Data Link Layer Issues

1
Data Link Layer Issues

• Dealing with Different Types of Networks

2
Types of Networks

• Network hardware can be categorized into
• Circuit-switched (e.g. telephone)
• Prior to communication, the hardware establishes
  a dedicated end-to-end connection
• Since there is a dedicated connection, a
  continuous stream of bytes can be sent
• Frequency or time-division multiplexing can be
  used to share links in such a network
• Packet-switched (e.g. Ethernet, ATM)
• Data is divided into packets of limited size, and
  each is forwarded through the network to the
  destination
• This can be done by routers or switches

3
Types of Networks

• Disadvantages
• Circuit-switched
• A dedicated connection that has no transmission
  means wasted bandwidth
• A connection is time consuming if short,
  infrequent, or sporadic communication is to occur
• Packet-switched
• Forwarding each packet means that each router
  must decide the next hop for every packet (even
  for the same destination)
• Routers are typically network slowdowns due to
  the amount of processing, as well as input/output
  buffering

4
Types of Networks

• Circuit-switching is used in a telephone
  conversation
• A connection to the receiver is established by
  the sender (the caller)
• The telephone company reserves a certain
  bandwidth (64 Kbps for voice communication) for
  this call
• If the bandwidth is not used by the callers, it
  is wasted
• Packet-switching is similar to the postal service
• Each message (envelope) is addressed to the
  recipient individually, and the postal service
  delivers each message to the recipient
• The postal service may deliver these envelopes
  through different cities and methods of transport
  (airplane, truck, )
• It can be said that these messages can be
  delivered using different routes

5
Circuit-Switching
A
B
Telephone Company Switching System
6
Circuit-Switching
A
B
Telephone Company Switching System
7
Packet-Switching
Quebec, QC
Buffalo, NY
Montreal, QC
A
B
Windsor, ON
Toronto, ON
Niagara Falls, ON
London, ON
Ottawa, ON
Kitchener, ON
Postal Network
8
Types of Packet-Switching

• Virtual circuit-switching
• A virtual circuit is created between source and
  destination
• This VC is used for all subsequent sending of
  packets
• Datagram
• Each packet is routed individually

9
Virtual Circuit Packet-Switching

• Advantages
• After the first message, routing is faster
• A route must only be determined once, for the
  first message
• Once the route has been determined, the path used
  by the router is reused for all messages
• As a result, routing tables are much smaller (and
  can be searched more quickly)
• Because a connection is created, the connection
  identifier can be used (alone) to address packets
• Typically, such as with ATM cells, this can
  reduce the size of a cell/packets header
• Messages do not arrive out of order
• As a result, receivers do not need to reorder the
  cells

10
Virtual Circuit Packet-Switching

• Disadvantages
• Connections take some time to create
• Routers/switches must intercommunicate in order
  to create the connection
• Infrequent messaging is not suitable for
  connection-based messaging
• The connection may be lost after a timeout, and
  will have to be recreated again and again
• The time delay for creating the connection may
  outweigh the speed benefits of using
  connection-based transport
• Routing tables will be dynamic, and routing
  algorithms are more complex

11
Datagram Packet-Switching

• Advantages
• Connections need not be created
• Infrequent messaging is perfect for
  connectionless messaging
• Connectionless messaging can be resumed after any
  amount of delay, any number of times, without any
  delays due to the resumption of communication
• Routing each message separately allows for load
  balancing
• Some messages may be sent through one route, but
  when that route becomes saturated, messages may
  then be sent through a different route in order
  to achieve the most optimal communication possible

12
Datagram Packet-Switching

• Disadvantages
• Each message takes a certain amount of time to
  transmit (including transmission, routing,
  reception, etc.)
• Nodes communicating large amounts of information
  in a short time will
• Use a lot of bandwidth for things such as header
  information
• Waste a lot of time routing messages to the same
  destination
• Messages may arrive out of order
• Messages must be reordered by the recipient

