UNIT-IV DATA COMMUNICATION TECHNIQUES. Data Link Protocols Asynchronous Protocols Synchronous Protocols Character-orientedBit-oriented Xmodem Ymodem Zmodem

UNIT-IVDATA COMMUNICATION

TECHNIQUES

Data Link Protocols

AsynchronousProtocols

SynchronousProtocols

Character-oriented Bit-oriented

• Xmodem• Ymodem• Zmodem• BLAST• Kermit

Asynchronous: treat each character in a bit stream independentlySynchronous: take whole bit stream and chop it into characters of equal size

The Use of the Word Asynchronous

Asynchronous Transmission Generally refers to the transmission

of characters with each character carrying information about timing

Asynchronous Communication Refers to overall communication

between two points An example in this case would be ATM

Asynchronous Transmission Applied to Characters

Stop Bit Start Bit

Each character is individually timed.

Character Frame

Asynchronous Transmission Applied to Packets

Packets of data Packets of data

Burst of Data

A B

Intermittent transmission of packets of data

Asynchronous Transmission/Communication Application

Character by character transmission

Data packet transmission at present

Speed Variations In Asynchronous Transmission Low and high-speed transmissions are

possible Low speed

Almost all modem based communications fall into this category

High speed Asynchronous Transfer Mode (ATM)

Internet is a good example where asynchronous communication is used predominantly to carry the information

Asynchronous Protocols Long, long…time ago Not complex and easy to implement Slow Required start/stop bit and space Now mainly used in modem Replaced by high speed

synchronous

Data Link Protocols



Character-oriented(Byte-oriented)

Bit-oriented


• BSC

Ymodem data unit changes to 1024 bytes (Xmodem=128)

use CRC16multiple files accepted

Zmodem combination of X and Ymodem

BLAST (Blocked Asynchronous Transmission) better than

Xmodem (full-duplex, sliding window flow conrol)

Kermit (Columbia U) most widely used asyn. Protocol (operation same as Xmodem)

Synchronous Protocols Character-oriented protocol

Based on one byte (8-bit) Use ASCII for control character Not efficient seldom used

Bit-oriented protocol Based on individual bits One or multiple bits for control More efficient

Binary Synchronous Communication (BISYNC)OR (BSC)

Character-oriented protocol Half-duplex, stop-and-wait ARQ 2 frame types

Data frame (data transmission) Control frame (connect/disconnect and flow/error

control)

A simple BSC data frame

SYN = Synchronous idle = 0010110STX = Start of text = 0000010ETX = End of text = 0000011

SYN : Alert the receiver for the incoming frameBCC : can be LRC (longitudinal redundancy check) or CRC (cyclic redundancy check)This simple frame is seldom used

A BSC frame with a header

Header Fields:• address (sender/receiver)• #frame identifier (0/1 for stop-and-wait ARQ)

A multiblock frame

ITB = Intermediate text block

Probability of error: Frame size increases, error increases multiple faults occurs Difficult to detect errors (error cancel each others)

Message is divided in several blocks Each block has STX, ITB and BCC Ending with ETX (end of text) Error detected, whole frame is discarded

(needs retransmission) ACK for entire frame one frame is entire message

Multiframe transmission

ETB = End of transmission Block

“Large Message” is broken down to multiple frame need ETB (End of transmission Block) need ETX (End of text) Half-duplex so ACK 0 and ACK 1

alternately

Control frame

Note: Control Frame is used to send command * Establish connection * Maintaining flow & error control * terminating connection

Control frames

Control frames

Control frames

Data Transparency BSC is designed for text message Now, non-text message (graphics,…)

Problem? BSC control character problem

Data transparency: should be able to send any data

Byte stuffing

DLE = data link escape

Byte Stuffing 2 activities:- Defining the transparent text region with

DLE- Preceding any DLE character within the

transparent region (extra DLE)Problem still exist if text = DLE ?

