? is the oldest layer 2 protocol
designed by IBM in 1975 to carry
traffic (see later). SDLC is the precursor to HDLC.
? was created out of SDLC by the
ISO in 1979 (see later).
? was developed by IBM to
enable SNA traffic over X.25.
? 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
? provides HDLC services for
? is used on the ISDN D
channel for packet delivery.
? is used on the ISDN Terminal Adapters to allow multiple users
across a single link.
? was developed by the IEEE 802.2, and is
used to provide HDLC style services on a LAN (see below).
? , some operations are similar to LAPD and V.120.
? is a derivation of HDLC and encapsulates Protocol Data Units
for transport across point-to-point links.
Although a subset of HDLC, SDLC was developed by IBM before HDLC, and was
the first link layer protocol based on synchronous, bit-oriented
operation. IBM defined SDLC for managing synchronous serially transmitted
bits over a data link, and these links can be full/half-duplex, switched
or unswitched, point-to-point, point to multipoint or even looped. SDLC
is designed for carrying SNA traffic.
In SDLC, a link station is a logical connection between adjacent nodes.
Only one is allowed on an SDLC line. A device can
be set up as a Primary or a Secondary link station. A device configured
as a Primary link station can communicate with both PU 2.0 nodes and PU
2.1 nodes (APPN) and controls the secondary devices. If the device is set
up as a secondary link station then it acts as a PU 2.0 device and can
communicate with Front End Processors (FEP), but only communicates with
the primary device when the primary allows it, i.e. the primary sets up
and tears down the connections and controls the secondaries.
In APPN configurations the device can support negotiable link stations
where XID frames are exchanged to decide which station is to be secondary
and which is to be primary.
A primary station issues commands, controls the link and initiates
error-recovery. A device set up as a secondary station can communicate
to a FEP, exist with other secondary devices on an SDLC link and exist
as a secondary PU 2.0 device.
SDLC supports line speeds up to 64Kb/s e.g. V.24 (RS-232) at 19.2Kb/s,
V.35 (up to 64Kb/s) and X.21.
The following diagram shows the frame format for SDLC, almost identical
? - Begins and ends the error checking procedure with which
is in binary. ? - 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.
? - this identifies the frame's function and can be one of
o - contains the which
is the number of the next frame to be sent, and the
which is the number of the next frame expected
to be received. The is also a which
performs error checking.
o - this can report on status, ask for and stop
transmission and acknowledge frames.
o - this does not have sequence numbers (hence
'unnumbered'), it can be used to start up secondaries and can
sometimes have an Information field.
? - can contain or
? - this check is carried out on the sending
AND receiving of the frame.
In a poll, the address field identifies the station being polled, in a
response, the address field contains the station transmitting, so this
field effectively is the secondary station's address. The control field .
HDLC is the protocol which is now considered an umbrella under which many
Wide Area protocols sit. ITU-T developed HDLC in 1979, and within HDLC
there are three types of stations defined:
? - this completely controls all data link operations
issuing commands from secondary stations and has the ability to hold
separate sessions with different stations.
? - this can only send responses to one primary
station. Secondary stations only talk to each other via a Primary
? - this can transmit and receive commands and
responses from one other station.
Configuring a channel for use by a station can be done in one of three
? - 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.
? - 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.
? - where two combined stations communicate over a
point-to-point link which can be full/half-duplex or
When transferring data, stations are in one of three modes:
? where the secondary station needs
permission from the primary station before it can transmit data.
Mainly used on multi-point lines.
? where the secondary station can
send data without receiving permission from the primary station.
This is hardly ever used.
? where either station can initiate
transmission without permission from the other. This is the most
common mode used on point-to-point links.
The following diagram details the HDLC frame format:
The HDLC frame begins and ends the error checking procedure with which
is in binary.
There are three types of HDLC frame types defined by the control field:
? 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
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.
? 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.
? are used for link initialisation or link
disconnection. The Unnumbered bits indicate the type of Unnumbered
frame being used.
LLC is a subset of and uses the
subclass of HDLC. It sits in the Data Link layer between the MAC layer and the layer 3 protocols and forms an
important part of the 802.2 specification.
There are three Classes of LLC:
1. - , this service sends and receives
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 operations which means that
are exchanged without connections, there
is no sequencing, acknowledgement or error-checking of PDUs.
2. - , this is a
connection-oriented service that provides a point-to-point link
between . 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 or
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. - , this is a
'connectionless-acknowledged' implementation that is rarely used.
The following diagram shows the structure of the LLC PDU.
This sits at the start of the data part of the Ethernet frame (see the
The following table lists the LLC command PDUs:
Type 1 Unnumbered Unnumbered 03 (CL) Information (UI) (U)
Exchange Exchange Unnumbered Identification Identification AF, BF (U) (XI) (XI)
Unnumbered Test (TEST) Test (TEST) E, F3 (U) Type 2 Information 00 00 to Information (I) Information (I) (CO) (I) FE FF
Receiver Ready Receiver Ready Supervisory 01 00 to (RR) (RR) (S) 01 FF
Receiver Not Ready Receiver Not Supervisory 05 00 to (RNR) Ready (RNR) (S) 05 FF
Supervisory 09 00 to Reject (REJ) Reject (REJ) (S) 09 FF
6F, 7F Set Asynchronous Unnumbered Unnumbered (SABME) Balance Mode Acknowledgement (U) and 63, Extended (SABME) (UA) 73 (UA)
Disconnected Unnumbered (DISC) Disconnect (DISC) Mode (DM) (U) and 0F,
Frame Reject Unnumbered 87, 97 (FRMR) (U)
Ack Ack Type 3 Unnumbered Connectionless, Connectionless, 67, F7 (AC) (U) seq 0 (AC0) seq 0 (AC0)
Ack Ack Unnumbered Connectionless, Connectionless, E7, F7 (U) seq 0 (AC1) seq 0 (AC1)
The LLC SAP's job is to sort the up and coming MAC frames and direct them
to the appropriate application or upper layer protocol. NetBIOS uses the
SAP address of , IP uses whilst DLSw uses and Network Management
uses . The , address does not identify any SAP to any protocol.
A more complete list follows:
00 Null SAP
42 Spanning Tree
7F ISO 802.2
FF Global SAP
The complete list can be found at
The Control field identifies sequencing and/or commands/responses. There
are three types:
? The carries out a numbered information
transfer in Type 2 operations.
? The carries out acknowledgement, retransmit
requests and suspends of I PDUs in Type 2 operations.
? The carries out some control functions and
unsequenced information transfer in Type 1 operations.
There is the special case of where the
DSAP and SSAP are set to and the Control field set to . Then,
immediately after the control field there are three bytes set aside for
the and then two bytes for the
(see the IPX/SPX document).
The SAP is generally used for the 802.x compliant protocols, however, the
idea of SNAP is to allow non-IEEE compliant protocols to become
pseudo-compliant without a major re-write of code for network drivers.
An OUI of 00-00-00 indicates that the frame is an Ethernet frame rather
than an 802.3 frame that is not assigned to any particular vendor. This
useful for when the frame crosses different media. On coming out the other
side it will be rebuilt as an Ethernet frame rather than something else.