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. updated 12/2001

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 updated 12/2001

Data Link Protocols updated 12/2001

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) updated 12/2001

Synchronous Protocols Bit Oriented SDLC Synchronous Data Link protocols HDLC High Level Data link protocols LAPs Link Access procedures LANs Local Area Networks updated 12/2001

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 updated 12/2001

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) updated 12/2001

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 updated 12/2001

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 updated 12/2001

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 updated 12/2001

HDLC Modes updated 12/2001

General HDLC Frame Format Sandwich the information between a header and trailer DLC Header DLC Trailer INFORMATION updated 12/2001

Initial Breakdown 01111110 01111110 INFORMATION FLAG FIELD ADDRESS CONTROL FCS updated 12/2001

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 updated 12/2001

FLAG FIELD Used to indicate beginning and end of the frame Pattern = 01111110 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 updated 12/2001

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 updated 12/2001

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 updated 12/2001

Specific HDLC Control Bits: I, S, U 1 2 3 4 5 6 7 8 9 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 updated 12/2001

Control Bits in S frames Frame type 8765 4 321 Receive ready 1000 P/F RRR Receive Not ready 1001 P/F RRR Reject 1010 P/F RRR Selective Reject 1011 P/F RRR updated 12/2001

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 updated 12/2001

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) updated 12/2001

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 351-7. An example is included. updated 12/2001

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 updated 12/2001

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 updated 12/2001

HDLC Operation - Disconnect Either sender or receiver can initiate a disconnect sends a DISC frame disconnect is accepted with a reply of UA updated 12/2001

HDLC - summary Extensive and flexible data link protocol Many subsequent link access procedures were derivatives of this updated 12/2001