13
Multiple Access Strategies

• Schemes for Sharing a Communication Medium

14
Multiple Access

• Most networks are shared medium
• This means that a single medium (e.g. radio
  frequency) is shared by all of a networks hosts
• We need a scheme to allow the hosts to share the
  medium, without collisions
• Collisions occur when two (or more) messages are
  transmitted at the same time
• The result is constructive and destructive
  interference in the carrier wave
• This causes the messages to be combined and
  scrambled

15
Contention

• In contention networks, any node that has a
  packet to send, merely sends the packet
• It is clear that this type of network frequently
  experiences collisions
• The more nodes trying to communicate, the higher
  the chance of collisions
• Thus, contention networks are severely limited in
  the number of hosts possible

16
Contention
Transmit
17
Contention
18
Contention
Transmit
19
Contention
20
Contention Collisions
Transmit
Transmit
21
Contention Collisions
Scrambled Signal
22
Contention

• No collision avoidance is present
• Messages are just sent
• When collisions occur, the messages are simply
  resent after some random (or pseudo-random)
  amount of time
• Collisions can occur anytime

23
Carrier Sensing
Test the medium for a signal
24
Carrier Sensing
Test the medium for a signal Available Transmit
25
Carrier Sensing
Test the medium for a signal
26
Carrier Sensing
Test the medium for a signal In use
27
Carrier Sensing
Transmission Complete
28
Carrier Sensing
Test the medium for a signal
29
Carrier Sensing
Test the medium for a signal Available
30
Carrier Sensing Collisions
31
Carrier Sensing Collisions
32
Carrier Sensing Collisions
Scrambled data
33
Carrier Sensing Collisions
Transmit
Transmit
34
Carrier Sensing (CSMA)

• To reduce the number of collisions, the medium is
  tested for a signal before each transmission
• If a signal exists, the node waits
• Signal testing can be anything from detection of
  an electrical signal, to testing for photons
• Collisions can still occur (although less often)
• If a node tests for a signal before a
  transmission from another node, and transmits
  after, a collision occurs

35
Carrier Sensing Hardware
Transmitter
Receiver
If the message is broadcast or the address is
this stations address, the message is forwarded
to the receiver
When a signal is detected, transmissions
are blocked by the signal detector
Signal Detector
36
CSMA/CA

• CSMA/CA networks (such as wireless 802.11g) also
  use carrier sensing and collision detect
• However, detecting collisions in wireless
  networks is significantly more complicated
• Also, after detecting carrier and determining
  there is no signal, a CSMA/CA network transmits a
  Do not broadcast message
• If this message is sent without a collision, the
  host can assume it is safe to transmit

37
Carrier Sensing Networks

• Advantages
• No tokens
• Simple hardware
• No need for token transmission
• Disadvantages
• Collisions
• Wasted bandwidth for re-transmits
• Require complicated re-collision avoidance schemes

38
Token Passing
Transmit
Transmit
Transmit
Transmit
T
39
Token Passing
Transfer Token
T
40
Token Passing
T
41
Token Passing
Transmit
T
42
Token Passing

• A small packet (the token) is passed from node to
  node
• When a node has the token, it has sole use of the
  network medium
• There are no collisions
• The nodes must have the token in order to
  transmit
• The network hardware ensures that there is only
  one token at any given time

43
Token-Based Networks

• Advantages
• No collisions, so no bandwidth is wasted by
  collisions and re-transmits
• No need for re-collision avoidance schemes
• Disadvantages
• Token transmission uses bandwidth
• More complicated hardware
• Hardware must be built to use tokens, dynamically
  determine token sequence, etc.