Insert an addition DLE next to the character (DLE DLE)

Data Link Protocols



Character-oriented(Byte-oriented)

Bit-oriented


• BSC

Bit-oriented protocol Represent more information into

shorter frame Avoid the transparency problems

Bit-orientedProtocols

SDLC HDLC LAPs LANs

SDLC: Synchronous data link control – IBMHDLC: High-level data link control – ISOLAPs : Link access procedure

HDLC Support half/full – duplex over

point-to-point and multipoint links HDLC system characterization

Station types Configurations Communication modes

Frames

HDLC station types Primary station

The station that controls the medium by sending “command”

Secondary station The station that “response” to the primary

station Combined station

The station that can both command and response

HDLC configurations The relationship of hardware

devices on a link 3 configurations of all stations

(primary/secondary/combined) Unbalanced Symmetrical Balanced

HDLC Configurations: Unbalanced (master/slave)

HDLC Configurations: Symmetrical

HDLC Configurations:Balanced

HDLC communication modes

NRM: Normal response mode (master/slave)

ARM: Asynchronous response mode (secondary can initiate if idle, all transmissions are made to primary station)

ABM: Asynchronous balanced mode (point-to-point equal)

Mode : describe “Who controls the link”

HDLC frame 3 frame types

Information frame (I-frame) Supervisory frame (S-frame)

For ACK, Flow/Error controls Unnumbered frame (U-frame)

For Mode setting, Initialize, Disconnect

HDLC Frame

HDLC Frame

HDLC Frame: Flag field

Flag: beginning and ending of a frame Last flag can be the start of the next flag

Flag similar to “Control Character” problem for transparency !!! Bit

Stuffing

Bit Stuffing How to differentiate data and flag? Adding one extra 0 whenever there are

five consecutive 1s in the data

HDLC: Bit stuffing

HDLC frame: Address field Primary station creates a frame destination address Secondary station creates a frame source address Can be one byte or more

HDLC Frame: Address field

HDLC Frame: Control field

N(R) can be think as “ACK”if correct N(R) = next frame seqelse N(R) = number of damaged frame

(need reTx)In S-Frame not transmit data, so do not need N(S)

S-Frame for response (return N(R) )Code flow and error control information

HDLC frame: Poll / Final

P/F: dual purposes1) P/F = 0 no meaning (regular data)2) P/F = 1 means “poll” when send by primary P/F = 1 means “final” when send by

secondary

HDLC Frame: Information field

HDLC Frame: FCS field

FCS: Frame check sequence

HDLC: S-Frame

HDLC: Use of P/F field


Piggybacking: data + ack



HDLC: S-FrameAcknowledgement

HDLC: S-Frame Positive Acknowledgement RR

Receiver sends “Positive Ack” (no data to send)

N(R) = seq of next frame RNR

Receiver sends “Positive Ack” N(R) = seq of next frame Receiver tells sender that sender cannot

send any frame until ‘RR’ frame is received

HDLC: S-Frame Negative Acknowledgement

Reject (REJ) Go-back-n ARQ N(R) = # of damage frame (and

follow) Selective-Reject (SREJ)

N(R) = # of damage frame

HDLC: U-Frame control field

For session management and control information

HDLC: U-Frame control field

HDLC: Polling example

HDLC: Selecting example

HDLC: Peer-to-peer example

SABM: Set asynchronous balanced modeUA: Unnumbered ack

HDLC: Peer-to-peer example

X.25 AND FRAME RELAY

X.25• X.25 is a packet-switching wide area network

developed by ITU-T in 1976.

• X.25 defines how a packet-mode terminal can be connected to a packet network for the exchange of data.

• X.25 is what is known as subscriber network interface (SNI) protocol.

• It defines how the user’s DTE communicates with the network and how packets are sent over that network using DCEs.

Figure 17-1

X.25

Data terminal equipment (DTE) is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment (DCE). The DTE/DCE classification was introduced by IBM.A data circuit-terminating equipment (DCE) is a device that sits between the data terminal equipment (DTE) and a data transmission circuit. It is also called data communications equipment and data carrier equipment. Usually, the DTE device is theterminal (or computer), and the DCE is a modem.In a data station, the DCE performs functions such as signal conversion, coding, and line clocking and may be a part of the DTE or intermediate equipment.

66

The X.25 Protocol

The ModelNetwork Has Multiple Nodes (DCEs)Host Computers (DTEs) OutsideHosts Have Addresses Like Phone NumbersVirtual Call SetupVirtual Call ClearData Transfer

DTE

DTE

DCE

DCE

DCE

DCEDCEX.25

X.25

X.25

Intra-Network Protocol

DTE

• X.25 network is a packet switching network that used X.25 protocol.

• X.25 is a standard packet switching protocol that has been widely used in WAN.

