SDLC is synchronous bit oriented protocol developed by IBM for serial-by-bit information transfer over a data communication channel. Using EBCDIC, data is transferred in frames. Primary station controls data transfer and issues command while secondary station receives command responses to primary.
HDLC is superset of SDLC. Hence, it gives added facilities of extended addressing, CRC-16, extended control field, where 127 frames can be sent together without receiving an acknowledgement. It also allows balance mode of operation analogous to point to point communication...
2. Prof Madhumita Tamhane
SDLC
✓ Synchronous Bit Oriented Protocol by IBM.
✓ Simplex/half duplex/full duplex.
✓ Sliding window protocol.
✓ Language EBCDIC.
✓ Data frame 256 characters long.
✓ All data link control functions performed by a single
control field.
✓ Master-slave environment only.
3. Prof Madhumita Tamhane
SDLC
✓ The primary station controls data exchange on the
communication channel and issues command.
✓ The secondary station receives commands and returns
responses to the primary.
✓ Three transmission states:-
✓ Transient state:-before and after the initial
transmission and after each line turnaround.
✓ Idle state:- after 15 or more consecutive 1s have been
received.
✓ Active state:- whenever either primary or secondary
station is transmitting information or control signals.
4. Prof Madhumita Tamhane
SDLC
✓ General frame structure:
Flag Address Control Data… FCS Flag
8 bits 8 bits 8 bits Variable
multiple
of 8
16 bits 8 bits
5. Prof Madhumita Tamhane
SDLC - Flag
✓ 7EH binary or “=“ in EBCDIC.
✓ For synchronization and indication of limits of frame.
✓ Multiple frames can have following variations:
✓ Beginning and ending flag of a frame.
✓ FCS 01111110 01111110 address …
✓ Ending flag of a frame can be common with beginning
flag of next frame.
✓ FCS 01111110 address control…
✓ Ending ‘0’ of ending flag of a frame can be common with
beginning ‘0’ of beginning flag of next frame.
✓ FCS 011111101111110 address control…
✓ Can be sent continuously to indicate idle line 1s.
Flag Flag
01111110 01111110
6. Prof Madhumita Tamhane
SDLC - Address
✓ 256 addresses possible.
✓ 00H null or void address, used for network testing.
✓ FFH is broadcast address.
✓ 254 unique station addresses or group addresses.
✓ No address given to primary as…
✓ Primary to secondary frame has destination secondary
address.
✓ Secondary to primary frame has source secondary
address.
Address
8 bits
7. Prof Madhumita Tamhane
Control Field
!
!
!
!
✓ 8 bit long.
✓ Used for polling, acknowledging previously received
frames and other data link management functions.
✓ Defined in three different frame formats.
✓ Information frame
✓ Control frame
✓ un-numbered frame
Control
8 bits
8. Prof Madhumita Tamhane
Control Field – Information frame
!
!
!
!
!
✓ LSB b7 – “0” identifies information frame.
✓ Information frame is used for transmitting sequenced
information.
✓ Packet numbered using 3-bit scheme.
!
P/P
Or
F/F
b0 b1 b2 b3 b4 b5
nr
0
b6 b7
ns
9. Prof Madhumita Tamhane
Control Field – Information frame
✓ ns =b4, b5, b6 - number of frame being sent.
✓ From 000 to 111 and repeats.
✓ nr =b0, b1, b2 - acknowledgement.
✓ It is number of next frame it expects to receive.
✓ Indicates all frames before nr are received correctly.
✓ Uses piggybacking as it is I frame.
✓ nr and ns together used for flow control and error
control ARQ.
P/P
Or
F/F
b0 b1 b2 b3 b4 b5
nr
0
b6 b7
ns
10. Prof Madhumita Tamhane
Control Field – Information frame
✓ Primary keeps track of ns and nr of all secondaries
while secondary keeps track of its own ns and nr.
✓ A station can not send more than 7 frames together
before receiving an acknowledgement to avoid
ambiguity in received frame number.
✓ Say 8 frames, 0 - 7 are sent and ACK0 is received.
✓ There is ambiguity whether
✓ receiver received all correctly and expecting new
frame “0” or
✓ frame “0” had error and receiver aborted and
expecting retransmission from old frame “0”.
✓ No ambiguity if max, only 7 frames, 0 - 6 are sent.
12. Prof Madhumita Tamhane
Control Field – Information frame
✓ Bit b3 is poll/not-a-poll (1/0) and sent from primary to
secondary.
✓ Bit b3 is sending final /not-a-final frame(1/0) when sent from
secondary to primary.
✓ A secondary can not transmit unless it receives a frame
addressed to it with P bit set.
✓ With I frame, primary can select a secondary station, send
formatted information, confirm previously received I frame,
and poll with a single transmission.
