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

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

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

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

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

ICSA 341 Data Communications & Networking Initial Breakdown 01111110 01111110 FLAG FIELD INFORMATION ADDRESS CONTROL FCS ICSA 341 Data Communications & Networking

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

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

Specific HDLC Control Bits: I, S, U 1 2 3 4 5 6 7 8 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

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

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

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

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

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

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

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

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