1
2. GETTING
CONNECTED (PART 2)
Rocky K. C. Chang
Department of Computing
The Hong Kong Polytechnic University
26 January 2016
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1. Automatic repeat request (ARQ)
• Approach to achieving link reliability:
• Ask for retransmission when a corrupted frame is detected.
• Mechanisms for asking a sender to retransmit:
• Receiver to send negative acknowledgments for corrupted frames,
or
• Receiver to send positive acknowledgments for good frames.
• Which one is better?
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1. Automatic repeat request (ARQ)
• Both approaches require a timeout mechanism.
• Negative-ack approach: A timer starts when a negative
acknowledgment is sent out.
• Positive-ack approach: A timer starts when a message is sent out.
• Retransmissions take place when timeouts occur:
• Negative-ack approach: The receiver retransmits a negative
acknowledgment.
• Positive-ack approach: The sender retransmits a frame.
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1. Automatic repeat request (ARQ)
• The ARQ implements a positive acknowledgment
approach.
• Only after an acknowledgment is received will a frame be removed
from a send buffer.
• An acknowledgment can be piggybacked on a message sent to the
other direction.
• An acknowledgment sometimes indicates the sequence number of
the next expected frame.
• Positive acknowledgment is usually accumulative, e.g., receiving
an acknowledgment for frame 4 implies that frames 1-3 are all
received correctly.
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1.1 Stop-and-wait ARQ
• The maximum number of unacknowledged frames is one.
• A sender cannot send a second frame before receiving an
acknowledgment for the first frame.
• The minimum number of sequence numbers needed to
identify the frames is two (0, 1), i.e., the first two in the
send buffer.
• 0: for the frame sent and waiting for its acknowledgment.
• 1: for the frame to be sent after receiving an acknowledgment for
the 0th frame.
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Sender
Receiver
Sender
Timeout
ACK
Timeout
Timeout
Fram
e
(a)
Timeout
Fram
e
Fram
e
AC K
Sender
Timeout
Receiver
Fram
e
AC K
(c)
Timeout
Sender
Timeout
Time
Fram
e
Receiver
Receiver
Fram
e
AC K
Fram
e
AC K
ACK
(b)
(d)
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1.1 Stop-and-wait ARQ
• Advantages:
• Very simple to implement, both on the sender and receiver sides.
• Disadvantages:
• Achieve a very low throughput, especially in a high-speed link
(does not keep the pipe full).
• In the best case, only one frame can be sent in a round-trip time.
• In other cases, an additional number of round-trip times is required
when errors occur.
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1.2 Sliding-window (or go-back-n) ARQ
• The maximum number of unacknowledged frames may
be more than one.
• A sender may continue to transmit frames even when
acknowledgments for frames previously sent are not received.
• A sliding window is used to keep track of the sender’s state and
receiver’s state.
• The window size determines the maximum number of
unacknowledged frames allowed on the sender side.
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1.2 Sliding-window (or go-back-n) ARQ
• On the sender side:
• Frames are labeled by a sequence number of integer values,
starting from 1.
• A static parameter: send window size (SWS)
• Two variable parameters:
• The maximum sequence number being acknowledged by the receiver
(LAR)
• The maximum sequence number sent by the sender (LFS)
• Note that LFS  LAR
• LFS = LAR: All frames sent have been acknowledged.
• LFS > LAR: LFSLAR frames are yet to be acknowledged.
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1.2 Sliding-window (or go-back-n) ARQ
• Also note that LFS  SWS + LAR.
• When LFS = SWS + LAR, the window is said to be full, and no more
new frames can be sent before (LAR+1)th frame is acknowledged.
• How do the values of LFS and LAR change?
• LFS is initialized to 0, and it is incremented after each new frame is
sent.
• LAR is initialized to 0, and it is shifted to the right according to the new
sequence number(s) acknowledged by the receiver.
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1.2 Sliding-window (or go-back-n) ARQ
• Sender side:
= SWS
…
…
LAR
LFS
• Receiver side:
= RWS
…
…
NFE
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1.2 Sliding-window (or go-back-n) ARQ
• On the receiver side:
• A static parameter: receive window size (RWS)
• RWS is the maximum number of out-of-order frames that the receiver is
willing to accept.
