1. ICSA 341 Data Communications & Networking
Data Link Protocols
ICSA 341 Data Communications & Networking

2. ICSA 341 Data Communications & Networking
HDLC Station Types
Primary Station (a station than can issue commands)
Secondary Station (a station that issues responses to commands)
Combined Stations (a station can operate as either primary or secondary, issuing either requests, or responses)
ICSA 341 Data Communications & Networking

3. HDLC Link configurations
Unbalanced (top)
(one primary and multiple secondary stations over pt-to-pt or multi-point link)
Symmetrical
(like unbalanced but control of link can shift between machines)
Balanced (bottom)
(combined station over pt-to-pt link -- a peer-to-peer arrangement)
ICSA 341 Data Communications & Networking

4. HDLC Data Transfer Modes
Normal Response Mode (NRM)
Unbalanced link configuration w/single primary and multiple secondary stations
Secondary stations can only transfer data when polled by the primary station
Asynchronous balanced mode (ABM)
Balanced link configuration w/combined stations
Either station can initiate data transfer at any time
Asynchronous response mode (ARM)
Secondary stations are allowed to transfer data without a poll from the primary station
ICSA 341 Data Communications & Networking

5. General HDLC Frame Format
Sandwich the information between a header and trailer
DLC Header
DLC Trailer
INFORMATION
ICSA 341 Data Communications & Networking

6. ICSA 341 Data Communications & Networking
Initial Breakdown
FLAG FIELD
INFORMATION
ADDRESS
CONTROL
FCS
ICSA 341 Data Communications & Networking

7. ICSA 341 Data Communications & Networking
HDLC Frame Types
Information (I) - carries user data and flow/error control information
Supervisory (S) - used to provide additional flow/error control functions
Unnumbered (U) used to provide additional link control functions
ICSA 341 Data Communications & Networking

8. ICSA 341 Data Communications & Networking
CONTROL FIELD
Either 8 or 16 bit field - used for flow management
different for each type of frame
If start bit = 0 it is an information frame
If first two bits = “10”, it is a supervisory frame
If first two bits = “11”, it is an unnumbered frame
ICSA 341 Data Communications & Networking

9. Specific HDLC Control Bits: I, S, U
N(S)
P/F
N(R) 1 S
P/F
N(R) 1 1
M(1)
P/F
M(2)
N(S) = sequence number of I frame being sent
N/R = sequence # of next expected I frame
P/F = poll/final bit
S = supervisory frame code
M(1) M(2) = unnnum. code
ICSA 341 Data Communications & Networking

10. Specific HDLC Control Bits: S
Selective Reject
RRR
P/F
1101
ICSA 341 Data Communications & Networking

11. Specific HDLC Control Bits: U
ICSA 341 Data Communications & Networking

12. Specific HDLC Control Bits: I
ICSA 341 Data Communications & Networking

13. ICSA 341 Data Communications & Networking
Other fields
Frame Check Sequence
Uses the standard CRC (16 bit)
or CRC-32 (32 bit)
Information
Variable length (usually multiple of 8 bytes)
I-frame = User Data
S-frame = does not exist
U-frame = management data (network management)
ICSA 341 Data Communications & Networking

14. Supervisory Frame Types
Receive Ready (RR)
ACK
Poll
Negative Poll
Positive Select
ICSA 341 Data Communications & Networking

15. Supervisory Frame Types
Receive Not Ready(RNR)
ACK
Select
Negative Select
Reject - Go Back n ARQ
Selective Reject - Selective Reject ARQ
ICSA 341 Data Communications & Networking

16. Data transfer example - Peers
U-frame, SABM, P=1, establishes link in ABM
Station B returns UA, F=1 indicating agreement on mode and the P/F bits will no longer be used
Station A sends I frame, numbered 0 then another numbered 1
Station A
Station B
SABM UA
data 0
data 1
data 0 ACK 2
data 1 ACK 2
data 2 ACK 2
RR, ACK 3
ICSA 341 Data Communications & Networking

17. Data transfer example - Peers
Station A
Station B
Station B now sends it’s own I frame but also ACKs I frames 0 and 1 from Station A
Station B follows with I frames 1 & 2, the ACK stays the same as no other data has been sent from Station A
Station A has no other data to send, but must ACK data from Station B, so an S-frame is sent, with RR, ACK 3
SABM UA
data 0
data 1
data 0 ACK 2
data 1 ACK 2
data 2 ACK 2
RR, ACK 3
ICSA 341 Data Communications & Networking