1. High level Data Link Layer Protocol - HDLC
Transmission Systems
High level Data Link Layer Protocol - HDLC
Note to instructors: This chapter is covered in it’s entirety except for in-depth discussion on character oriented protocols and the detailed analysis frame types and bit patterns. The idea is to have the students understand the concepts, operation and issues surrounding frame based communication. HDLC is the basis for many data link protocols and we use it as our foundation.
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2. High level Data Link Control
Data Link Control – layer 2
Specifies flow and error control for communication
Arranges data into frames, supplemented by control bits
Receiver checks control bits:
No problem, strips them and passes data;
Problem detected, communicate with the sender to correct the problem
Recall chapter 10
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3. Data Link Protocols
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4. Data Link Protocols Asynchronous protocols Synchronous protocols
Feature Start & Stop Bits, variable-length gaps
Synchronous protocols
Interprets a transmission frame as a series of characters
Control information is in the form of an existing character encoding system (ASCII)
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5. Synchronous Protocols Bit Oriented
SDLC
Synchronous Data Link protocols
HDLC
High Level Data link protocols
LAPs
Link Access procedures
LANs
Local Area Networks
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6. High-Level Data Link Control HDLC
Specifications developed by the ISO
Superset of SDLC (used in IBM’s SNA)
Specified as the OSI layer two protocol
Supports half & full duplex over point-to-point and multi-point links
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7. HDLC Station categories
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)
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8. HDLC Link configurations
Unbalanced (top)
Master /Slave
Symmetrical
One physical station and two logical functions
Balanced (bottom)
Both stations are the combined type – point-to-point
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9. 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
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10. HDLC Data Transfer Modes
Asynchronous response mode (ARM)
Unbalanced link configuration w/single primary and multiple secondary stations
Secondary stations are allowed to transfer data without a poll from the primary station
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11. HDLC Modes
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12. General HDLC Frame Format
Sandwich the information between a header and trailer
DLC Header
DLC Trailer
INFORMATION
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13. Initial Breakdown 01111110 01111110 INFORMATION FLAG FIELD ADDRESS
CONTROL
FCS
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14. 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 for system management
May carry information for management
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15. FLAG FIELD Used to indicate beginning and end of the frame
Pattern =
Zero bit stuffing
inserting an extra zero whenever there are five consecutive 1s in the data
receiver does not mistake the data for a flag
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16. ADDRESS Address of the secondary station for this transmission
8 bit field, 7 bit address (if LSB is a 0, next 8 bits extend the address)
address is all 1’s - broadcast address
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17. 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
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18. 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
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19. Control Bits in S frames
Frame type
Receive ready P/F RRR
Receive Not ready P/F RRR
Reject P/F RRR
Selective Reject P/F RRR
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20. Unnumbered Frames SNRM – Set Normal Response Mode
SARM – Set Asynchronous Response Mode
SABM – Set Asynchronous Balanced Mode
UP – unnumbered polling
UA – unnumbered acknowledge
DISC – disconnect
RD – request disconnect
DM – Disconnect mode
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21. Other fields Frame Check Sequence Information
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)
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22. HDLC Operation - Initialization
Alerts the other side that initialization is requested
Indicates which of the three modes (NRM, ARM, ABM) is to be used
Indicates whether a 3 or 7 bit sequence number is to be used during the exchange
Accomplished through the unnumbered control format
The examples that were part of this slide set were removed because they are not consistent with the format provided in the book. There are several examples of operation frame flow on pages An example is included.
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23. 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
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24. Data transfer example - Peers
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
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
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25. HDLC Operation - Disconnect
Either sender or receiver can initiate a disconnect
sends a DISC frame
disconnect is accepted with a reply of UA
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26. HDLC - summary Extensive and flexible data link protocol
Many subsequent link access procedures were derivatives of this
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