UNIT-IV
DATA COMMUNICATION
TECHNIQUES
Data Link Protocols
Asynchronous
Protocols
•
•
•
•
•
Xmodem
Ymodem
Zmodem
BLAST
Kermit
Asynchronous:
Synchronous:
Synchronous
Protocols
Character-oriented
Bit-oriented
treat each character in a bit stream independently
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
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Low and high-speed transmissions are
possible
Low speed
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High speed

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Almost all modem based communications fall into
this category
Asynchronous Transfer Mode (ATM)
Internet is a good example where
asynchronous communication is used
predominantly to carry the information
Asynchronous Protocols
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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
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•
•
•
•
Xmodem
Ymodem
Zmodem
BLAST
Kermit
Synchronous
Protocols
Character-oriented
(Byte-oriented)
• BSC
Bit-oriented
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
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Based on one byte (8-bit)
Use ASCII for control character
Not efficient  seldom used
Bit-oriented protocol
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Based on individual bits
One or multiple bits for control
More efficient
Binary Synchronous Communication
(BISYNC)OR (BSC)
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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 = Synchronous idle = 0010110
STX = Start of text
= 0000010
ETX = End of text
= 0000011
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
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
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
Asynchronous
Protocols
•
•
•
•
•
Xmodem
Ymodem
Zmodem
BLAST
Kermit
Synchronous
Protocols
Character-oriented
(Byte-oriented)
• BSC
Bit-oriented
Bit-oriented protocol


Represent more information into shorter
frame
Avoid the transparency problems
Bit-oriented
Protocols
SDLC
HDLC
LAPs
SDLC: Synchronous data link control – IBM
HDLC: High-level data link control – ISO
LAPs : Link access procedure
LANs
HDLC
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Support half/full – duplex over point-topoint and multipoint links
HDLC system characterization
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Station types
Configurations
Communication modes
Frames
HDLC station types
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Primary station
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Secondary station
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The station that controls the medium by sending
“command”
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)
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Unbalanced
Symmetrical
Balanced
HDLC Configurations:
Unbalanced (master/slave)
HDLC Configurations:
Symmetrical
HDLC Configurations:
Balanced
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)
HDLC frame

3 frame types
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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
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
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 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
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: Use of P/F field
HDLC: S-Frame
Acknowledgement
HDLC: S-Frame
Positive Acknowledgement

RR
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
Receiver sends “Positive Ack” (no data to send)
N(R) = seq of next frame
RNR

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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)
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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 mode
UA: Unnumbered ack
HDLC: Peer-to-peer example
X.25 AND FRAME RELAY
X.25
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•
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•
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
interface (SNI) protocol.
network
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.
The X.25 Protocol
The Model
Network Has Multiple Nodes (DCEs)
Host Computers (DTEs) Outside
Hosts Have Addresses Like Phone Numbers
Virtual Call Setup
Virtual Call Clear
DCE
Data Transfer
X.25
DTE
X.25
DCE
Intra-Network Protocol
DCE
DCE
66
DTE
DCE
X.25
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 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 – 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
Figure 17-6
Frame Layer and Packet Layer Domains
The X.25 Protocol
LAPB Link Setup and Disconnect
•SABM = Set Asynchronous Balanced
Mode
Local
DTE
Local
DCE
•UA Acknowledges SABM
•DISC Requests Disconnect
•UA Acknowledges DISC
SABM
UA
Now in Data Transfer Mode
DISC
UA
Now in Disconnected Mode
•Exchange on Local Link Only
The X.25 Protocol
LAPB Data Transfer
Local
DTE
Local
DCE
I-Frame #1
RR N(R)=2
I-Frame #2
RR N(R)=3
I-Frame #3
I-Frame #0
N(R)=4
•I-Frame Contains
Packet
•Seq from 0 - 7
and back to 0
•RR Gives Next
Expected I-Frame
•I-Frame Can also
Acknowledge
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 Protocol
Call Setup
Local
DTE
Local
DCE
Remote
DCE
Remote
DTE
Call Request
•Each Channel is Distinct
•Select Unused Channel
Locate Remote DCE
•Different Channel Numbers
on Each End
Incoming Call
Internal
Protocol
Call Connected
•End to End is “Virtual
Circuit”
Call Accepted
•VC = Local Chnl + Network
Route + Remote Chnl
•Internal Network Protocol
Not Specified
•Call Setup is End to End
The X.25 Protocol
Call Clearing
Local
DTE
Local
DCE
Clear Request
Remote
DCE
Remote
DTE
Remote DCE from
Call Setup
•Each Channel is Distinct
•Channels Become Available
Clear Indication
Internal
Protocol
Clear Confirm
•End to End is “Virtual
Circuit”
•Internal Network Protocol
Not Specified
Clear Confirm
•Clearing May be End to End
or Local
•Clear Packet Used to
Report Procedure Errors
The X.25 Protocol
Data Transfer w/End to End Ack
Local
DTE
Local
DCE
Data Packet #1
Remote
DCE
Remote
DTE
Remote DCE from
Call Setup
•Each Channel is Distinct
•End to End is “Virtual
Circuit”
Data Packet #1
Internal
Protocol
RR P(R)=2
•Internal Network Protocol
Not Specified
•Each Data Pkt Has Seq Nr
RR P(R)=2
•Each RR Has Next
Expected Seq Nr
•Example Shows End to End
Acknowledgement
The X.25 Protocol
Data Transfer w/Local Ack
Local
DTE
Local
DCE
Remote
DCE
Remote
DTE
Remote DCE from
Call Setup
Data Packet #1
RR P(R)=2
•Each Channel is Distinct
•End to End is “Virtual
Circuit”
Data Packet #1
Internal
Protocol
•Internal Network Protocol
Not Specified
•Each Data Pkt Has Seq Nr
RR P(R)=2
Data Packet #2
•Each RR Has Next
Expected Seq Nr
•Example Shows Local
Acknowledgement
RR P(R)=3
Data 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 17-8
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
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.
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•
•
•
•
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.