• X.25 is a standard for interface between the host system with the packet switching network in which it defines how DTE is connected and communicates with packet switching network.

• It uses a virtual circuit approach to packet switching (SVC and PVC) and uses asynchronous (statistical) TDM to multiplex packets.

Figure 17-2

X.25 Layers in Relation to the OSI Layers

X.25 protocol specifies three layers:

i. Physical Layer (X.21)

ii. Frame Layer (LAPB)

iii. Packet Layer (PLP) (Packet Layer Protocol)

X.25 Layers

-specifies the physical interface between the node (computer, terminal) and the link that connected to X.25 network.

-specifies a protocol called X.21 or X.21bis (interface).

-similar enough to other PHY layer protocols, such as EIA-232.

X.25 – Physical Layers

X.21 hardware interface

X.25 Frame Layer

- provides a reliable data transfer process through data link control which used link access procedure, balanced (LAPB) protocol.

- there are 3 categories of frame involved in the LAPB frame format:

I-Frames – encapsulate PLP packets from the network layer and before being passed to the physical layer

Figure 17-3

Format of a Frame in X.25

Cont…

S-Frames – flow and error control in the frame layer

U-Frames- used to set up and disconnect the links between a DTE and a DCE.

In the frame layer, communication between a DTE - DCE involves three phases: 1: Link Setup ; 2: Packet Transfer ; 3: Link Disconnect

Figure 17-6

Frame Layer and Packet Layer Domains

The X.25 ProtocolLAPB Link Setup and Disconnect

Local DCE

Local DTE

SABM

UA

•SABM = Set Asynchronous Balanced Mode

•UA Acknowledges SABM

•DISC Requests Disconnect

•UA Acknowledges DISC

•Exchange on Local Link Only

DISC

UA

Now in Data Transfer Mode

Now in Disconnected Mode

The X.25 ProtocolLAPB Data Transfer

Local DCE

Local DTE

I-Frame #1

RR N(R)=2

•I-Frame Contains Packet•Seq from 0 - 7 and back to 0•RR Gives Next Expected I-Frame•I-Frame Can also Acknowledge

I-Frame #2

RR N(R)=3

I-Frame #3

I-Frame #0 N(R)=4

X.25 Packet layer (PLP)

Packet Layer Protocol (PLP) - it is the network layer in X.25- this layer is responsible for establishing the connection, transferring the data, and terminating the connection between 2 DTEs. - it also responsible for creating the virtual circuits and negotiating network services between two DTEs.

• Virtual circuits in X.25 are created at the network layer (not the data link layers as in some other wide area networks such as Frame Relay and ATM)

Figure 17-6

Frame Layer and Packet Layer Domains

The X.25 ProtocolCall Setup

Local DCE

Remote DCE

Local DTE

Remote DTE

Call Request

Incoming Call

Call Accepted

Call Connected

Internal Protocol

•Each Channel is Distinct

•Select Unused Channel

•Different Channel Numbers on Each End

•End to End is “Virtual Circuit”

•VC = Local Chnl + Network Route + Remote Chnl

•Internal Network Protocol Not Specified

•Call Setup is End to End

Locate Remote DCE

The X.25 ProtocolCall Clearing

Local DCE

Remote DCE

Local DTE

Remote DTE

Clear Request

Clear Indication

Clear Confirm

Clear Confirm

Internal Protocol


•Channels Become Available



•Clearing May be End to End or Local

•Clear Packet Used to Report Procedure Errors

Remote DCE from Call Setup

The X.25 ProtocolData Transfer w/End to End Ack

Local DCE

Remote DCE

Local DTE

Remote DTE

Data Packet #1

Data Packet #1

RR P(R)=2

RR P(R)=2

Internal Protocol




•Each Data Pkt Has Seq Nr

•Each RR Has Next Expected Seq Nr

•Example Shows End to End Acknowledgement


The X.25 ProtocolData Transfer w/Local Ack

Local DCE

Remote DCE

Local DTE

Remote DTE

Data Packet #1

Data Packet #1RR P(R)=2

Internal Protocol




•Each Data Pkt Has Seq Nr

•Each RR Has Next Expected Seq Nr

•Example Shows Local Acknowledgement

RR P(R)=2

Data Packet #2

RR P(R)=3Data Packet #2

RR P(R)=3


Implementation of X.25

• X.25 protocol is a packet-switched virtual circuit network.

