Data Link Layer - Computer Science | Academics | WPI

[Pages:35]Data Link Layer

Networks: Data Link Layer

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Data Link Layer

? Provides a well-defined service interface to the network layer.

? Determines how the bits of the physical layer are grouped into frames (framing).

? Deals with transmission errors (CRC and ARQ). ? Regulates the flow of frames. ? Performs general link layer management.

Networks: Data Link Layer

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Packets

Packets

(a)

Data link

Data link

Layer

A

Physical

Frames

Layer

Physical B

Layer

Layer

(b)

1

1

2

2

12 3 21

12 3 21

Medium

2

A

B1

1 Physical layer entity 2 Data link layer entity

3 Network layer entity

Copyright ?2000 The McGraw Hill Companies

Leon-Garcia & Widjaja: Communication Networks

Networks: Data Link Layer

Figure 5.2

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End to End

ACK/NAK

1

2

3

4

5

Data

Data

Data

Data

Hop by Hop

Data

Data

Data

1

2

3

ACK/ NAK

ACK/ NAK

ACK/ NAK

Data

4

5

ACK/ NAK

Figure 5.7

Leon-Garcia & Widjaja: Communication Networks

Networks: Data Link Layer

Copyright ?2000 The McGraw Hill Companies

4

Tanenbaum's Data Link Layer Treatment

? Concerned with communication between two adjacent nodes in the subnet (node to node).

? Assumptions:

? The bits are delivered in the order sent. ? Rigid interface between the HOST and the node

? the communications policy and the Host protocol (with OS effects) can evolve separately. ? He uses a simplified model.

Networks: Data Link Layer

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Host

LaLyaeyre4r 4

Host

A

B

Node 1

Layer 2 frame

Node 2

Data Link Layer Model Assume the sending Host has infinite supply of messages. A node constructs a frame from a single packet message. The CRC is automatically appended in the hardware. The protocols are developed in increasing complexity to help students understand the data link layer issues.

Networks: Data Link Layer

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Basic Elements of ARQ

Packet sequence

Transmitter

Information frames

Receiver

Error-free packet sequence

Station A

Control frames

Station B

CRC

Information Header packet

CRC

Header

Control frame

Information Frame

Copyright ?2000 The McGraw Hill Companies

Leon-Garcia & Widjaja: Communication Networks

Networks: Data Link Layer

Figure 5.8

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Tanenbaum's Protocol Definitions

Continued ?

Figure 3-9. Some definitions needed in the protocols to follow. These are located in the file protocol.h.

Networks: Data Link Layer

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