Stop-and-Wait Data Link Protocols
• Such elementary protocols are also called PAR (Positive Acknowledgment with Retransmission)
or ARQ (Automatic Repeat reQuest).
• Data frames are transmitted in one direction (simplex protocols ) where each frame is individually
acknowledge by the receiver by a separate acknowledgment frame.
• The sender transmits one frame, starts a timer and waits for an acknowledgment frame from the
receiver before sending further frames.
• A time-out period is used where frames not acknowledged by the receiver are retransmitted automatically
by the sender.
• Frames received damaged by the receiver are not acknowledged and are retransmitted by the sender
when the expected acknowledgment is not received and timed out.
• A one bit sequence number (0 or 1) is used to distinguish between original data frames and duplicate
retransmitted frames to be discarded .
• Such protocols result in a substantial percentage of wasted bandwidth
and may fail under early time-out situations.
Simplex Stop-and-Wait Protocol
Simplex Positive Acknowledgment with Retransmission (PAR) Protocol
Simplex PAR Protocol (Effect of Errors)
Data Link Layer: Flow Control
(Sliding Window Protocols)
• These protocols allow both link nodes (A, B) to send and receive data and
acknowledgments simultaneously.
• Acknowledgments are piggybacked into an acknowledgment field in the
data frame header not as separate frames.
• If no new data frames are ready for transmission in a specified time, a
separate acknowledgment frame is generated to avoid time-out.
• Each outbound frame contains a sequence number ranging from 0 to
2 n-1 (n-bit field). N = 1 for stop-and-wait sliding window protocols.
• Sending window: A set of sequence numbers maintained by the sender
and correspond to frame sequence numbers of frames sent out but not
acknowledged.
• The maximum allowed size of the sending window w correspond to the
maximum number of frames the sender can transmit before receiving
any acknowledgment without blocking (pipelining).
• All frames in the sending window may be lost or damaged and thus must
be kept in memory or buffers until they are acknowledged.
Sliding Window Data Link Protocols
• Receiving window: A set of sequence numbers maintained by the receiver and
indicate the frames sequence numbers it is allowed to receive and acknowledge.
• The size of the receiving window is fixed at a specified initial size.
• Any frame received with a sequence number outside the receiving window is
discarded.
• The sending window and receiving window may not have the same upper or
lower limits or have the same size.
• When pipelining is used, an error in a frame is dealt with in one of two ways:
- Go back n:
• The receiver discards all subsequent frames and sends no acknowledgments.
• The sender times out and resends all the discarded frames starting with faulty frame.
- Selective repeat:
• The receiving data link stores all good frames received after a bad frame.
• Only the bad frame is retransmitted upon time-out by the sender.
Working of Sliding Window Protocol
Difference
Data Transmission
Channel Utilization & Data Throughput
For Sliding Window Protocols
b = Channel bandwidth or transmission rate bits/sec
FS = Frame size = # of data bits + # overhead bits = d + h
R = Channel round trip time
N = Send/receive window size
p = Probability frame a data frame is lost or damaged
• Ignoring errors, condition to maximize Utilization/Throughput:
Time to transmit N frames Round trip time
FS/b * N = (d + h)/b * N R
Under this condition:
Maximum channel utilization data size/frame size = d/(d + h)
Maximum data throughput d/FS = d/(d + h ) * b
• Including the effect of errors only on data frame; assuming selective
repeat:
On the average p data frames have to be retransmitted
Under these condition: Total Data Frame overhead = h + p * FS
Maximum channel utilization d/[(1 + p)*FS ]
Maximum data throughput d/[(1 + p)*FS] * b
Operation Sequences for Sliding Window Protocol
Effect of Errors:
• A protocol may be represented by a finite state machine (protocol machine).
• States are chosen when the protocol machine is waiting for the next event (i.e
sending or receiving a protocol data unit PDU).
• The state of the complete protocol is the combination of the state of the two
protocol machines and the channel.
• The state of the channel depends on its content.
• Each state may have one or more transitions to other states when protocol events
occur.
• Incomplete state machine specification.
• Deadlock states.
Data Link Protocol Example:
HDLC - High-Level Data Link Control
• Bit-oriented protocol derived from IBM’s SNA data link
protocol SDLC (Synchronous Data Link Control).
• Frame Types: Information, Supervisory, Unnumbered.
• Uses sliding window with 3-bit sequence numbers.
• Uses CRC-CCITT for error control.
• Protocol commands include:
- DISC (DISConnect) used to disconnect a machine from the line. - SNRM (Set Normal Response
Mode) brings a machine online and sets one machine as channel master and the other as slave ( was
used for dumb terminals when connected to mainframes).
- SABM (Set Asynchronous Balanced Mode).
- FRMR (FRaMe Reject) rejects a frame with correct checksum with impossible structure.
Internet Data Link Protocols:
Serial Line IP (SLIP) RFC 1055
• Send raw IP packets with a flag byte (0xC0) at the end for framing with character
stuffing (data 0xC0 replaced with 0xDB 0xDC).
• Recent versions use header compression by omitting header fields in consecutive
packets and frames.
• Does not include any form of error detection or correction.
• Supports only one network protocol: IP (Internet Protocol).
• Dynamic IP address assignment not supported.
• Lacks any form of authentication.
Internet Data Link Protocols:
Point-to-Point Protocol (PPP)
• Uses standard HDLC framing byte (01111110) with error detection.
• Uses Link Control Protocol (LCP) for brining lines up, option negotiation, and to bring
lines down.
• Network layer options and configurations are negotiated independent of the network layer
used by utilizing different
NCPs (Network Control Protocol) packets for each supported network layer.
• Support for several packet types by using a protocol field:
- Network protocols (protocol field starts with 0): IP, IPX, AppleTalk etc.
- Negotiating protocols (protocol field starts with 1): LCP, NCP.
• PPP is used for both dial-up network access and for router-to- router communication in subnets.
PPP Frame Format
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