Chapter 9

dargspurNetworking and Communications

Oct 27, 2013 (4 years and 12 days ago)

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Chapter 9

Logical

Link

Control

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Figure 9
-
1

IEEE 802 Project

LLC:



allows
interoperability

between different LAN protocols.



makes LAN
-
specific aspects such as access method, encoding,


signaling, and transmission medium
transparent

to upper users.

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Figure 9
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2

Multiplexing Using LLC

When stations use multiple upper
-
layer protocols such as
IP

and
IPX
,

there is a need for multiplexing. LLC can take a message from either

of the protocols at
S

and deliver it to the appropriate protocol at
D
.

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Figure 9
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3

Concept of Services

LLC provides services to the user of the local area network.

The user here usually means the network layer.

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Figure 9
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4

Categories of LLC Services

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5

Data Transfer Using

Unacknowledged Connectionless

Primitives

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6

Data Transfer Using

Acknowledged Connectionless

Primitives

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7

Connection Using

Connection
-
Oriented

Primitives

connection setup step

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8

Data Transfer Using

Connection
-
Oriented Primitives

data transfer step

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Figure 9
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9

Disconnection Using

Connection
-
Oriented Primitives

Connection release step

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Figure 9
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10

LLC PDU Types

I
-
PDU S
-
PDU U
-
PDU



I
-
PDU: used to transport user data in conn.
-
oriented services



S
-
PDU: used for flow and error control



U
-
PDU: used to carry user data in connectionless services or


management information in conn.
-
oriented services

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Figure 9
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11

PDU Format

DSAP: Destination service access point

SSAP: Source service access point


DSAP and SSAP are addresses (predefined code)
used by the LLC


to identify the upper
-
layer protocol

that uses/generates the data


(logical user of LLC, such as IP, IPX, ARP, RARP, …).

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Figure 9
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12

DSAP and SSAP

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Figure 9
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13

Control Fields

N(S):

the seq. number of the PDU being sent

N(R):

the seq. number of the PDU expected

P/F
: has meaning only when it is set (=1). When set,


it means
poll

when the PDU is sent by the
primary


it means
final

when the PDU is sent by the
secondary

Piggybacking: combining

data with ACK info. in a

single PDU

NRM (M/S)

ARM (M/S)

ABM (P
-
to
-
P)

3 Modes:

RR, REJ, RNR,

SREJ

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Figure 9
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14

PDU Used in Unacknowledged

Connectionless Services

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15

PDUs Used in Acknowledged

Connectionless Services

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Figure 9
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PDU Used in Connection

Establishment

Set ABM

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17

PDU Used in Data Transfer

N(R)

N(S)

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Figure 9
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18

Piggybacking

N(R)

N(S)

not yet receiving any PDU

Note
: Seq. no. can start randomly

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Figure 9
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19

PDUs Used in Disconnection