44
Local Area Networks

• Networks which span a small geographic area
• They typically represent high bandwidth, short
  delays, few errors
• They commonly support features such as
  broadcasting, multicasting
• They are typically limited to hundreds of network
  nodes (maximum)

45
Typical Local Area Networks

• A collection of computers in the same room
• e.g. The basement of the computer centre
• All computers within an office building
• e.g. The computers in the offices of the
  professors and staff in Lambton tower

46
Local Area Network Topologies

• Structures of LANs

47
Token Bus Networks

• The token is passed in a specific sequence
• Nodes must know the address if the next node in
  the sequence
• The token sequence is not necessarily in the same
  order as the physical order of nodes on the
  communication medium
• When a node has completed transmission, it
  forwards the token, addressed to the next node in
  the token sequence
• The token sequence forms a logical ring

48
Common Token Bus Networks

• IEEE 802.4 networks
• Nodes are share a communication medium similar to
  that of Ethernet (IEEE 802.3)
• Coaxial cable connection

49
Token Bus Operation
Transmit
A
C
B
D
Token sequence C,A,D,B
50
Token Bus Operation
Transmit Token
A
C
B
D
Token sequence C,A,D,B
51
Token Bus Operation
Receive Token
A
C
B
D
Token sequence C,A,D,B
52
Token Bus Operation
Transmit
A
C
B
D
Token sequence C,A,D,B
53
Token Bus Operation
Transmit Token
A
C
B
D
Token sequence C,A,D,B
54
Token Bus Operation
A
C
B
D
Receive Token
Token sequence C,A,D,B
55
Token Bus Operation
A
C
B
D
Transmit
Token sequence C,A,D,B
56
Token Ring Networks

• The token is passed to each node, in the physical
  order on the network
• The physical medium must be a closed loop to meet
  this network category
• So the token can keep going around the network

57
Common Token Ring Networks

• IEEE 802.5 networks
• Nodes are share a coaxial communication medium
  similar to that of Ethernet (IEEE 802.3)
• FDDI networks (fibre distributed data interface)
• Nodes use 2 fibre optic rings as the
  communication medium
• CDDI networks (copper dist. data interface)
• Based on FDDI technology, but uses copper wiring
  similar to 802.4
• However, CDDI uses 2 rings like FDDI

58
Token Ring Operation
A
Transmit
D
B
C
59
Token Ring Operation
A
Transmit Token
D
B
C
60
Token Ring Operation
A
Receive Token
D
B
C
61
Token Ring Operation
A
D
B
Transmit
C
62
Token Ring Operation
A
Transmit Token
D
B
C
63
Token Ring Operation
A
D
B
Receive Token
C
64
Token Ring Operation
A
D
B
C
Transmit
65
Bus and Ring Networks

• Advantages
• Less wiring is necessary
• Disadvantages
• Node failure can mean partial (or complete) LAN
  failure
• This can mean locating network problems is also
  more difficult

66
Star Topology

• Star networks send all messages through a central
  hub
• Each node on the network is wired separately to
  the hub
• Star networks are not a shared bus technology,
  but a private bus technology
• However, nodes still share the hub

67
Common Star Networks

• Twister pair Ethernet (logical star)
• All nodes connect to a central hub (an Ethernet
  hub) via Cat5 cables
• The hub forwards messages to all wires, and the
  destination node keeps the message
• Other nodes ignore the message
• An Ethernet switch (similar to an ATM switch)
  forwards only in the one correct direction (or
  not, if appropriate)

68
Star Network Operation
Transmit
A
B
Hub
C
D
69
Star Network Operation
A
B
Hub
C
D
Receive
70
Star Network Operation
Transmit
A
B
Hub
C
D
71
Star Network Operation
A
B
Hub
C
D
Receive
72
Twisted Pair Ethernet

• Physically, all Ethernet types are bus networks
• However, the actual layout of the cables in
  twisted pair Ethernet forms a star topology
• Twisted pair is called a logical star topology,
  while still a physical bus topology

73
Twisted Pair Ethernet as a Bus
Short Shared Bus
B
C
A
D
Hub
F
G
E
H
Long Private Lines
74
Traditional Ethernet as a Bus
Long Shared Bus
B
C
A
D
F
G
E
H
Short Private Lines
75
Star Topology

