Data Link Layer - Computer Science | Academics | WPI

[Pages:36]Data Link Layer Review

Advanced Computer Networks

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.

Advanced Computer 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)

12 3 21

12 3 21

Medium

2

A

B1

1 Physical layer entity 2 Data link layer entity

Advanced Computer Networks

3 Network layer entity

Data Link Layer

Leon-Garcia & Widjaja: Communication Networks

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End to End

ACK/NAK

Transport Layer

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

Advanced Computer Networks Data Link Layer

Leon-Garcia & Widjaja: Communication Networks

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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. ? A 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.

Advanced Computer Networks Data Link Layer

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Host

LaLyaeyre4r 4

Host

A

B

Node 1

Layer 2 frame

Node 2

Tanenbaum's 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.

Advanced Computer 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

Advanced Computer Networks Data Link Layer

Figure 5.8

7

Tanenbaum's Protocol Definitions

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

8 Advanced Computer Networks Data Link Layer

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