SDLC, HDLC and LLC

Overview

• Synchronous Data Link Control (SDLC) is the oldest layer 2 protocol designed by IBM in 1975 to carry Systems Network Architecture (SNA) traffic (see later). SDLC is the precursor to HDLC.
• High-Level Data Link Control (HDLC) was created out of SDLC by the ISO in 1979 (see later).
• Qualified Logical Link Control (QLLC) was developed by IBM to enable SNA traffic over X.25.
• Link Access Protocol Balanced (LAPB) is one of the three layers making up X.25 and was a development by ITU-T, of LAP which itself came from HDLC. LAPB operates like HDLC, however it is restricted to using the ABM transfer mode i.e. point-to-point links (see later).
• Link Access Protocol for Modems (LAPM) provides HDLC services for V.42 modems.
• Link Access Protocol for D-Channel (LAPD) is used on the ISDN D channel for packet delivery.
• V.120 is used on the ISDN Terminal Adapters to allow multiple users across a single link.
• Logical Link Control (LLC) was developed by the IEEE 802.2, and is used to provide HDLC style services on a LAN (see below).
• Frame Relay, some operations are similar to LAPD and V.120.
• PPP is a derivation of HDLC and encapsulates Protocol Data Units for transport across point-to-point links.

Synchronous Data Link Control (SDLC)

• Flag - Begins and ends the error checking procedure with 0x7E which is 01111110 in binary.
• Address - This is only the secondary address since all communication occurs via the single primary device. The address can be an individual, group or broadcast address.
• Control - this identifies the frame's function and can be one of the following:
  • Information (I) - contains the Send Sequence Number which is the number of the next frame to be sent, and the Receive Sequence Number which is the number of the next frame expected to be received. The is also a Poll Final Bit (P/F) which performs error checking.
  • Supervisory (S) - this can report on status, ask for and stop transmission and acknowledge I frames.
  • Unnumbered (U) - this does not have sequence numbers (hence 'unnumbered'), it can be used to start up secondaries and can sometimes have an Information field.
• Data - can contain Path Information Unit (PIU) or Exchange Identification (XID).
• Frame Check Sequence (FCS) - this check is carried out on the sending AND receiving of the frame.

High-Level Data Link Control (HDLC)

• Primary Station - this completely controls all data link operations issuing commands from secondary stations and has the ability to hold separate sessions with different stations.
• Secondary Station - this can only send responses to one primary station. Secondary stations only talk to each other via a Primary station.
• Combined Station - this can transmit and receive commands and responses from one other station.

• Unbalanced - this configuration allows one primary station to talk to a number of secondary stations over half-duplex, full-duplex, switched, unswitched, point-to-point or multipoint paths.
• Symmetrical - where commands and responses are multiplexed over one physical channel when two stations with primary and secondary parts have a point-to-point link joining them.
• Balanced - where two combined stations communicate over a point-to-point link which can be full/half-duplex or switch/unswitched.

• Normal Response Mode (NRM) where the secondary station needs permission from the primary station before it can transmit data. Mainly used on multi-point lines.
• Asynchronous Response Mode (ARM) where the secondary station can send data without receiving permission from the primary station. This is hardly ever used.
• Asynchronous Balanced Mode (ABM) where either station can initiate transmission without permission from the other. This is the most common mode used on point-to-point links.

• Information Frames are used for the data transfer between stations. The send sequence, or next send N(S), and the receive sequence, or next receive N(R), hold the frame sequence numbers. The Poll/Final bit is called Poll when used by the primary station to obtain a response from a secondary station, and Final when used by the secondary station to indicate a response or the end of transmission.
• Supervisory Frames are used to acknowledge frames, request for retransmissions or to ask for suspension of transmission. The Supervisory code denotes the type of supervisory frame being sent.
• Unnumbered Frames are used for link initialisation or link disconnection. The Unnumbered bits indicate the type of Unnumbered frame being used.

Logical Link Control (LLC)

1. LLC Class 1 - Connection-less (CL), this service sends and receives Link Service Data Units (LSDU) without the need for acknowledgement. It supports point-to-point, multipoint and broadcast communication and is suitable for higher level protocols that do all the sequencing, addressing, routing and recovery. These include IPX, TCP/IP, Vines, XNS, AppleTalk etc. LLC1 is less intense on network resources than LLC2 or LLC3 as there is less requirement for the data-integrity measures included within the following two methods. The responsibility for data flow and integrity rests with the layer 3 protocols. LLC1 operates Type 1 operations which means that Protocol Data Units (PDU) are exchanged without connections, there is no sequencing, acknowledgement or error-checking of PDUs.
2. LLC Class 2 - Connection-Oriented (CO), this is a connection-oriented service that provides a point-to-point link between Link Service Access Points (LSAP). Although LLC2 responds to the higher layer protocol with respect to opening and closing connections, LLC2 is responsible for flow control, sequencing the frames and error recovery. LLC2 can operate Type 1 or Type 2 operations, where a connection has to be established, acknowledgement, sequencing and error checking all take place in the LLC sublayer. LLC2 is generally required in environments that run protocols such as NetBIOS and SNA.
3. LLC Class 3 - Acknowledged-Connectionless (AC), this is a 'connectionless-acknowledged' implementation that is rarely used.

• The Information Transfer (I) PDU carries out a numbered information transfer in Type 2 operations.
• The Supervisory (S) PDU carries out acknowledgement, retransmit requests and suspends of I PDUs in Type 2 operations.
• The Unnumbered (U) PDU carries out some control functions and unsequenced information transfer in Type 1 operations.