• Advantages
• Simple installation and wiring
• Node failures do not affect the rest of the
  system
• Disadvantages
• All traffic passes through same hub, so network
  bandwidth is limited by hub speed
• This can be reduced with buffers inside hubs
  which store messages that come in when the hub is
  busy
• Hub failure LAN failure
• More wiring
• Duplication of messages

76
LAN Service Models

• In general, most LANs implement (in some sense)
  the OSI reference model
• The IEEE committee on LAN technology (IEEE 802)
  chose to subdivide the Data Link Layer into 2
  sub-layers
• MAC (Medium Access Control) Deals with issues
  specific to each type of LAN
• Such as token passing, collision detection, error
  detection, etc.
• LLC (Logical Link Control) Deals with issues
  common to all LAN types
• Such as data transmission, etc.

77
Data Link Addressing

• The data link layer is represents the network
• e.g. Ethernet
• Addressing, then, is specific to the network
  hardware
• MAC addresses are typically used for this purpose
• These addresses are not used in routing
• They are only used on a single network
• Thus, they are used for hop to hop delivery
• End-to-end delivery is the domain of the Network
  layer

78
MAC Addresses

• Officially the IEEE 802 committee standardized
  addresses to be 16bit, 48bit, and even 60bit
• 48bit addresses (in use by most LANs covered by
  the 802 committee) allow for globally unique
  identifiers (GUIDs) to be assigned to each
  network card by the manufacturer
• As a result, each NIC can be uniquely identified
  on any network
• These are called MAC addresses, due to the Data
  Link sub-layer that deals with them
• e.g. 8D-F0-A6-75-9C-13

79
Data Link Flow Control

• Flow control is limiting the packet rate so that
  both the source or destination can keep up
• At the data link layer, source and destination
  are on the same LAN
• Thus, limiting the packet rate is relatively easy

80
Data Link Reliability

• Reliability
• Best effort The network takes no steps to
  ensure packets arrive
• The majority of packets should be received
  without problems
• Reliable The network uses acknowledgements to
  ensure packets arrive
• When packets are lost (for whatever reason), they
  are handled appropriately
• Error handling Corrupt packets should be
  re-sent
• Reliability at the Data Link layer is usually
  unnecessary, since the Transport layer will
  typically be able to do it more efficiently

81
Error Control

• Error control is achieved using one of the
  following methods
• Checksum An n-bit sum is taken of the binary
  stream
• In other words, a checksum counts the ones
• What if one 0 became a 1 and a 1 became a 0??
• Cyclical redundancy check
• Should generate different CRC values, despite the
  same number of 0s and 1s

82
Ethernet

• An Early Incarnation of LANs

83
What Started It All
Robert Metcalfe (from Xerox PARC)
84
Ethernet History

• In 1973, Xerox PARC developed a packet-switched
  LAN, called Ethernet
• In 1978, IEEE created a standard (802.3) based on
  the research of Xerox, Intel, and DEC
• IEEE Institute of Electrical and Electronics
  Engineers
• 802.3 Ethernet uses a coaxial cable to connect
  nodes (called 10Base5 or ThickNet)
• Since then, several new forms of Ethernet have
  evolved

85
ThickNet (10Base5)
Outer Insulating Jacket
Inner Insulating Layer
Braided Metal Shield (Ground)
Transmission Wire
10Base5 5 gt 0.5
½ Inch Diameter
86
ThickNet (10Base5)
10Base5 10 gt 10 Mbps

• Each network node uses a transceiver
• A transceiver taps into the wire through holes
• Maximum throughput is 10 million bits per second
  (10 Mbps)

Transceiver
87
ThinNet (10Base2)

