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
2
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
3
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
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. ? 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
5
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
6
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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