Example Data Link Protocols

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Quick review

• Reference Models?

• Layers?

• Flow Control?

• Bit stuffing?

• Connection Oriented?

• Synchronous transmission?

• Full-duplex communication?

• Data Link protocol?

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Example Data Link Protocols

High-Level Data Link Control -HDLC

The Data Link Layer in the Internet

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SDLC protocol

(Synchronous Data Link Control)

High-Level Data Link Control -HDLC

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(HDLC )History

• The original ISO standards for HDLC are:– ISO 3309 – Frame Structure– ISO 4335 – Elements of Procedure– ISO 6159 – Unbalanced Classes of Procedure– ISO 6256 – Balanced Classes of Procedure

• The current standard for HDLC is ISO 13239, which replaces all of those standards.

CCITT

)ADCCP (Advanced Data Communication

Control Procedure

)HDLC( High-Level Data

Link Control

)LAP (Link Access procedure

)LAPB(

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(HDLC )Features

• One of the most common data link layer protocols (CISCO routers).

• Bit-oriented protocol.

• Use bits to stuff flags occurring in data.

• Provides both connection-oriented and connectionless service.

• Synchronous Data link control protocol.

• Implements error and flow control (adjust window size based on

receiver capability)).

• Used for point-to-point and multipoint links access.• Reliable protocol:

– Use Selective repeat and go-back-N,with a 3-bit sequence number Up to seven unacknowledged frames may be outstanding at any instant.

• Full-duplex communication (receive and transmit at the same time).16-10-2012

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Data Transfer Mode• There are three modes of data transfer operations:

– Normal Response Mode (NRM) • The secondary station initiate a transmission after permission from the

primary station (usually half-duplex).• Typically use SDLC.• Mainly used in terminal-mainframe networks.

– Asynchronous Response Mode (ARM) • The secondary station can initiate a transmission without receiving

permission from the primary station. • Used mostly in point-to-point configurations.

– Asynchronous Balanced Mode (ABM) • Both sides can send and receive.• Mainly used in full duplex point-to-point links, for communication

between combined stations.• Used in the protocol set known as X.25.

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HDLC – Frame Format

Two 8-bit flag fields to identify the start and end of frame (The value of the flag is always (01111110)).

Address field: Multipoint links: contains the address of the destination . Point-to-point links: used to distinguish command from response

(Why?). This field is set to (11111111) in broadcasting (to all-stations).

Control field: (Identify the frame type). Data or information field contains user data from the network layer. The Frame Check Sequence  (FCS)

Computed over the Address, Control, and Information fields. Error detection field.

Flag FCS Data Control Address Flag

8 16 Grater than or = 0 8 8 8

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HDLC frame types

Information frame (I-frame):

Carries data from the upper network layer it can also carry flow and error control information (piggybacking)

Supervisory Frame (S-Frame):

Carries only flow and error control Information (ACK, NAK)

Used when piggybacking can’t be used or no data is sent with acknowledgement or sending commands…

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HDLC frame types

Unnumbered Frames (U-frame) carries link management:

Set up the logical link between the primary station and a secondary station.

Inform the secondary station about the mode of operation which is used).

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I_Frame Control Field

First bit is 0 to indicate I-frame.

Seq: holds the frame sequence number (3-bits).

P/F:

P/F = 1: means Poll (If the frame is sent by the primary )

P/F = 0: means final (If the frame is sent by the secondary).

Next: holds the ACK number (piggybacking) to be sent to the sender (3-bits).

Seq Next

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S-frame control field

There are four different supervisory frames:

Type Next

Next Name Type

ACK( Indicate the next frame expected) Receiver Ready 00

Negative ACK (Retransmit all outstanding frames starting at Next (Go Back N))

Reject 01

ACK(Indicate the next frame expected + stop sending)

Receiver Not Ready 10

Negative ACK (Retransmission oonly the frame specified in Next(Selective Repeat))

Selective Reject 11

Type 3 frames are undefined in SDLC and LAPB 16-10-2012

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U-frame control field

Set up the logical link between the primary station and a secondary station.

Inform the secondary station about the mode of operation which is used).

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Protocol operation

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The Data Link Layer in the Internet

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Point-to-Point communication

• Point-to-point communication is primarily used in the internet in two situations:

Router to router

Point-to-Point Links

Router

Router

Router Router

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Point-to-Point communication

• Point-to-point communication is primarily used in the internet in two situations:

Dial-up host-router

PPP—The Point-to-Point Protocol

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PPP-The Point-to-Point Protocol

A data link layer protocol.

• Character(Byte) -oriented protocol

• Use byte stuffing to stuff flags occurring in data.

Used for Point-to-Point Connections only.

Used as data link to connect :

Home users (PC) to ISP using a telephone line and a modem.

Two routers.

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PPP Features

• Defines the format of the frame to be exchanged between devices.

• Defines the link establishment and data transfer between devices.

• Supports multiple protocols.

• Defines how two devices authenticate each other

• Error detection – (not correction).

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PPP Support protocolsLink management:The link control protocol (LCP) is responsible

for : Establishing, configuring, and negotiating a data-link connection. Monitors the link quality. Terminate the data-link connection.

Authentication: Authentication is optional (negotiated between the two devices). PPP supports two authentication protocols:

Password Authentication Protocol (PAP). Challenge Handshake Authentication Protocol (CHAP).

Network protocols configuration: (NCPs) responsible for: Establishing and configuring different network layer protocols

(such as IP, AppleTalk , Decnet )For IP network protocol, NCP handles dynamic IP address

assignment.16-10-2012

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Frame format

Flag • 01111110 (Byte).• Identify the frame boundaries.

Address• No need for an address• 11111111 for prodcasting. • Can be removed – negotiated.

Control • 11000000• Can be removed – negotiated

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Frame format

Protocol• It tells what kind of packet is in the

Payload field.• Length is 2 or 1byte(negotiated)

Data field (payload)

• Contains user’s data or other information• Default maximum length 1500 bytes.• length negotiated (at link configuration)

Checksum (FCS)

• length negotiated

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Multiplexing in PPP

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PPP Transition states

• Dead (Idle state): – The link is not being

used.– No physical layer

connection exists.

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PPP Transition states

• Establish:– Performed by LCP.

– Options are negotiated between endpoints (frame data length, keeping address and control fields, use Authentication or not, test the

line quality,.. etc).

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PPP Transition states

• Authenticate: – Performed by

Authentication Protocol

– The user identity is verified.

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PPP Transition states

• Network:– Performed by NCP(to

configure the network layer.– Control (dynamic IP

address).

• Open: - Exchange of Data

packets.

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PPP Transition states

• Terminate:

– The link is terminated (NCP first then LCP)

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LCP Frame Types

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An example

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An example (continued)

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References

• http://en.wikipedia.org/wiki/High-Level_Data_Link_Control#History• http://www.pulsewan.com/data101/hdlc_basics.htm• http://www.javvin.com/protocolHDLC.html• http://searchnetworking.techtarget.com/definition/HDLC• http://vkalra.tripod.com/hdlc.html

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Questions ?

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Thank you

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