• Create as an inexpensive alternative to ThickNet
  (or 10Base2)
• Called thin-wire Ethernet, because it uses a thin
  cable with less shielding
• Less shielding means more interference, so cable
  placement is important
• 10Base2 does not use transceivers, which are
  expensive, which further reduces cost

88
ThinNet (10Base2)
10Base2 2 gt 0.2
10Base2 10 gt 10 Mbps
Node A
Node D
Node B
Node C

• The signal passes through each node
• The network interface card (NIC) retransmits the
  signal, so transceivers are not required
• Maximum throughput is 10 million bits per second
  (10 Mbps)

89
Twisted Pair Ethernet (10BaseT)

• Uses 4 pairs of twisted wires inside an
  unshielded cable
• The twisting of the wires reduces interference
• The absence of shielding makes the cable flexible
  and inexpensive
• The cable is capable of 10Mbps

90
Twisted Pair Ethernet

• Connectors on twisted pair Ethernet (RJ45) look
  similar to telephone wire connectors (RJ11)
• This kind of Ethernet uses unshielded twisted
  pair (UTP)
• UTP cable has various categories
• Cat3 Can only be used for 10BaseT
• Cat5 Can be used for 10BaseT, 100BaseT
• Cat5e, Cat6 Can be used for up to 1000BaseT

91
ThinNet Ethernet
011100110
011100110
92
Twisted Pair Ethernet
011100110
011100110
accept message
011100110
011100110
011100110
ignore
ignore
ignore
93
10 Mbps Ethernet Overview

• 10Base2 and 10Base5 both used coaxial cable which
  joined each node in a line
• 10BaseT uses UTP cabling, where each node is
  directly connected with the hub
• The hub receives messages and forwards them to
  all nodes
• The one that is connected to the recipient node

94
Fast Ethernet

• Using the same Cat5 cabling used for 10BaseT, an
  Ethernet-based LAN that operates at 100 Mbps
  (100BaseT) is possible
• Standard IEEE 802.3u
• While using the same cable, network hubs and
  network interface cards (NICs) must be upgraded
  to transmit messages at 100 Mbps

95
Fast Ethernet

• While very few computers can handle 100 Mbps
  throughput (bus speeds of computers are often
  slower than this), multiple computers can share
  this bandwidth
• 10/100 Ethernet (or 10/100 switched Ethernet)
  allows you to use the same NICs and hubs for both
  10BaseT and 100BaseT
• If a NIC and hub can both handle 100BaseT, that
  speed is used, otherwise 10BaseT is used
• 10/100 Ethernet allows you to slowly upgrade your
  network with minimal downtime

96
Gigabit Ethernet

• Gigabit Ethernet allows for 1000 Mbps throughput
• Gigabit Ethernet (Gig-E) can use Cat5 cabling
  (1000BaseT) or shielded Cat5E cabling
  (1000BaseTX)
• Standard IEEE 802.3ab
• Gig-E pushes the limits of the speed capable with
  Cat5 cabling, due to interference with the
  electrical signal, Cat5E cabling results in
  better performance
• Gigabit Ethernet is so fast, that it is sometimes
  used as a backbone for a Wide Area Network (WAN)
  instead of more expensive optical networks
• e.g. One of the backbones of the network here at
  the U

97
Ethernet Future

• Another form of Gigabit Ethernet which uses fibre
  optic cabling has been proposed (802.3z)
• Using multimode (multiple channel 1000BaseSX),
  or single mode (1000BaseLH, 1000BaseZX)
• Research groups are in the process of developing
  10 Gigabit Ethernet (802.3ae)
• This research is managed by the 10 Gigabit
  Ethernet Alliance
• http//www.10gea.org

98
LAN Service Models

• LLC (Logical Link Control), for LANs, can be one
  of two types
• Type 1 A straight datagram scheme
• The packet is delivered using best-effort service
• No acknowledgements are used to ensure packet
  arrival
• Type 2 A reliable scheme
• Packets are numbered
• Packets are acknowledged as they are received

