TRUYỀN SỐ LIỆU VÀ MẠNG Ch03 data link layer protocols

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Telecomm. Dept.

Faculty of EEE

DCN

HCMUT

Data Communication and

Networking

Dr. –Ing. Vo Que Son

Email: [email protected]

2

Telecomm. Dept.

Faculty of EEE

DCN

HCMUT

Content

Chapter 3: Data Link Layer Protocols

Flow Control

Error Control

Connection Management

Data Link Protocols

Chapter 4: Telecommunication Networks

802.x standard and TCP/IP Model

Ethernet, Token Pass, Token Ring

IP Addressing: Classfull and VLSM

Network devices

Switching and Routing

STP, VLAN

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Telecomm. Dept.

Faculty of EEE

DCN

HCMUT

Framing

The data link layer needs to pack bits into frames, so

that each frame is distinguishable from another.

Fixed-Size Framing

Variable-Size Framing

4

Telecomm. Dept.

Faculty of EEE

DCN

HCMUT

Flow and Error Control

The most important responsibilities of the data link

layer are flow control and error control. Collectively,

these functions are known as data link control.

Flow control refers to a set of procedures used to restrict

the amount of data that the sender can send before

waiting for acknowledgment.

Error control in the data link layer is based on automatic

repeat request, which is the retransmission of data.

 Now let us see how the data link layer can combine framing,

flow control, and error control to achieve the delivery of data

from one node to another. The protocols are normally

implemented in software by using one of the common

programming languages.

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Telecomm. Dept.

Faculty of EEE

DCN

HCMUT

Error Control

Forward Error Control (FEC)

Usually used in real-time application (e.g. voice, video)

Accept errors if can not correct

List some FEC methods?

Error Detection + ARQ (Automatic Retransmission Request)

Require absolutely correct data transmission

Used in non real-time application (e.g. email, file

transmission)

2 basic ARQ protocols:

• Idle RQ: Bisync (IBM), XModem

• Continuous RQ:

– Go-back N: e.g. HDLC, V.42

– Selective-Repeat: TCP, Service Specific Connection Oriented

Protocol

6

Telecomm. Dept.

Faculty of EEE

DCN

HCMUT

Notations

 P: Primary, sending data frames

 S: Secondary, receiving data frames and sending ACK back to

Primary

 I frame: data/Information frame

 I(N): Data frame with sequence N

 ACK (Acknowledge) frame: response/reply sent from S back to

P, confirm receiving a good I-frame

 NAK (Negative Acknowledge) frame: response/reply sent from

S back to P, confirm receiving a corrupted I-frame (error

frame)

P S

I -frame

ACK frame or NAK frame

7

Telecomm. Dept.

Faculty of EEE

DCN

HCMUT

Idle RQ

 Principle:  P can have only 1 I

-frame

outstanding (awaiting for

an ACK) at a time

 On receipt of an error

-free

ACK frame, P transmits

another I

-frame

 If S receives an I

-frame or

P receives an ACK

containing transmission

error: the frame is

discarded

 If P does not receive an

ACK within a given timeout

interval: retransmits the

waiting I

-frame

 If an ACK is corrupted, the

S receives another copy of

the frame and it is rejected

by S.