P/P
Or
F/F
b0 b1 b2 b3 b4 b5
nr
0
b6 b7
ns
13. Prof Madhumita Tamhane
Excercise
✓ Determine bit pattern for the control field of a frame
sent from the primary to a secondary for following
conditions..
✓ Primary is sending frame 3.
✓ It is a poll.
✓ Primary is confirming the correct reception of frames
2,3,4 from secondary.
!
!
✓ Solution: 10110110 = B6H
14. Prof Madhumita Tamhane
Control Field – Supervisory frame
✓ Information sending is not allowed.
✓ Frame is used to :
✓ 1. Confirm previously received I-frames.
✓ 2. Convey ready or busy condition.
✓ 3. Reports missing or error frames.
✓ Hence used for flow and error control and Poll/Select
when I frame not available.
15. Prof Madhumita Tamhane
Control Field – Supervisory frame
✓ Frame identified by 01 in b6, b7 position.
✓ nr =b0, b1, b2 -- frame expected
✓ acknowledgement of correct reception.
✓ sequence number of frame in error
✓ b3 is Poll/Not Poll or Final/Not final .
✓ Bits b4 and b5 are indicate receive status of transmitting
stations, request transmission or retransmission of
sequenced information frames.They can be…
P/P
Or
F/F
b0 b1 b2 b3 b4 b5
nr
1
b6 b7
X X 0
16. Prof Madhumita Tamhane
Control Field – Supervisory frame
b b Receive Status
0 0 Ready to receive RR
Positive acknowledgement when I frame not available.
0 1 Ready Not to receive RNR
1 0 REJ(Reject) Go back N ARQ
1 1 Not Used in SDLC
17. Prof Madhumita Tamhane
Control Field – Supervisory frame
Options
✓ RR with P set.
✓ Polling.
✓ Ack when primary wants secondary to send more frames.
✓ RR with F set.
✓ EOT after poll
✓ Positive Ack to selection
✓ Negative response to polling
✓ RNR with P set.
✓ Selection
✓ RNR with F set.
✓ Negative Ack to selection
✓ RR with F = 0.
✓ ARQ REJ of nr (only FD)
18. Prof Madhumita Tamhane
Excercise
✓ Determine bit pattern for the control field of a frame
sent from the a secondary to primary for following
conditions..
✓ Secondary is ready to receive.
✓ It is a final frame.
✓ Secondary is confirming the correct reception of
frames 3,4,5 from secondary.
!
!
✓ Solution: 11010001 = D1H
19. Prof Madhumita Tamhane
Excercise
✓ Determine bit pattern for the control field of a frame
sent from the a secondary to primary for following
conditions..
✓ Secondary is ready to receive.
✓ It is not a final frame.
✓ Secondary is conveying correct reception of frames
2,3,4.
✓ Secondary is conveying that frame 5 is in error.
!
!
✓ Solution: 10100001 = D1H
20. Prof Madhumita Tamhane
Control Field – Un-numbered Frame
!
!
!
!
✓ Used for network management operations.
✓ Un-numbered Management Information sending is
allowed.
✓ Frame identified by 11 in b6, b7 position.
✓ b3 is Poll/Not Poll or Final/Not final
✓ b0, b1, b2 , b4, b5 – 32 command/responses possible.
P/P
Or
F/F
b0 b1 b2 b3 b4 b5
1
b6 b7
X X 1
X X X
21. Prof Madhumita Tamhane
Control Field – Un-numbered Frame
✓ Possible modes of secondary
✓ The Initialization mode:System specific
✓ Normal Response mode:A secondary can not initiate
unsolicited transmission.
✓ Can transmit only in response to a frame received
with P bit set.
✓ Normal Disconnect mode: Secondary is off-line.
✓ Secondary can receive only a TEST, XID, CFGR,
SNRM, or SIM command from primary and can
respond only if P bit is set.
22. Prof Madhumita Tamhane
Control Field – Some Un-numbered commands
✓ Un-numbered Information (UI)
✓ 000 P/F 0011
✓ Higher level status, operational interruptions, time of day,
link initialization parameters etc.
✓ Set Initialization Mode (SIM)
✓ 000 P 0111
✓ SIM resets nr and ns counter at primary and
secondary.
✓ Secondary Responses by UA.
✓ Used if network management error noted.
✓ To bring a station out of disconnect mode.
23. Prof Madhumita Tamhane
Control Field – Some Un-numbered commands
✓ Request Initialization Mode (RIM)
✓ 000 F 0111 Response
✓ Request sent by secondary for SIM from primary if
error noted or needs to come out of DISC.
✓ Set Normal Response Mode (SNRM)
✓ 100 P 0011 Command
✓ Primary places secondary in normal response mode.
✓ Resets nr and ns counter at primary and secondary.
✓ Secondary responses by UA.