99
IEEE 802 Committees

• Five 802 committees were developed to research
  various technologies associated with LANs
• 802.1 Issues common to all LANs
• e.g. addressing, management, bridges
• 802.2 Issues related to the LLC sub-layer
• e.g. reliability schemes, packet transmission
• 802.3 Issues related to CSMA/CD category LANs
• e.g. Ethernet
• 802.4 Issues related to token bus category LANs
• 802.5 Issues related to token ring category LANs

100
LAN Addresses

• The 48 bit addresses (often called MAC addresses)
  are the ones used by Ethernet LANs
• e.g. 02-60-8C-08-E1-0C
• All Ethernet cards contain a globally unique MAC
  address

101
Ethernet Overview

• Ethernet is not a reliable service
• There are no acknowledgements for packet receipt
• Ethernet uses best-effort delivery
• Most Ethernet networks use broadcasting to
  achieve messaging
• Each message is received by each node
• Ethernet is one network in a category of networks
  known as shared bus networks
• Each node shares a single communication medium

102
Ethernet Overview

• Ethernet is a carrier-sensing network
• Carrier-sensing networks use distributed access
  control methods
• Each station determines whether it can access the
  communication medium
• Each station senses whether or not the
  transmission medium (wire) is charged
• If not, an attempt at transmission is made
• If so, the node will wait and sense again

103
Ethernet Overview

• Sometimes, more than one station will attempt to
  transmit at roughly the same time
• This is called a collision
• Due to the finite speed of electrons traversing a
  wire
• 70 of the speed of light
• Or due to the finite speed of photons moving
  through glass
• The speed of light
• The two (or more) messages collide or interfere
  with one another, creating scrambled data packets

104
Collision Detection in Ethernet

• When scrambled messages are read by the
  transmitting stations, it is determined to be a
  collision
• Both (or all) of the stations involved will
  detect the collision
• This type of network is known as CSMA/CD
• Carrier-sensing, multiple access with collision
  detection
• Each station must retransmit their packets

105
Collision Avoidance in Ethernet

• After a collision occurs, if both stations tried
  to transmit after the same period of time,
  another collision would occur
• To combat this, Ethernet uses a binary
  exponential back-off policy
• Each subsequent collision would cause the station
  to wait double the amount of time before
  reattempting transmission

106
Ethernet Packets (Frames)

• Size 64 octets 1518 octets
• An octet is another term for an 8-bit byte
• The frame contains more than just data
• The source and destination addresses
• An identifier, signifying that the frame is in
  fact an Ethernet frame
• A Cyclical Redundancy Check (CRC) to ensure data
  integrity upon arrival

107
Ethernet Frames

• Sequence of 01010101 used to synchronize the
  receiving station
• The MAC address of the destination node
• The MAC address of the sender node
• The identifier used to identify the frame as an
  Ethernet frame
• The data to be sent to the destination
• A cyclical redundancy check (CRC) used to
  determine if data has been corrupted

Preamble
8 octets 6 octets 6 octets 2
octets 46-1500 4 octets
Dest Address
Source Address
Frame Type
Data
CRC
108
Ethernet Distance Limitations

• Coaxial Ethernet cables have a maximum length
• Due to signal deterioration
• This length could be extended using repeaters
• Machines that read signals through a port and
  recreate them (at full strength) out another port
• The use of more than 2 repeaters between any 2
  stations would interfere with times used in
  CSMA/CD schemes
• As a result, a maximum of 2 repeaters can be
  placed between any 2 nodes

109
Ethernet Distance Limitations

• Ethernet LAN sizes could also be increased by
  using Bridges to connect separate LANs into a
  single LAN
• Bridges filter out erroneous frames, as well as
  line noise
• Some bridges (adaptive bridges) are even
  intelligent enough to know when a frame must be
  forwarded or not
• e.g. If the destination node is not on the other
  side of a Bridge, the frame need not be forwarded