• Virtual Circuit in X.25 created at the network layer. unlike Frame Relay and ATM which both VC created at Data Link Layer.

• Fig 17.7 shows an X.25 network in which 3 virtual circuits have been created between DTE A and 3 other DTEs.

Figure 17-7

Virtual Circuits in X.25

Virtual Circuit in X.25• Each virtual circuit in X.25 should be

identified for use by the packets.• The VC in X.25 is called logical

channel number (LCN). See fig 17.8

Figure 17-8LCNs in X.25

PVC and SVC in X.25• PVC = permanent Virtual Circuit• SVC = Switched virtual circuit• X.25 applied both PVC and SVC.• PVCs are established by the X.25

network providers. (similar to the leased line in telephone networks.)

• SVCs are established at each session. Involve 5 events (like 3-phase). Setup, transfer & connection released.

5 events in SVC• A Link is setup between DTE and DCE

also between REMOTE DTE and DCE• A virtual circuit is established between

the local DTE and the remote DTE.• Data are transferred between the two

DTEs.• The virtual circuit is released• The link is disconnected

Frame Relay• Packet-switching with virtual-circuit

technology• Improvement of previous technology

X.25• Operate only at the PHY and Data

link layer.

Frame Relay: Why it is needed?• Higher Data Rate at Lower Cost• Allow Bursty Data• Less Overhead Due to Improved

Transmission Media (compared to prev. tech X.25)

Higher Data Rate at Lower Cost

•Fig. Frame Relay versus Pure Mesh T-Line Network•To connect all the highspeed LANs, it is better used frame-relay network rather than T-Line Network which cost a lot of money and impractical

Frame Relay Operation• Frame relay provides permanent

virtual and switched virtual circuit connections (PVC and SVC)

• The devices that connects users to the network are DTEs.

• The switches that route the frames thru the network are DCEs (see fig 18.5)

Figure 18-5Frame Relay Network

Virtual Circuit in FR• FR is a virtual circuit network. It

therefore does not use PHY addresses to define the DTEs connected to the network.

• It uses VCI called Data Link Connection Identifier (DLCI).

• DLCI is assigned to the DTEs when Virtual Circuit is established for connection

Figure 18-6DLCIs

FR Operation: SVC and PVC• It uses a virtual circuit identifier that is known as data

link connection identifier (DLCI).• Two types of connection:

1. Permanent virtual connection (PVC)• The connection is already exist for 2 DTE in the

network• 2 DLCI is given for each end of the connection

2. Switched virtual connection (SVC)• Everytime when one DTE needs to connect to

other DTE, VC will be established. It needs a protocol that has network layer function and network layer addressing like IP.

• Generally, local DTE will send a SETUP message to the remote DTE which will response by sending message CONNECT.

• VC will be establish for sending the data• Message RELEASE is sent to terminate the

connection.

Figure 18-7

PVC DLCIs

Figure 18-9SVC DLCIs

Figure 18-8

SVC Setup and Release

Figure 18-13Comparing Layers in Frame Relay and X.25

Figure 18-25FRAD

To handle frames arriving from other protocols, Frame Relay uses a device called a FRAD.

A FRAD assembles and disassembles frames coming from other protocols to allow them to be carried by Frame Relay frames.

A FRAD can be implemented as a separate device or as part of a switch.

Figure 12.3 Frame Relay frame

Adv of Frame Relay tech.• Higher speed than X.25 (44.376 Mbps)• Application that used TCP/IP protocol such

as email/http/chat can easily use Frame relay as it backbone bcoz FR operates at only 2 layer (DL and PHY).

• Allow bursty data• Allow frame size of 9000 bytes, which can

accommodate all LAN frames• Less expensive compared to other WANs

tech.

Disadv. Of Frame Relay• Max. transfer rate is at 44.376. Not enuff

speed compared to nowadays demand• allows variable-length frames which may

cause varying delays for different users.• Because of the varying delays, which are

not under user control, Frame relay is not suitable for sending delay sensitive data such as real time voice or video. E.g. FR not suitable for teleconferencing.

Documents

UNIT-IV DATA COMMUNICATION TECHNIQUES. Data Link Protocols Asynchronous Protocols Synchronous Protocols Character-orientedBit-oriented Xmodem Ymodem Zmodem