Computer Networks
Week 3
Link Layer Issues.
Mr. Bilal Ahmad
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Module contents:
• Protocol stacks
• Protocol basics
• Application/Network QoS
• Link Layer issues
• Error control (ARQ, CRC, FEC)
• Flow control
• Internet Protocol
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Addressing
IPv6 (IPv4 interoperability)
Routing (link-state, distance vector, Internet, routers)
QoS Routing
Routing problems in wireless networks
Geographic Routing
• QoS Support
• IntServe
• DiffServe
• MPLS
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Link layer Issues
• The Link Layer sits directly above the physical layer (PHY).
• At the PHY we have defined the connectors, the interconnection
types, the required voltage levels, the clock speeds, the type of
modulation etc. The link layer has to make use of this to actually
exchange useful data over the links.
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Link layer Issues
• The study here is the mechanisms used for successful exchange of
data. In particular:
• Error control
• ARQ
• CRC
• FEC
• Flow Control
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Link layer Issues
• Our aim is to
• Convert raw ‘PHY layer bit stream’ to stream of ‘frames for network layer’.
• Take raw transmission facility and make it free of undetected transmission
errors.
• Break up data stream into ‘data frames’
• Process the ‘acknowledgment frames’.
• Creates and recognizes frame boundaries (adding special bit patterns to
beginning and end of each frame).
• We must consider that:
• An error burst might destroy/corrupt a frame; the acknowledgement
mechanism should cause a re-transmission. Care is needed to ensure they are
recovered in the correct order and duplication does not occur.
• We need to use:
• Traffic regulation.
• Single/Multiple lines, piggybacking.
• Medium access control (MAC).
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Link layer Issues
• Error control
• LANs and the Internet both discard packets if an error is
detected (at the link layer).
• In the link layer of a leased line from PSTNs error recovery
is performed.
• In wireless links some form of error recovery is
performed.
• In the simplest form error control requests a repeat of any
packets found to contain errors. This is an Automatic
Repeat reQuest (ARQ) mechanism.
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ARQ
• The most basic ARQ mechanism is ‘idle-RQ’:
ARQ error control scheme: (a) error free
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Idle-RQ
ARQ error control scheme: (b) corrupted I-frame
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Idle-RQ
ARQ error control scheme: (c) corrupted ACK-frame
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Idle-RQ
• Note that for this ARQ system to work:
• P (the source) must retain a copy of the sent message.
• In order that S (the destination), can determine a duplicate, each frame
must contain a unique identifier, N(S).
• S must know which was the last frame received without error.
• The scheme is known as ‘Stop and Wait’.
• We could have a system where the unique identifier is just a
single bit in length, i.e. 1 or 0. This is what is used in the
Bluetooth system.
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Idle-RQ
The utilisation, U, will be
U=Tix/Tt ,
Where, Tt=Tix + time transmitter is waiting for an ACK.
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Idle-RQ
• Since the processing time will be short and the ACK
will be short as compared to the data frame. The
utilisation can be approximated to:
U= Tix .
(Tix +2Tp)
Letting a=Tp/Tix
U=
1 .
1 + 2a
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Idle-RQ
QUIZ NO. 1 “Total Time 30 minutes”
• A series of 1000-bit frames are to be transmitted using the idle-RQ protocol.
Determine the link utilization, U, for the following 3 types of data link,
assuming a transmission bit rate of:
(i) 1 kbps,
and (ii) 1 Mbps.
Assume that the velocity of propagation of the first two links is
2x
108 m/s and for the third link is 3 x 108 m/s.
Assume bit error rate is negligible.
Data link a) A twisted pair cable of length 1km.
Data link b) A leased line 200km in length.
Data link c) A satellite link of 50,000km.
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Idle-RQ
• Answer: U = 1.
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Answer: (i) U = 1.
(ii) U = 0.33
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• Answer: (i) U = 0.75
(ii) U = 0.003
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Idle-RQ efficiency observations:
• For relatively short links U ~ 100%, and independent
of the bit rate, i.e. idle-RQ is adequate for short
links and modest bit rates.
• For longer terrestrial links the link utilisation is high
for low bit rates but reduces significantly as the bit
rate increases.
• For very long links, e.g. extra-terrestrial links, then
even for low bit rates, utilization is poor.
• Hence idle-RQ protocol is unsuitable for high bit
rate terrestrial links and extra-terrestrial links.
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Continuous RQ
• The next ARQ scheme we examine gets improved link
utilization at the expense of increased buffer storage
requirements.
• Note:
• Duplex link is required.
• Sender, P, sends data frames continuously.
• P holds a copy of sent frames awaiting acknowledgement.
• An ACK is sent for each correctly received data frame.
• When a data frame is acknowledged it is removed frames
held by P.
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Continuous-RQ
(no errors).
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Continuous-RQ (with errors).
•
If errors occur then the receiver, S can:
1. Just request repeat transmission of corrupted frames,
i.e. ‘selective repeat’ or,
2. Request that the transmission goes back to the where
the last correctly received frame, ‘go-back-N’.
•
•
In both cases corrupted frames are discarded and
retransmission requests are issued after the next
error free frame is received.
A timeout is still necessary in case the last frame
of a sequence is corrupted.
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1. Selective repeat. Effect of corrupted data-frame.
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1. Selective repeat. Effect of corrupted ACK-frame.
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2. Go-back-N. Effect of corrupted data-frame.
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2. Go-back-N. Effect of corrupted ACK-frame.
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Flow Control
• The transmitter and the receiver entities maintain buffers
of frames for operation of the ARQ mechanisms.
• The rate of frame transmission must be limited to avoid
buffer overflow at the receiver.
• To control the rate of flow a sliding window is used.
• This limits the number of unacknowledged frames which
may be transmitted. This limit is set in the Send Window,
K for the link.
• Factors determining size of K: frame size, available buffer
storage, link propagation delay, transmission bit rate.
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Sliding window example:
Send and receive window limits.
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Sequence numbers
• The counters used to determine the received and
acknowledged frames etc. are (obviously) finite in
size.
• In the idle-RQ scheme only 1 or 0 was used to
identify the frames and hence only one frame was
required to buffered.
• With a go-back-N scheme and send window of K,
then K+1 identifiers are needed.
• With selective repeat and send and receive
windows of K, then 2K identifiers are required.
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• Sequence numbers: a) max for each scheme,
b) example assuming 8 sequence numbers.
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Sequence numbers
• In fact sequence numbers will use a binary register so a certain number
of bits will be reserved.
• Idle-RQ requires little buffering but has poor link efficiency (though OK
for short, slow links)
• Selective repeat and Go-back-N schemes are more efficient and more
generally used but require more available memory for buffers and
counters.
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Summary
• The link layer takes the PHY bit stream and using
basic error control and flow control protocols
creates a useful data stream.
• Error control mechanisms include repeat request
schemes (Selective repeat, Go-back-N).
• Flow control is maintained through use of buffers
and counters to control the sliding window.
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