Data Link Protocols(HDLC & PPP) PowerPoint Presentation, PPT

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Data Link Protocols. The set of specifications used to implement the DLL.. Timing. Co-ordination . No Timing Co-ordination. Character-Oriented Protocols: interpret a transmission frame as a succession of characters, each usually composed of one byte. (Byte-oriented Protocols). ID: 720061

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Data Link Protocols(HDLC & PPP)

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Data Link Protocols(HDLC & PPP)
Slide2
Data Link Protocols
The set of specifications used to implement the DLL.
Timing
Co-ordination
No Timing Co-ordination
Slide3
Character-Oriented Protocols: interpret a transmission frame as a succession of characters, each usually composed of one byte. (Byte-oriented Protocols)
Example: BSC (Binary Synchronous Communication)
Developed by IBM
Specifies half-duplex transmission with stop-and-wait ARQ.
Character-oriented Protocols
Slide4
Bit-oriented Protocols
A
bit-oriented protocol
is a communications protocol that sees the transmitted data as an
opaque stream of bits with no semantics, or meaning. Control codes are defined in terms of bit sequences instead of characters. Bit oriented protocol can transfer data frames regardless of frame contents.
It can also be stated as "bit stuffing" this technique allows the data frames to contain an arbitrary number of bits and allows character codes with arbitrary number of bits per character.Examples: HDLC & PPP
Slide5
High Level Data Link Control
Most widely used DLL protocol
Published by ISO
Supports both half-duplex & full-duplex modes in Point-to-point & Multipoint Configurations.
Systems using HDLC can be characterized by :station types,configurations,
response modes.
Slide6
HDLC Station Types
Primary station
Controls operation of link
Issues commands (frames)Maintains separate logical link to each secondary stationSecondary station
Under control of primary stationIssues responses (frames)Combined stationMay issue commands and responsesCombines the features of primary and secondary stations
Slide7
HDLC Link Configurations
Configuration refers to the relationship of hardware devices on a link.
Primary, Secondary & Combined stations can be configured in 3 ways:
Unbalanced:
One primary and one or more secondary stations.Supports full duplex and half duplexBalanced:
Two combined stationsSupports full duplex and half duplexSymmetrical:Each Physical station on a link consists of two logical stations, one a Primary & the other a Secondary
Slide8
HDLC Link Configurations
Slide9
HDLC Link Configurations
Slide10
HDLC Transfer Modes
Normal Response Mode (NRM)
Unbalanced configuration
Primary can only initiate transmission
Secondary may only transmit data in response to command (poll) from primaryUsed on multi-drop linesHost computer as primaryTerminals as secondary
Slide11
HDLC Transfer Modes
Asynchronous Balanced Mode (ABM)
Balanced configuration
Either station may initiate transmission without receiving permission
Most widely usedNo polling overheadAsynchronous Response Mode (ARM)
Unbalanced configurationSecondary may initiate transmission without permission form primaryPrimary is responsible for connect, disconnect, error recovery, and initializationrarely used
Slide12
HDLC Transfer Modes ( In Nutshell)
NRM
ARM
ABM
Station Type
Primary & SecondaryPrimary & SecondaryCombinedInitiatorPrimaryEither
Any
Slide13
Frame Structure
01111110
01111110
Defines 3 types of frames (I,S,U frames)
Slide14
Flag Fields
Delimit frame at both ends
01111110
Receiver hunts for flag sequence to synchronize
Bit stuffing used to avoid confusion with data containing 01111110The transmitter inserts 0 bit after every sequence of five 1s with the exception of flag fieldsIf receiver detects five 1s it checks next bit
If 0, it is deletedIf 1 and seventh bit is 0 (i.e., 10), accept as flagIf sixth and seventh bits 1 (i.e., 11), sender is indicating abort
Slide15
Bit Stuffing
Example with
possible errors
Slide16
Address Field
Identifies secondary station that sent or will receive frame
Usually 8 bits longMay be extended to multiples of 7 bits
LSB of each octet indicates that it is the last octet (1) or not (0)All ones (11111111) is broadcast
Slide17
Control Field
Different for different frame type
I-frame (information frame) data to be transmitted to user (next layer up)
Flow and error control piggybacked on information framesS-frame (Supervisory frame)Used for flow and error controlU-frame (Unnumbered frame)
supplementary link controlFirst one or two bits of control filed identify frame type
Slide18
Control Field Diagram
Slide19
Poll/Final Bit
Use depends on context
Command frameP bit : used for poll from primary1 to solicit (poll) response from peer
Response frameF bit : used for response from secondary1 indicates response to soliciting command
Slide20
I-frame
Contains the sequence number of transmitted frames and a piggybacked ACK
1 2 3 4 5 6 7 8
0
N(S)
P/F
N(R)
Slide21
S-frame
Used for flow and error control
1 2 3 4 5 6 7 8
1
S
P/F
N(R)
RR --- receive ready
RNR --- receive not ready
REJ --- reject on frame N(R)
SREJ --- selective reject on N(R)
0
Slide22
U-frame
Mode setting, recovery, connect/diconnect
1 2 3 4 5 6 7 8
1
M
P/F
M
1
Unnumbered function bits
Slide23
Unnumberred frames
Set normal response mode (SNRM)
Set asynchronous response mode (SARM)Set asynchronous balanced mode (SABM)
Disconnect (DISC)Unnumberred acknowledgement (UA)Disconnect mode (DM)
Request disconnect (RD)Unnumberred poll (UP)Reset (RSET)Exchange identification (XID)Test (TEST)Frame reject (FRMR)
Slide24
Information Field
Only in information and some unnumbered frames
Must contain integral number of octetsVariable length
Slide25
Frame Check Sequence Field
FCS
Error detection16 bit CRCOptional 32 bit CRC
Slide26
HDLC Operation
Exchange of information, supervisory and unnumbered frames
Three phasesInitializationData transfer
Disconnect
Slide27
PPP
( Point-to-Point Protocol)
Slide28
Transition States
Idle State:
link is not being used.
Establishing State:
When one of the end-points start the communication, the connection goes into the establishing state.Authenticating State: is optional, the two end-points decide during establishing state whether to opt for authenticating state or not.Networking State:
the exchange of user control and data packets.Terminating State: Close the link.
Slide29
Transition States
Idle
Establishing
Authenticating
Terminating
Networking State
Detect Carrier
Success
Success
Finish
Fail
Fail
Drop Carrier
Slide30
PPP Flags
Flag
Address
Control
ProtocolData & paddingFCSFlag