• A variable parameter:
• Sequence number of the next frame expected (NFE)
• If a frame, whose sequence number is larger than NFE+RWS, is
received, the frame will be discarded; otherwise, it will be buffered in
a receive buffer.
• When a frame is received correctly, NFE is either updated or
unchanged.
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1.3 Three uses of ARQs
• Provide reliable service
• Provide in-order service
• The original order of the frames is preserved even when out-oforder frames are received.
• Provide flow control
• Prevent the sender from flooding the receiver.
• The sender needs to vary the value of SWS by taking into account
of the state of the receive buffer.
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2. Local area networks (LANs)
• Early 1980s
• IBM’s token ring vs. DIX (Digital, Intel, and Xerox) Ethernet
• IEEE 802.2 (logical link control), 802.3 (Ethernet), 802.4 (Token Bus),
802.5 (Token Ring)
• Late 1980s
• Fiber Distributed Data Interface (FDDI)
• Distributed Queue Dual Bus (DQDB)
• Early 1990s
• ATM LANs vs Fast Ethernet (switched Ethernet)
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2. Local area networks (LANs)
• Mid-1990s
• IEEE 802.11 (wireless LAN)
• Now and future (who knows?)
• Gigabit Ethernet vs ATM LANs
• Optical Ethernet, wireless Ethernet
• Development trends:
• From shared medium to switched LANs
• From router-based backbone to switched backbone
• From wired to wireless
• From single-medium to multi-media
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3.1 Ethernet LAN: physical connectivity
• Components:
• Cable (passive)
• Transceivers (transmitter + receiver)
• Adaptor (active). Each adaptor card is uniquely identified by a 48-bit
(physical or MAC) address, e.g., 00:40:26:5A:67:88.
• Design principles:
• Cost-effective resource sharing
• Reliability
• Inexpensive
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3.1 Ethernet LAN: physical connectivity
Transceiver
Ethernet cable
Adaptor
Host
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3.1 Ethernet LAN: physical connectivity
• Both DIX and IEEE 802.3 Ethernets do not require
switching elements.
• Hosts are connected to a cable (10base2/5/T) through network
adaptors.
• Several segments may be connected (horizontally) to another
segment (vertically) through hubs, which serve as repeaters.
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3.1 Ethernet LAN: physical connectivity
…
…
…
…
Repeater
Host
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3.1 Ethernet LAN: physical connectivity
Hub
Hub
from the datalink layer and
up
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3.2 Ethernet LAN: Datalink sublayers
• A new multiple access control (MAC) problem:
• How do multiple hosts share a single transmission medium
efficiently?
• This problem occurs in token ring, FDDI, and wireless LAN.
• An additional MAC sublayer was created for this purpose.
• A logical link control (LLC) sublayer:
• Provide similar services as a datalink layer except that error
detection is provided at the MAC sublayer.
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3.2 Ethernet LAN: Datalink sublayers
• Provide three types of services:
• Unacknowledged connectionless (datagram) service: Basically no
additional service.
• Acknowledged connectionless service: Reliability through a stop-andwait-ARQ-like mechanism.
• Connection-mode service: A connection is set up between two hosts
with flow control and reliability services.
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3.2 Ethernet LAN: Datalink sublayers
• The datalink layer consists of
• LLC sublayer and MAC sublayer
IEEE 802.2 Logical Link Control (LLC) Sublayer
IEEE
802.3
IEEE
802.4
IEEE
802.5
IEEE
802.11
IEEE
802.12
ANSI
FDDI
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3.2 Ethernet LAN: frames
• DIX Ethernet frame structure:
• The 7-byte preamble is sent before the frame to allow the receiver to
synchronize with the signal.
1-byte start 6-byte dest 6-byte src
7-byte
2-byte
frame
preamble
address
address type
delimiter
type
0800
Data
4-byte
CRC
IP datagram
• IEEE 802.3 Ethernet frame structure:
802.3 MAC
Preamble
802.2 LLC
802.2 SNAP
dest
src
DSAP SSAP cntl org code type
address address len
03
00
AA
AA
Data
4-byte
CRC
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3.3 Ethernet LAN: MAC protocol
• Types of MAC addresses:
• Unicast address: hardwired into ROM
• Broadcast address: all 1 bits
• Multicast address: First bit set to 1 and configurable.
