + N, must wait till t0 + 2a + 1 to send again, thus get N time units busy during 2a + 1
=> U = N/ (1 + 2a)

18. Performance of Sliding Window (N<1+2a)B t0 A
(assuming Tf = 1) fa
. . . f2 f1
t0+a
fa+1 f3 f2
. . .
t0+a+1
ack1 N
fa+3
fa+2
t0+2a+1
. . .
ack1
=> N frames sent in 1+2a time => U = N/ (1+2a)

19. Performance of Selective Repeat with Errors
without errors:
U = 1 if N> 1+2a else = N/ (1+2a)
U = time to send 1 frame / total time per frame *Nr
where Nr = number of times must be resent
U = 1/ Nr if N> 1+2a else N/ (1+2a)*Nr
again Nr = f(i) P i-1(1-P) = 1/ (1-P)
thus U = (1-P) if N>1+2a else (1-P)N/ (1+2a)

20. Performance of Go-back-N ARQ : with errors
again just consider the Nretrans, ie
U = Tf / Nr (Tf + 2 )
U = 1/ Nr (1 + 2a)
here each erroneous frame requires retransmission of K frames
Nr = Exp{#frames xmitted to send 1}
Nr = f(i) P i-1(1-P)
where f(i) = total # frames xmitted if 1st frame must be sent i times = 1 + (i-1)K
Nr = 1 - K + K/ (1 - P)

21. Performance of Go-back-N ARQ
U = 1/ Nr(1 + 2a)
Nr = 1 - K + K/ (1 - P)
U = 1/ (1 - K + K/ (1 - P))(1 + 2a)
note K ~= 2a + 1 for N> 2a + 1 etc, thus:
U = (1 - P)/ (1 + 2aP) if N > 1 + 2a
U = N(1 - P)/ (1 + 2a)(1 - P + NP) if N < 1 + 2a

22. Performance Comparison
1.0
Go-back-N=127
Go-back-N=7
0.5
stop&wait
Utilization
0.1
1.0 10
1000
a =  / Tf

23. Sliding Window with Selective Repeat & NAKs
window size = 2, sequence range =4
frames
ACK0
ACK3
ACK2
ACK1
NAK2
ACK0 X
Tanenbaum Section 3.4.3
error

24. HDLC-High Level Data Link Control-ISO 3309
Three stations types:
primary - issues command frames
secondary - response frames
combined - primary & secondary fcns
Two Link configurations
unbalanced - 1 primary & full/half duplex
balanced - only point to point combined
Three Data Transfer Modes
normal response mode (NRM) - unbalanced
asynchronous balanced mode (ABM)
asynchronoous response mode (ARM) -unbal.

25. HDLC -synchronous xmission -1 frame format
flag - 8 bits -
(bit stuffing used to avoid in data)
address - one or more octets
control 8 or 16 bits
information - variable
frame check sequence CRC-16 or 32
flag - 8 bits - 1 flag can close & open frames
flag address control data FCS flag

26. Control Field Format - three frame types
Information frame (ARQ piggybacked in data)
send-seq# P/ F rec-seq#
(3 or 7 bits) (3 or 7 bits)
Supervisory (ARQ if not piggybacked)
sup-fcn P/ F rec-seq#
Unnumbered (other functions)
unn-fcn P/ F unn-fcn

27. HDLC Commands & Responses
I - Information exchange user data
S - Supervisory
RR ACK, ready to receive I
RNR ACK, not ready to receive
REJ NAK, go back N
SREJ NAK selective reject
U - Unnumbered
SNMR/ SNME set mode, 2 octet control
SARM/ SARME set mode, 2 octet control
SABM/SABE, SIM, DISC, UA, DM, RD, RIM, UI,UP, RSET, XID, TEST, FMR

28. HDLC example - one way, reject recovery
I - 1
I - 2 X
I - 3
REJ - 2
I - 2
I - 3

29. HDLC example - busy condition
RNR - 2
RR - P
RNR - 2
I - 1
RNR - 2
RR - P

30. Protocol Analysis - Finite State Machines
FSM model of communication protocol objects
state defined for sending, receiving, link objects
typical state = waiting (some event)
very large number of states for simple protocols
state transitions occur when events occur
FSM = (S, M, I, T) where:
S is set of states of all objects
M is set of all types of messages
I is the set of object initial states
T is the set of state transitions
correctness, reachability, deadlock

31. • Petri Net Models places can be occupied by tokens
directed arcs connect places and transitions
transitions are enabled if all its input arcs come from places occupied by tokens
enabled transitions can fire removing all incoming tokens and placing tokens at all places connected to it by output arcs •

32. Tanenbaum’s Protocol 3 - Petri Net Model
link
send 0
0 on link
loss
wait
ack 0
sender
timeout
receiver
ack on link
loss
send 1
wait
ack 1
timeout
1 on link
loss