CS 635, Lecture 3, Part 4 Logical Link Control
The basic LLC protocol is modeled after HDLC and have similar functions
LLC Types and Classes
There are thre LLC protocols defined in the standard, one for each form
It is possible for a single station to support more than one form of service
and employ more than one of types of protocols. The combination of service
supprted is given by the station class (table 5.2, page 164). Note that
all allowable classes support type 1. This ensures that all stations on
a LAN will have a common service mode that can be used for management operations.
Type 1 operation supports unacknowledged connectionless service
Type 2operation supports connection-mode service
Type 3 operation supports acknowledged connectionless service.
LLC Protocol Data Units
All three LLCS protocols employ the same PDY format (figure 5.8, page 165)
There are three types of PDU, each with a different control field format:
used to carry user data. The control field includes a 7-bit sequence number
, N(S), associated with this PDU; it also include a piggybacking acknowledgement
sequence number, N(R)
used for flow and error control. It includes an acknowledgement sequence
number and a 2-bit S field to distinquish three different PDUs: receive
ready (RR), receive not ready (RNR), and reject (REJ)
Various protocol control PDUs. The 5-bit M field indicates which PDU.
Type 1 Operation
Type 1 operation supports the unacknowledged connectionless service.
The UI PDU is used to transfer user data. There is not ack, flow control,
or error control. The XID and TEST PDUs support management functions associated
with all three types of operation. An LLC entity may issue a command (C/R
bit = 0) XID or TEST. The receiving LLC entity issues a correcsponding
XID or TEST is response.
The XID PDU is used to exchange two types of operation: types of operation
supported and window size.
The TEST PDU is used to conduct a loopback test of the transmission. Upon
receipt of a TEST command PDU, the addressed LLC entity issues a TEST response
PDU as soon as possible.
If the DSAP and SSAP fields are null, the information field indicates which
LLC class is provided by the sending LLC entity.
If the XID includes specific DSAP and SSAP addresses, the information field
indicates which types of operation may be provided for that particular
SSAP. For a SAP that supports type 2 operation, and for a particular connection,
the information field also includes the receive window size used in the
sliding-window flow control mechanism.
Type 2 Operation
Type 2operation supports the connection-mode service. Type 2 operation
involves three phases: connection establishment, data transfer, and connection
A user issues DL-CONNECT.request to the LLC. The LLC issues a SABME PDU
(set asynchronous balanced mode extended). If the connection is accepted
by the LLC designated in DSAP, the destination LLC returns a UA PDU (unnumbered
acknowledgement). The connection from that point is uniquely identified
by the pair of user SAPs. If the destination LLC user rejects the connection
request, its LLC entity returns a DM PDU (Disconnect mode).
Once the connection is established both side can start sending data in
I PDUs, starting with a sequence number 0. An LLC sending data will number
PDUs sequentially, modulo 128, and place the sequence number is N(S). N(R)
is the ack for I-PDUs received.
S-PDUs (supervisory) are used for flow control and error control.
RR-PDU (receive ready) is used to acknowledge the last I-PDU received by
indicating the next I-PDU expected. The RR is used when there is
no reverse user data traffic to piggyback the ack.
RNR-PDU (received not ready) acks an I-PDU, as with RR, but also asks the
peer entity to suspend transmission of I-PDUs. When the LLC that issues
RNR is again ready, it sends an RR.
REJ-PDU (reject) initiates the go-back-N ARQ. It indicates that the last
I-PDU received has been rejected and that retransmission of all I-PDUs
beginning with number N(R) is required.
Either LLC can request a reset, either on their own initative or in response
to a user's DL-RESET.request. An LLCs requests a reset on a particular
connection by issuing a SABME. The remote user either accepts the reset,
which causes its LLC entity to reply with a UA, or rejects it, which causes
its LLC entity to reply with a DM (disconnection mode). When a reset occurs,
both LLC entities reset their send and receive sequence numbers to zero.
A connection can be terminated by a user issuing a DL-DISCONNECT.request
or LLC terminating the connection itself. In either case, the LLC
issues a DISC PDU to the other LLC on the connection. Ther remote LLC must
accept the disconnect by replying with a UA and issuing a DL-DISCONNECT.indication
to its user. No assumption can be made about outstanding I-PDUs.
Type 3 Operation
With type 3 operation, each PDU transmitted is acknowledged. A new unnumbered
PDU, the Acknowledged Connectiuonless (AC) Information PDU is defined.
User data are send in AC command PDUs and must be ack'ed using an AC response
PDU. To guard against lost PDUs, a 1-bit sequence number is used.
The sender alternates the use of 0 and 1 in its AC command pDU, and the
receiver responds with an AC PDU with the opposite number of the corresponding
command. Only on PDU is each direction may be outstanding at any time.
For the DL-DATA-ACK servicem the P/F bit is always set to 0. The AC
command PDU contains user data and the AC response PDU does not. For the
DL-REPLY service, the P/F/ bit is always set to 1. The AC command may or
may not contain user data. The AC response contains user data if they are
available; otherwise, it does not, which signals the other side that the
reply has failed.