UNIT-IV DATA COMMUNICATION TECHNIQUES

UNIT-IV DATA COMMUNICATION TECHNIQUES
Protocol: set of rules or specifications used to implement one or more layers of the OSI model Data Link Protocol: a set of specifications used to implement data link layer rules –> line discipline, flow control, and error handling

Data Link Protocols Asynchronous Protocols Synchronous Protocols
Xmodem Ymodem Zmodem BLAST Kermit Asynchronous: treat each character in a bit stream independently Synchronous: take whole bit stream and chop it into characters of equal size Character-oriented Bit-oriented Asynchronous: treat each character in a bit stream independently Synchronous: 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 Character Frame Each character is individually timed.

Asynchronous Transmission Applied to Packets
Burst of Data Packets of data Packets 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 Asynchronous Protocols Synchronous Protocols
Xmodem Ymodem Zmodem BLAST Kermit Ymodem  data unit changes to 1024 bytes (Xmodem=128) use CRC16 multiple 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) Character-oriented (Byte-oriented) Bit-oriented BSC

Ymodem  data unit changes to 1024 bytes (Xmodem=128)
use CRC16 multiple 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 Speed  better than asynchronous

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 : Alert the receiver for the incoming frame BCC : can be LRC (longitudinal redundancy check) or CRC (cyclic redundancy check) This simple frame is seldom used SYN : Alert the receiver for the incoming frame BCC : can be LRC (longitudinal redundancy check) or CRC (cyclic redundancy check) This simple frame is seldom used SYN = Synchronous idle = STX = Start of text = ETX = End of text =

A BSC frame with a header
Header Fields: address (sender/receiver) #frame identifier (0/1 for stop-and-wait ARQ) SOH: start of Header

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

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
“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 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
Control frame  not control character Note: Control Frame is used to send command * Establish connection * Maintaining flow & error control * terminating connection

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) 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 Asynchronous Protocols Synchronous Protocols Xmodem Ymodem Zmodem BLAST Kermit Character-oriented (Byte-oriented) Bit-oriented BSC

Bit-oriented protocol
Represent more information into shorter frame Avoid the transparency problems

Bit-oriented Protocols
SDLC HDLC LAPs LANs SDLC: Synchronous data link control – IBM HDLC: High-level data link control – ISO LAPs : Link access procedure Most of the bit-oriented protocols are proprietary Most famous  HDLC

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 Secondary station Combined 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)
Can be point-to-point (for 2 devices) or multipoint

HDLC Configurations: Symmetrical

HDLC Configurations: Balanced
Only point-to-point

HDLC communication modes
Mode : describe “Who controls the link” 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) ARM: need to communicate through the Primary (even for secondary to secondary primary)

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: 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 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 TX more than 5 consecutives “1”  insert (stuffs) one redundant bit “0” after the fifth “1” Example:  No Matter that the sixth bit is one or not ! 3-Exceptional: 1. it’s really a Flag (6 consecutive “1”) 2. Tx is being aborted (7-14 consecutive “1”) 3. Channel is in Idle state (>= 15 consecutive “1)

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
One byte = 128 stations (one bit is used for another purpose) Large network needs multiple byte address

HDLC Frame: Control field
N(R)  can be think as “ACK” if correct  N(R) = next frame seq else  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

N(R)  can be think as “ACK”
if correct  N(R) = next frame seq else  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 purposes 1) 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 P/F: dual purposes 1) 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

HDLC: Use of P/F field Piggybacking: data + ack

HDLC: Use of P/F field

HDLC: S-Frame Acknowledgement

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” 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
SNRM : set normal response mode DISC: Disconnect

HDLC: Polling example

HDLC: Selecting example

HDLC: Peer-to-peer example
SABM: Set asynchronous balanced mode UA: 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.

Intra-Network Protocol
The Model Network Has Multiple Nodes (DCEs) Host Computers (DTEs) Outside Hosts Have Addresses Like Phone Numbers Virtual Call Setup Virtual Call Clear Data Transfer The X.25 Protocol DCE DTE X.25 Intra-Network Protocol DCE X.25 DCE DTE DCE X.25 DTE DCE

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.

X.25 Layers in Relation to the OSI Layers
Figure 17-2 X.25 Layers in Relation to the OSI Layers

X.25 Layers X.25 protocol specifies three layers:
Physical Layer (X.21) Frame Layer (LAPB) Packet Layer (PLP) (Packet Layer Protocol)

X.25 – Physical 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.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

Frame Layer and Packet Layer Domains
Figure 17-6 Frame Layer and Packet Layer Domains

Now in Data Transfer Mode Now in Disconnected Mode
The X.25 Protocol LAPB Link Setup and Disconnect SABM = Set Asynchronous Balanced Mode UA Acknowledges SABM DISC Requests Disconnect UA Acknowledges DISC Exchange on Local Link Only Local DCE Local DTE SABM UA Now in Data Transfer Mode DISC UA Now in Disconnected Mode

The X.25 Protocol LAPB Data Transfer I-Frame Contains Packet
Seq from and back to 0 RR Gives Next Expected I-Frame I-Frame Can also Acknowledge Local DCE Local DTE I-Frame #1 RR N(R)=2 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)

Frame Layer and Packet Layer Domains
Figure 17-6 Frame Layer and Packet Layer Domains

The X.25 Protocol Call Setup Local DCE Remote DCE Local DTE Remote DTE
Call Request 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 Incoming Call Internal Protocol Call Accepted Call Connected

Remote DCE from Call Setup
The X.25 Protocol Call Clearing Local DCE Remote DCE Local DTE Remote DTE Remote DCE from Call Setup Clear Request Each Channel is Distinct Channels Become Available End to End is “Virtual Circuit” Internal Network Protocol Not Specified Clearing May be End to End or Local Clear Packet Used to Report Procedure Errors Clear Indication Internal Protocol Clear Confirm Clear Confirm

The X.25 Protocol Data Transfer w/End to End Ack Local DCE Remote DCE Local DTE Remote DTE Remote DCE from Call Setup Data Packet #1 Each Channel is Distinct End to End is “Virtual Circuit” Internal Network Protocol Not Specified Each Data Pkt Has Seq Nr Each RR Has Next Expected Seq Nr Example Shows End to End Acknowledgement Data Packet #1 Internal Protocol RR P(R)=2 RR P(R)=2

The X.25 Protocol Data Transfer w/Local Ack Local DCE Remote DCE Local DTE Remote DTE Remote DCE from Call Setup Data Packet #1 Each Channel is Distinct End to End is “Virtual Circuit” Internal Network Protocol Not Specified Each Data Pkt Has Seq Nr Each RR Has Next Expected Seq Nr Example Shows Local Acknowledgement RR P(R)=2 Data Packet #1 Internal Protocol RR P(R)=2 Data Packet #2 Data Packet #2 RR P(R)=3 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 17-8 LCNs 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 18-5 Frame 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 18-6 DLCIs

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 18-9 SVC DLCIs

Figure 18-8 SVC Setup and Release

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

Figure 18-25 FRAD 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 /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 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.

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