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Datornätverk A – lektion 8
Kapitel 11: Flow control and Error control.
(Kapitel 12: Point-to-point access PPP.
Översiktligt.)
11.1 Flow and Error Control
Flow Control (Flödesstyrning)
Error Control (Felhantering)
•
•
Båda dessa funktioner hanteras av vissa
datalänkprotokoll (lager 2), i LLC-sublagret, t.ex.
vid trådlös kommunikation eller vid modem.
End-to-end flödesstyrning och felkontroll
hanteras av transportprotokollet TCP (lager 4).
Flow control
Necessary when data is being sent faster than it can be
processed by receiver to avoid that the receiver’s buffer is
overwhelmed.
Felhantering med hjälp av
felrättande koder
FEC = Forward Error Correction.
Baseras på felrättande istället för felupptäckande koder.
Kräver ingen backkanal.
Två typer:
1. Faltningskoder (convolutional codes).
Ex:Vid Faltningskod med kodtakt (code rate) 1/3 infogas två redundanta bitar
mellan varje bit i nyttomeddelandet. Dessa felrättande bitar beräknas
kontinuerligt för varje inkommande bit i nyttomeddelandet.
2.
Blockkoder (block codes)
Ex: I digital-TV-systemet används en s.k. Read Salomon-kod med
beteckningen RS(204, 188, 8). Det innebär att nyttoinformationen delas
upp i 188 byte stora block. För varje block beräknas en felrättande kod,
som läggs till blocket så att blocket blir 204 byte. Redundanden är alltså
204 – 188 = 16 byte. Koden klarar 8 felaktiga byte.
Felhantering med hjälp av
felupptäckande koder
Alternativ 1: Bortkastning av felaktiga paket.
Alternativ 2: ARQ = Automatic Repeat reQuest
= automatisk omsändning av paket vid bitfel, eller om paketet
inte når fram.
I fortsättning kommer vi med begreppet ”error control”
eller ”felkontroll” att avse ARQ.
Protocols to be presented
•
•
•
•
Stop-and-wait ARQ
Sliding Window Flow Control
Go-back-N ARQ
Selective Repeat ARQ
Sliding Window
Protocols
The Stop-and-Wait Protocol
The simplest protocol for error and flow control
How the protocol operates:
○ Source may not send a new frame until the receiver acknowledges
previous one.
○ The receiver sends only positive acknowledgements (ACK) to notify
the sender that the frame was received.
○ If the frame 0 was received, the ACK 1 is sent. In that way the sender
is notified that the receiver is expecting frame 1.
○ The ID of the frame is called a sequence number.
○ 1 bit sequence numbers is sufficient. Sequence: 0 1 0 1 0 ... .
11.1
Normal operation
ACK n = Acknowledgement. Expecting frame number n
11.2
Stop-and-Wait ARQ, lost frame
Lost or Damaged Frame
• The sender starts a timer when it sends each frame
• If the ACK is not received before the timer expires, the
sender resends the same frame again
11.3
Stop-and-Wait ARQ, lost ACK frame
Lost or damaged ACK
• Lost ACK causes duplicate frames
• A duplicate frame is recognized by the sequence number
and is discarded
• The receiver sends the same ACK again
11.4
Stop-and-Wait ARQ, delayed ACK
Note:
Numbered acknowledgments are
needed if an acknowledgment is
delayed and the next frame is lost.
Piggybacking
• Usually the communication is in both ways – this means that
the sender is a receiver and the receiver is the sender, too.
(both send and receive data)
• To save on the processing and bandwidth the short ACKs
messages are not sent as separate frames. Instead, they may be
included in the frames with data.
• This technique is called piggybacking
11.5
Piggybacking
Efficiency of Stop-and-Wait
• Very inefficient, having in mind that most of the time the sender is idle
• Example: 40 km copper cable, 10 Mbps rate, 1000 bit frame,
○ Signal in copper propagates at 2 x 108 m/sec
○ Transmission time is 1000/10000000 (Takes 0.1 msec to transmit frame)
○ Propagation time is 40000/ 2 x 108 (0.2 msec delay to begin arriving at the
receiver)
○ Total time is 0.3 msec. to get to the receiver
○ ACK transmission time is approximately 0 (assuming the ACK is very short
(length  0)
○ 0.2 msec is the time for the ACK to arrive at the sender
• Total time is 0.5 msec before the sender can transmit again
• 0.5 ms for 0.1 msec frame or efficiency is 20%
Sliding-Window flow control
• Several frames can be sent without acknowledgement being received
• N is the window size – the maximum number of frames that can be sent
and not being acknowledged.
• The receiver must be able to buffer N frames.
• Sequence numbers are used to identify each frame. They are carried in the
header.
• The number of different sequence numbers must be at least N+1.
• If the field for sequence numbers allows m bits, the number of different
sequence number is 2m and the sequence numbers range from 0 to 2m-1.
