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 9-2
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 9-3
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 9-4
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Categories of LLC Services
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Figure 9-5
Data Transfer Using
Unacknowledged Connectionless Primitives
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Figure 9-6
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Data Transfer Using
Acknowledged Connectionless Primitives
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Figure 9-7
Connection Using
Connection-Oriented Primitives
connection setup step
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Figure 9-8
Data Transfer Using
Connection-Oriented Primitives
data transfer step
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Figure 9-9
Disconnection Using
Connection-Oriented Primitives
Connection release step
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Figure 9-10
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 9-11
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 9-12
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DSAP and SSAP
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Figure 9-13
Control Fields
NRM (M/S)
3 Modes: ARM (M/S)
ABM (P-to-P)
RR, REJ, RNR, SREJ
Piggybacking: combining
N(S): the seq. number of the PDU being sent data with ACK info. in a
N(R): the seq. number of the PDU expected single PDU
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
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Figure 9-14
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PDU Used in Unacknowledged
Connectionless Services
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Figure 9-15
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PDUs Used in Acknowledged
Connectionless Services
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Figure 9-16
PDU Used in Connection
Establishment
Set ABM
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Figure 9-17
PDU Used in Data Transfer
N(R)
N(S)
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Figure 9-18
Piggybacking
not yet receiving any PDU
N(S)
N(R)
Note: Seq. no. can start randomly
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Figure 9-19
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PDUs Used in Disconnection
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