Logical Link Control (CLASS NOTES) Many LANs use a Data Link protocol which is broken up into two pieces, a Logical Link Control (LLC) sublayer and a Media Access Control (MAC) sublayer.

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Logical Link Control (CLASS NOTES)


I

Many LANs use a Data Link protocol which is broken up into two pieces, a
Logical Link Control (LLC) sublayer and a Media Access Control (MAC)
sublayer.
















A

The MAC
sub
layer controls access to the physical me
di
a.

1

CSMA/CD

2

Token passing

3

Polling.

4

The MAC layer also provides frame synchronization, data link
addressing

(physical)

and error detection

(CRC)
.

B

The LLC sublayer provides an interface
between

the MAC sublayer
and

the layers above the Data Link layer.

1

The
LLC makes LAN specific access methods

of the MAC sublayer

transparent to upper layers.

2

When nodes use multiple upper layer protocols, the LLC layer makes
sure the message from the upper layer protocol on the transmit side of
the link is delivered to the ap
propriate layer protocol on the receive
side of the link.
















DATA LINK LAYER


NETWORK and HIGHER LEVEL LAYERS


LOGIC LINK LAYER


MEDIA ACCESS LAYER


PHYSICAL LAYER


LLC



LLC

UNIX

IP

Novell
IPX

IBM

Netbios

IBM

Netbios

UNIX

IP

Novell
IPX

Network Layer

Network Layer


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C

The LLC, being above the MAC layer in a protocol stack, is encapsulated
within the MAC layer frame.





II

The LLC layer header consists of three fields as follows:










A

The SAPs are Service Access Points used to identify which transmitter
protocol is talking to which receiver protocol in a multiple protocol LAN.
These identifiers ensure that source and destination protocols match, i.e.
IP talks to IP, IPX talks to I
PX, etc.

1

DSAP carries a Destination Service Access Point code.

2

SSAP carries a Source Service Access Point code.

3

The SAPs are eight bit/
one byte fields.

B

The control field is
an

eight bit/
one byte field which identifies the type of
frame being sent.

1

Frame ty
pes are referred to as
Protocol Data Units (PDUs)

2

LLC defines three types of PDUs

a

Information PDUs (I
-
PDUs): used for information transport. ACKs
and NAKs may also be piggybacked onto I
-
PDUs in the case of two
way data transfer.

b

Supervisory PDUs (s
-
PDUs)
: used for ACK/NAK (flow and error
control) when two way information transfer is not occurring making
piggybacking impossible.

c

Unnumbered PDUs (U
-
PDUs): used for manage
ment/control
information when ACKs/
NAKs
are
not being t
ransmitted
(unnumbered = no ACK/
N
AK frame numbers included in the control
field
)
.

3

The I
-
PDU control field has the following format:









MAC LAYER

HEADER

LLC LAYER


HEADER

MAC LAYER

TRAILER

INFORMATION FROM

UPPER LAYERS

LLC LAYER


HEAD
ER


DSAP


SSAP


CONTROL


CONTROL

0

PDU

Type ID


N(S)

THREE

Bits


P / F

Poll/ Final

Bit

N(R)

THREE

Bits



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a

The I
-
PDU indicates
the

frame con
tains information following the

control field.

b

An I
-
PDU frame is indicated by an initial zero in the control field.

c

N(S) and N(R) indicate sent and received frame numbers
respectively.

These bits are used with the sliding window flow
control process.

d

The Poll
/
Final bit only has meaning when it is a logic level 1. It is
used by the source to solicit a response (Poll)

from

the destination.
It is used by the destination to respond to a
poll
.

4

The S
-
PDU control field has the following format:










a

The P/F and N(R) bits are used as above.

b

The
two bit code is used to identify one of three types of
supervisory frames.

(i)

Receive Ready (RR): used by
an

information receiving node to
send an ACK/NAK when it has no information to send and
cannot piggyback the ACK/NAK on an I
-
PDU.