24. Prof Madhumita Tamhane
Control Field – Some Un-numbered commands
✓ Request disconnect mode (RD)
✓ 010 F 0011 Response
✓ Disconnect (DISC)
✓ 010 P 0011 Command
✓ In this mode secondary can not receive/send I frames.
✓ Can receive only SIM, SNRM.
✓ Secondary responses by UA.
✓ Disconnect mode (DM)
✓ 000 F 1111 Response
✓ Disconnected secondary reminds primary about its status if later
tries to send un-numbered frame.
✓
25. Prof Madhumita Tamhane
Control Field – Some Un-numbered commands
✓ Un-numbered acknowledgement (UA)
✓ 011 F 0011 Response
✓ In response to SIM, SNRM, DISC
✓ TEST 111 P 0011
✓ Command/Response to solicit/reply TEST response.
✓ Secondary returns back information field.
✓ Exchange for link testing purpose.
26. Prof Madhumita Tamhane
Control Field – Some Un-numbered commands
✓ Exchange Station Identification (XID)
✓ 101 P 1111 Command/Response
✓ XID solicits identification of secondary station.
✓ I field can be included in the frame to convey
identification data of either primary or secondary
station.
✓ EX- In dial-up circuits, it is necessary that secondary
station identifies itself before primary will exchange
information frames with it.
!
27. Prof Madhumita Tamhane
Control Field – Some Un-numbered commands
✓ Frame Reject (FRMR) 100 F 0111 Response
✓ Reasons:
✓ Control field has invalid command.
✓ Data exceeds buffer .
✓ Information sent in frame that doesn't allow it.
✓ ns and nr do not match.
✓ Secondary keeps sending FRMR till it receives SNRM,
DISC or SIM
28. Prof Madhumita Tamhane
Control Field – Some Un-numbered commands
I field of FRMR
!
!
!
✓ W = 1, Invalid command
✓ X = 1, Prohibited information field received.
✓ Y = 1, Buffer overrun.
✓ Z = 1, Received nr disagrees with transmitted ns.
XXXXXXXX 0XXX0XXX WXYZ 0 0 0 0
C field of rejected
command
ns nr
secondary’s present ns and nr
30. Prof Madhumita Tamhane
Transparency
✓ Flag 01111110 should be avoided in data.
✓ Using zero-bit-insertion or zero stuffing.
✓ In Data, after every five 1’s, a zero is inserted.
✓ Hence, if we get six 1’s, it is a valid flag.
✓ Example:
✓ 01111110 01101111 11010011
Flag Address Control
✓ 1110001100110101 00101011 01111110
Data FCS Flag
31. Prof Madhumita Tamhane
Transparency
✓ Transmitted:
✓ 01111110 01101111 101010011
Flag Address Control
✓ 11100001100110101 00101011 01111110
Data FCS Flag
!
✓ Received:
✓ 01111110 01101111 11010011
Flag Address Control
✓ 1110001100110101 00101011 01111110
Data FCS Flag
32. Prof Madhumita Tamhane
Message Abort
✓ Used to permanently terminate a frame.
✓ Generally done to accommodate high-priority message
such as emergency link recovery procedure,…
✓ Message abort is occurrence of 7-14 consecutive 1s.
✓ Zeros are nor inserted in message abort.
✓ Terminates an existing frame and immediately begins
a high priority frame.
✓ If more than 14 consecutive 1s occur in succession, it
places into idle line condition.
✓
34. Prof Madhumita Tamhane
HDLC
✓ In SDLC - ISO defines high order bit as bo.
✓ In HDLC - ISO defines low order bit as bo.
✓ Hence all patterns are reversed in HDLC.
b0 b1 b2 b3 b4 b5 b6 b7
SDLC
b7 b6 b5 b4 b3 b2 b1 b0
HDLC
35. Prof Madhumita Tamhane
HDLC - Modes
✓ Normal Response mode NRM:
✓ One primary, rest secondary.
✓ Primary polls/selects secondary.
✓ Unsolicited responses are not allowed.
!
✓ Asynchronous Response mode ARM :
✓ A secondary may initiate a transmission without permission
from primary if channel in idle.
✓ Primary-secondary relationship otherwise not altered.
✓ Secondary to secondary through primary only.
36. Prof Madhumita Tamhane
HDLC - Modes
✓ Asynchronous Balance mode ABM :
✓ Special arrangement.
✓ If point to point link between two secondaries is present then
Peer to peer is allowed.
✓ Mode to be invoked when desired.
✓ Primary brings a secondary in ABM mode.
37. Prof Madhumita Tamhane
HDLC - Frame
!
!
!
!
✓ General frame structure as defined by
✓ ISO 3309 – 1976 E (Frame structure, Flag and transparency
mechanism.)
✓ ISO 4335 – 1979 E (Control, I, S and U fields)
!