• Promiscuous mode
• CSMA/CD (carrier sense multiple access with collision
detection)
• Each adaptor is able to distinguish a busy link from an idle link.
• Each adaptor is able to detect “frame collisions,” if occurred, as it
transmits.
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3.3 Ethernet LAN: MAC protocol
• To send a frame,
• transmits it immediately when the link is detected idle.
• The maximum length of the payload is 1500 bytes for a 10-Mbps DIX
Ethernet.
• To receive a frame,
• Every adaptor attached to the link will receive a copy of a frame
transmitted on the link.
• The frame will be discarded if the destination address does not match
• its unicast address, broadcast address, and any configured multicast
addresses.
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3.3 Ethernet LAN: MAC protocol
• Carrier sense is not sufficient for avoiding frame collisions.
• A frame must be long enough to detect collisions:
• A sufficient condition: A frame occupies the entire pipe back and forth.
• For example, for a 10-Mbps Ethernet segment of 2500m long (a roundtrip propagation delay of 51.2 s):
• Minimum frame length = 51.2s  10Mbps = 512 bits (64 bytes), or a 14-byte
header + a 46-byte payload + a 4-byte CRC.
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3.3 Ethernet LAN: MAC protocol
A
B
A
B
A
B
A
B
(a)
(b)
(c)
(d)
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3.3 Ethernet LAN: MAC protocol
• When more than one adaptor transmits frames “almost at
the same time,”
• The frames are “collided” and can be detected by the adaptors
involved.
• The adaptors involved then send a 32-bit jamming sequence, and
stop transmission.
• The adaptors use exponential backoff for retransmission (up to a
limited number of attempts).
• After first collision: either 0 or 51.2 s.
• After second collision: 0, 51.2, 102.4, 153.6 s.
• After nth collision: k51.2 s for k = 0..2n1.
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3.4 Ethernet’s performance
• Throughput decreases with a and the number of hosts,
where
• a = propagation delay/transmission delay.
• Implications:
• Limit on the number of stations and the maximum length of the
Ethernet segment
• Effect of increasing link’s data rate
• Support for delay-sensitive data
• Methods of improving its performance
• Each collision domain consists of only one host.
4. Wireless Links
• Wireless links transmit electromagnetic signals
• Radio, microwave, infrared
• Wireless links all share the same “wire” (so to speak)
• The challenge is to share it efficiently without unduly interfering with
each other
• Most of this sharing is accomplished by dividing the “wire” along
the dimensions of frequency and space
• Exclusive use of a particular frequency in a particular
geographic area may be allocated to an individual entity
such as a corporation
4. Wireless Links
A wireless network using a base station
4. Wireless Links
• Mesh or Ad-hoc network
• Nodes are peers
• Messages may be forwarded via a chain of peer nodes
A wireless ad-hoc or mesh network
4.1 Wireless LAN (IEEE 802.11)
• Also known as Wi-Fi
• Like its Ethernet and token ring siblings, 802.11 is
designed for use in a limited geographical area (homes,
office buildings, campuses)
• Primary challenge is to mediate access to a shared communication
medium – in this case, signals propagating through space
• 802.11 supports additional features
• power management and
• security mechanisms
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4.1 Wireless LAN (IEEE 802.11)
• MAC problem:
• Hidden node (terminal) problem: Frame collision occurs but the
senders involved are unaware of it.
• AB and CB (A and C are not connected)
• Exposed node problem: A node is unnecessarily prevented from
transmitting frames.
• BA and CD (A and D are not connected)
• Both problems are due to the fact that, unlike Ethernet, the nodes
are not always connected together.
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4.1 Wireless LAN (IEEE 802.11)
A
B
C
D
• Multiple Access with Collision Avoidance (MACA)
• To send a frame,
• the sender first sends a Request to Send (RTS) frame to the receiver.
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4.1 Wireless LAN (IEEE 802.11)
• The receiver then replies with a Clear to Send (CTS) frame back to the
sender.
• Any node that sees the CTS frame will refrain from sending any
frames (for the hidden node problem).
• Any node that sees the RTS frame but not the CTS frame is free to
transmit (for the exposed node problem).
• When two or more nodes transmit an RTS frame at the same time,
their RTS frames collide.
• Retransmission of RTS frames takes place after the nodes involved do
not receive the CTS frames, i.e. no direct collision detection supported.