(ii)

Receiver Not Ready (RNR): used by a information receiving
node to send an ACK/NAK and request tha
t no more PDUs be
sent until an RR S
-
PDU is delivered

(iii)

Reject (REJ): this is the Negative Acknowledgement (NAK) sent
when
there is no information to be sent and piggybacking on an
I
-
PDU is impossible
.

5

The U
-
PDU control field has the following format:










a

The P/F bit is used as above.

b

Each unnumbered PDU has 5 bits to identify 32 possible functions.
Some of the functions are used in connection oriented services.
Some specific functions identified by the codes are the following:

(i)

AC: for acknowledged co
nnectionless information.


CONTROL

Two bits
=

10

PDU

Type ID


TWO

BIT

CODE

P / F

Poll/ Final

Bit

N(R)

THREE

Bits


CONTROL

Two bits
=

11

PDU

Type ID


TWO

BIT

CODE

P / F

Poll/ Final

Bit

THREE

BIT

CO
DE



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(ii)

DISC: disconnect.

(iii)

FRMR: frame reject.

(i v)

SABME: set asynchronous balanced mode.

(v)

TEST: loopback test.

(vi )

UI: Unnumbered information.

(vii)

UA: Unnumbered acknowledged.

(viii)

XID: Exchange IDs.


III

As indicated above, the LLC sublayer supplies services

or interfaces between
upper layer functionality and the MAC layer. Two type of services offered are
for connection oriented protocols and for connectionless protocols. With
connectionless protocols, the LLC sublayer can service both those with
acknowledg
ements and those without.

The LLC service tree looks as follows:

















A

Certain types of supervisory/management instructions must pass between
the upper layers and the LLC sublayer for each type of service rendered.
These instructions are often

referred to as primitives.

B

Connectionless without acknowledgement has but two primitives.

1

DL
-
UNITDATA.request

2

DL
-
UNITDATA.indication.

3

D
ata flow with
these
primitives can be pictured as follows:








SERVICE


CONNECTIONLESS

CONNECTION

ORIENTED

WITHOUT

ACKNOWLEDGEMENT

WITH

ACKNOWLEDGEMENT

Upper

layers


LLC

Upper

layers


LLC


Peer
-

to


Peer


Process

DL
-
UNITDATA.

request

DL
-
UNITDATA.

indication


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C

Connectionless with acknowledgement has
more

prim
itives.

Three such
primitives are as follows:

1

DL
-
DATA
-
ACK.request.

2

DL
-
DATA
-
ACK.indication.

3

DL
-
DATA
-
ACK
-
STATUS.indication.

4

D
ata flow with
these
primitives can be pictured as follows:











D

Connection oriented systems contains many primitives. Some o
f these
are as follows:

1

DL
-
CONNECT.request.

2

DL
-
CONNECT.indication.

3

DL
-
CONNECT.response.

4

DL
-
CONNECT.confirm.

5

DL
-
DISCONNECT.request.

6

DL
-
DISCONNECT.indication.

7

DL
-
DISCONNECT.response.

8

DL
-
DISCONNECT.confirm.

9

DL
-
DATA.request.

10

DL
-
DATA.indication.

11

D
ata flow with
these primitives can be pictured as follows (nex
t page)
:

Upper

layers


LLC

Upper

layers


LLC


Peer
-

to


Peer


Process

DL
-
DATA
-
ACK
.

requ
est

DL
-
DATA
-
ACK
.

indication

DL
-
DATA
-
ACK
-
STATUS
.

indication


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of 6






















Upper

layers


LLC

Upper

layers


LLC


Peer
-

to


Peer


Process

DL
-
DATA
.

request

DL
-
CONNECT
.

indication

DL
-
CONNECT
.

request

DL
-
CONNECT
.

indication

DL
-
CONNECT
.

confirm

DL
-
CONNECT
.

response

DL
-
DATA
.

request

DL
-
CONNECT
.

indication

DL
-
DATA
.

request

DL
-
CONNECT
.

indication

DL
-
DATA
.

request

DL
-
CONNECT
.

indication

DL
-
DISC
ONNECT
.

request

DL
-
DISCONNECT
.

indication

DL
-
DISCONNECT
.

confirm

DL
-
DISCONNECT
.

response