✓ Flag same as SDLC.
Flag Address Control Data… FCS Flag
8 bits extended extended Variable
multiple of 8
16 bits 8 bits
38. Prof Madhumita Tamhane
HDLC - Address
!
!
!
!
!
!
✓ 8 bit or extended.
✓ If bo = 0, next byte is also a part of address.
✓ If bo = 1, end of address.
Address
extended
b7 b6 b5 b4 b3 b2 b1 b0
39. Prof Madhumita Tamhane
HDLC - Address
✓ Example:
!
✓ 01111110 XXXXXXX0 XXXXXXX1
Flag Byte1 Byte2 -end of address
!
✓ Can extend up to n number of bytes.
40. Prof Madhumita Tamhane
HDLC - Control – Information Frame
Normal Mode
!
!
!
!
!
!
Control
normal
P/P
Or
F/F
b7 b6 b5 b4 b3 b2
ns
b1 b0
0
nr
41. Prof Madhumita Tamhane
HDLC - Control – Information Frame
Extended Mode
!
!
!
!
!
!
Control
extended
ns
0
P/
F nr
42. Prof Madhumita Tamhane
HDLC - Control – Supervisory Frame
Normal Mode
!
!
!
!
!
!
Control
normal
P/F
b7 b6 b5 b4 b3 b2 b1 b0
1
nr
0 X X
43. Prof Madhumita Tamhane
HDLC - Control – Supervisory Frame
Extended Mode
!
!
!
!
!
!
Control
extende
d
1
P/
F nr
0 X X 0 0 0 0
44. Prof Madhumita Tamhane
HDLC - Control – Supervisory Frame
Normal Mode
!
!
!
!
!
!
b b Receive Status
0 0 Ready to receive RR
Positive acknowledgement when I frame not available.
0 1 REJ(Reject) Go back N ARQ
1 0 RNR
1 1 SREJ (selective reject)
45. Prof Madhumita Tamhane
HDLC - Control – Supervisory Frame
✓ OPTIONS with RR:
✓ RR with ACK
✓ RR with P = 1
✓ RR with F = 1
✓ RR with F = 1
!
✓ OPTIONS with RNR:
✓ RNR with ACK
✓ RNR with P = 1
✓ RNR with F = 1
46. Prof Madhumita Tamhane
HDLC - Control – Un-numbered Frame
!
!
!
!
!
!
✓ No extended field available.
✓ Control 5 bits are reversed vis-a-vis SDLC
✓ SIM – 10000
✓ SNRM - 00001
Control
normal
P/F
b7 b6 b5 b4 b3 b2 b1 b0
1 X1 X X X X
47. Prof Madhumita Tamhane
Draw poll response sequence frames for HDLC
✓
Main A
Flag Flag
F
C
S A
S - Frame
0 P = 1 RR
Flag Flag
A
F
C
S
I - Frame
0 F = 0 0
Data
Flag Flag
A
F
C
S
I - Frame
1 F = 1 0
Data
Flag Flag
F
C
S A
S - Frame
2 P = 0 RNR
48. Prof Madhumita Tamhane
Draw select response sequence frames for
HDLC
✓
Main A
Flag Flag
F
C
S A
S - Frame
0 P = 1 RNR
Flag FlagA
FC
S
S - Frame
RR F = 1 0
Flag FlagA
F
C
S
I - Frame
0P = 00
Data
Flag FlagA
FC
S
S - Frame
RR F = 1 1
49. Prof Madhumita Tamhane
Peer to peer communication using
HDLC
✓
A B
Flag FlagB
F
C
S
I - Frame
00
Data
Flag FlagB
F
C
S
U - Frame
11P = 1100
Data
Flag Flag
A
F
C
S
U - Frame
00 F = 1 110
Data
Flag FlagB
F
C
S
I - Frame
10
Data
50. Prof Madhumita Tamhane
Contd.
✓
A B
Flag Flag
A
F
C
S
I - Frame
0 2
Data
Flag Flag
A
F
C
S
I - Frame
1 2
Data
Flag Flag
A
F
C
S
I - Frame
2 2
Data
Flag Flag
F
C
S B
S - Frame
3 RR
51. Prof Madhumita Tamhane
Peer to peer communication with error
✓
A B
Flag Flag
A
F
C
S
I - Frame
0 2
Data
Flag Flag
A
F
C
S
I - Frame
1 2
Data
Flag Flag
A
F
C
S
I - Frame
2 2
Data
Error
Flag Flag
F
C
S B
S - Frame
1 REJ
52. Prof Madhumita Tamhane
Contd.
✓
A B
Flag Flag
A
F
C
S
I - Frame
1 2
Data
Flag Flag
A
F
C
S
I - Frame
2 2
Data
Flag Flag
F
C
S B
S - Frame
3 RR