In that case the maximum window size is N = 2m-1.
11.6 Sender sliding window
The sender window is the the set of frames that may be
transmitted before an ACK. It slides when the sender
has received an ACK and sent next frame.
11.7
Receiver sliding window
The receiver window is the the set of frames that may
be accepted before the buffer is full. While the buffer
is full, the receiver sends no ACK. The window of a
stuffed receiver slides when the receiver has
”consumed” a frame and thus sent an ACK.
Stop-and-Wait vs. Sliding Window
Window size N=3.
Sender
Sender
Receiver
Transmission +
propagation
Transmiss
time for the
ion time
packet
for the
packet
Transmission +
propagation
time for the
ACK
.
.
.
Sequence numbers are 1
bit long (0 or 1)
Frame
consumption
delay
Receiver
propagation
time
..
.
Frame
consumption
delay
...
Sequence numbers from 0 to 2m1. m-bit field for the seq. num.
Sender and Receiver Prospective
The window size
is 7
Sliding Window Flow Control
0
1
2
3
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5
6
7
0
1
2
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5
6
7
0
0
1
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0
1
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0
7
0
0
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0
1
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0
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0
0
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0
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0
0
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0
0
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0
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0
0
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0
1
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0
0
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0
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0
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0
0
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0
1
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0
F0
F1
ACK1
F2
0
1
2
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0
1
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0
ACK2
F3
0
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0
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ACK3
F4
0
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0
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0
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0
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0
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0
ACK4
F5
ACK5
ACK6
N=6
ARQ with Sliding Window
Problems arise when some of the frames are discarded (errors or lost
frames). Two strategies are developed to deal with this problem:
• Go-back-N strategy
○ The reciever simply discards all frames after the damaged frame without
sending acknowledgement.
• Selective repeat strategy
○ The receiver keeps all the frames after the damaged frame. It sends negative
acknowledgement (NACK) for the damaged frame. When the sender finaly
notice that something is wrong it retransmits the bad frame.
The two strategies are trade-offs between bandwidth and data-link buffer
space.
Go-Back-N Strategy
• If a frame is lost, the lost frame and all the frames sent after it are
sent again.
• Sending window of size N, receiving window of size 1.
• The sender has to buffer N frames
• Bandwidth is wasted.
11.9
Go-Back-N ARQ, normal operation
11.10
Go-Back-N ARQ, lost frame
11.11 Go-Back-N ARQ: sender window size
Selective Repeat Strategy
• Only retransmit the frames that are in error
• Both sending and receiving window are of size N
11.13
Selective Repeat ARQ, lost frame
11.14
Selective Repeat ARQ, sender window size
Bandwidth – Delay Product
• The product of the bit rate (bandwidth expressed as bits per
seconds) and the propagation time gives the number of bits
that can be on the channel and thus can give orientation
about the window size
• When propagation time is high (for example in satellite
channels), the window size need to be larger
Example 1
In a Stop-and-Wait ARQ system, the bandwidth of the line is 1 Mbps, and 1 bit
takes 20 ms to make a round trip. What is the bandwidth-delay product? If the
system data frames are 1000 bits in length, what is the utilization percentage of
the link?
Solution
The bandwidth-delay product is
1  106  20  10-3 = 20,000 bits
The system can send 20,000 bits during the time it takes for the data to go
from the sender to the receiver and then back again. However, the system
sends only 1000 bits. We can say that the link utilization is only 1000/20,000,
or 5%. For this reason, for a link with high bandwidth or long delay, use of
Stop-and-Wait ARQ wastes the capacity of the link.
Example 2
What is the utilization percentage of the link in Example 1 if the link uses GoBack-N ARQ with a 15-frame sequence?
Solution
The bandwidth-delay product is still 20,000. The system can send up to 15
frames or 15,000 bits during a round trip. This means the utilization is
15,000/20,000, or 75 percent. Of course, if there are damaged frames, the
utilization percentage is much less because frames have to be resent.
PPP (Point-to-Point Protocol)
• Based upon HDLC
• Used for point-to-point access
• Common protocol used for connecting home users to the Internet (via
dial-up, DSL or cable modem or leased line)
• Defines the negotiation for establishment of the link
• Defines the protocol carried on the network layer
• Includes authentication and a field about the type of network protocol
carried within the frame
PPP Frame Format
Number of bytes in a field
1
1
Flag
Address
01111110 11111111
1
Control
00000011
1 or 2
variable
Protocol
Payload
2 or 4
CRC
1
Flag
01111110
 Physical Address field with all 1s indicate
broadcasting, i.e. that all stations accept the
frame
 Since the Address and Control fields are
constant, the two parties can negotiate to omit
them, thus saving 2 bytes
 Protocol field defines what is carried in the
payload field (user data or other information)
 CRC bits are error control bits