Transcript CH. 6 Data Link Control

CH. 7 Data Link Control
Protocols
Data and Computer
Communications by Stallings
Requirements & Objectives of
Data Link Control
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Frame Synchronization
Flow Control
Error Control
Addressing
Control and Data on Same Link
Link Management
7.1 Flow Control
• Definition:
– A technique for assuring that a transmitting station
does not overwhelm a receiving station with data.
• Stop-and Wait Flow Control
– The sender sends a frame and then waits until the
receiver acknowledges the frame.
– Works well for large blocks of data--but large
blocks are usually broken into smaller blocks.
– Does not always perform well for small blocks.
7.1 Flow Control (p.2)
• Characterization of Stop-and-Wait
– Let B = length of the link in bits--the number of bits
present on the link when a stream of bits fully
occupies the link.
– Let R =data rate of the link, in bps.
– Let d = length, or distance, of the link in meters.
– Let V = velocity of propagation, in meters/second.
– Then B = R x d/V (Equation 7.1).
– Variable a = B/L (Equation 7.2)
– Utilization--Fig. 7.2
7.1 Flow Control (p.3)
• Example 7.1 Stop and Wait
– a. Fiber Optic Link (d = 200 meters; R = 1 G bps; V
= 2 x 108 meters/second; 1,000 bytes/frame)
• B =1,000 bits; L = 8,000 bits; tframe = 8 s.
• a = 1,000/8,000 = .125 < 1 (see Fig. 7.2b)
• Total Normalized Time--1 + 2a = 1.25 (10s)
– b. Satellite Relay Link ( d = 2 x 36,000 km; R = 1 M
bps; V = 3 x 108 meters/second; 1,000 bytes/frame.)
• B =240,000 bits; L = 8,000 bits; tframe = 8ms.
• a =30 > 1 (see Fig. 7.2a)
• Total Normalized Time--1 + 2a = 61 (488 ms).
7.1 Flow Control (p.4)
• Sliding Window Flow Control (Fig.7.3 and 7.4)
– Allows more than one frame to be sent at a time.
– More than one frame may be acknowledged at a time.
– Source A keeps a list of sequence numbers that are
allowed to be sent.
– Destination B maintains a list of sequence numbers
that it is prepared to receive.
– If k bit sequence numbers are used, frames are
numbered modulo(2k) and maximum window size is
2k -1.
7.1 Flow Control (p.5)
• Example 7.3 Sliding Window
– a. Fiber Optic Link
• Time until ACK for 1st frame was 10s.
• tframe was 8s.
• A. window size of 2 would be all that is needed for
transmission to be continuous.
• Note W = Time until ACK/tframe will be the maximum
window needed for continuous transmission.
– b. Satellite Link
• W = 488ms/8ms=61 for continuous transmission.
• If W = 7 (3-bit window), sends 7 frames and must then
wait for an ACK.
7.2 Error Control
• Two Types of Errors: lost and damaged frames.
• Elements of Error Control – automatic repeat
request (ARQ)
– Error Detection: use a CRC
– Positive ACK
• Destination returns a positive ACK for error-free frames.
– Retransmission after Timeout
• Source retransmits a frame that has not been acknowledged
after a predetermined amount of time.
– Negative ACK and Retransmission
• Destination returns a negative ACK for frames in which an
error is detected; the source retransmits the frames.
7.3 Error Control (p.2)
• Stop-and Wait ARQ (Fig. 7.5)
– Based on stop-and-wait flow control.
– Source transmits a single frame and then must
wait for an ACK.
– If an error is detected a NAK could be sent.
– If there is no response, then source times-out
and resends the message.
– To avoid duplications, 1 bit sequence number
could be added to frame and or ACKs.
– It is simple but sometimes inefficient.
7.3 Error Control (p.3)
• Go-back-N ARQ (Fig. 7.6a)
– Source transmits multiple frames (sliding window)
and then waits for ACK (RR).
– Destination sends ACKs up to the last correct frame
received.
– One ACK can acknowledge several frames.
– The error frame is retransmitted along with all
subsequent frames.
• Selective-reject ARQ (Fig. 7.6b)
– Only error frames are retransmitted.
7.3 High Level Data Link
Control (HDLC)
• ISO 3309, ISO 4335.
• Three Types of Stations
– Primary--has the responsibility for controlling
the operation of the link.
– Secondary--controlled by primary station.
– Combination--combines features of primary
and secondary
7.3 HDLC (p.2)
• Two Link Configurations
– Unbalanced (Primaries and Secondaries)
• Used in point to point and multipoint operation.
– Balanced (Combinations)
• Used only in point to point operation.
7.3 HDLC (p.3)
• Three Data Transfer Modes
– Normal response mode (NRM): unbalanced
configuration; poll and selection (used on
multidrop lines).
– Asynchronous balanced mode (ABM): balanced
configuration; either combined station may initiate
transmission (used on pt-to-pt lines).
– Asynchronous response mode (ARM):
unbalanced configuration; secondary may initiate
transmission( rarely used).
7.3 HDLC (p.4)
• Frame Structure(Fig. 7.7)
– Flag Field (8 bits)
• 01111110 (Ox7E)
• Transparency is handled using bit- stuffing.
• Transmitter will stuff a 0 after 5 1's--see Fig.
7.8.
– Address Field (One or more Octets)
• Used to identify secondary stations
7.4 HDLC (p.5)
• Frame Structure (cont.)
– Control Field (8 or 16 bits)
• Information frames: carry data and ACKs.
• Supervisory frames: ACKs.
• Unnumbered frames : supplemental control.
– Information Field (variable number of bits)
– Frame Check Sequence Field (16 or 32 bits)
7.4 HDLC (p.6)
• HDLC Operations (Fig. 7.9)
– Initialization, Data Transfer, Disconnect.
– Table 7.1 Commands and Responses
• Reject supports "Go-Back-N".
• Selective Reject supports "Selective Reject".
• Receiver Not Ready, Receiver Ready
– Used as ACKs and for flow control.
• Mode setting commands.
• Information transfer commands.
• Recovery commands.
– Error types can be reported.