110
FDDI

• Fiber Distributed Data Interconnect

111
FDDI

• Use optical fibre cabling as a shared
  communication medium
• Optical fibre cables are made of glass
• Because they are so thin, they are fairly
  flexible
• Capable of 100 Mbps
• Light is used to transmit data
• Light is not susceptible to electrical
  interference
• Optical cabling can span longer distances
• Optical cabling does not need to be shielded near
  devices which generate electromagnetic
  interference
• Light waves (photons) travel faster than electrons

112
FDDI

• Is a token-ring category network
• A token is passed from station to station
• When a station receives the token, it may
  transmit data
• If a station has no data, it allows the token to
  pass to the next station
• FDDI uses 2 rings of cabling, moving in opposite
  directions
• The second ring is used to allow twice the flow
  of data
• The purpose of the second ring is to allow data
  to reach its destination, even when one station
  has failed (and cannot forward messages)

113
FDDI Ring Technology
114
FDDI With Node Failure
115
FDDI Token Passing
1
2
3
4
S12 D07
S12 D07
S12 D07
S12 D07
S12 D07
S12 D07
S12 D07
S12 D07
S12 D07
S12 D07
12
5
T
S12 D07
11
6
S12 D07
S12 D07
10
9
8
7
S12 D07
116
FDDI Token Passing
T
1
2
3
4
T
12
5
11
6
10
9
8
7
117
FDDI Frames
Preamble
Data Used to Synchronize Stations
octets 2
Start Delimiter
Indicates Start of Frame
1
Frame Control
Identifies the Type of Frame
1
Dest Address
Address of the Destination Node
2 or 6
Source Address
Address of the Source Node
2 or 6
Routing Info
Routing Information
0-30
Data
Frame Data
0-4500
FCS
Frame Check Sequence
4
End Delimiter
Indicates End of Frame
0.5
Frame Status
Status of Frame
1.5
118
Wireless Networks

• Radio-Based LANs

119
Wireless LANs

• Contrary to ones initial guess, wireless LANs
  are very similar to wired LANs
• Wireless LANs are a shared media network, just
  like Ethernet
• However, in a wireless LAN, the shared medium is
  not the air, but something called a base station
  or wireless access point

120
Wireless LANs (WLANs)

• The wireless access point, which is similar to a
  hub, is the shared medium
• Despite the fact that radio waves using the same
  frequency will cause mutual interference, the air
  is not generally considered a shared medium
• Technically speaking, twisted pair Ethernet is
  similar to WLANs
• The cables themselves are just point-to-point
  connectors and are not shared
• The hub/switch, however, is shared

121
Wireless LANs (WLAN)

• Wireless Access Point (WAP) A base station that
  coordinates transmission between one or more
  wireless hosts
• Analogous to a cell tower in a mobile phone
  network
• Wireless hosts must be a certain distance away
  from a WAP to participate on a WLAN
• The communicable area of all of the WAPs in a
  WLAN, define the coverage area for the WLAN
• Some WLANs do without a WAP, but pass messages
  directly to one another
• These are typically small (2-3 hosts) networks,
  and are called ad hoc networks

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802.11 Operation

• 802.11 networks (such as 802.11g) use CSMA/CA
  multiple access scheme
• Hosts try to detect carrier before sending (CS)
• This is not adequate, since there could be hidden
  hosts
• These are hosts out of range of this host, but in
  range of the same base station

123
802.11 Operation

• To avoid collisions with hidden hosts
• The host will send a request to send (RTS)
  frame before transmitting
• The base station will respond with a clear to
  send (CTS) frame if the channel is clear
• Once a base station sends a CTS, it will reject
  any further RTS requests until the data is
  received by the host who sent the first RTS
• This is called collision avoidance (CA)
• Frames are acknowledged at the data link layer in
  802.11 networks

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802.11 Frame Format

• Flags
• MAC Address of sending host
• MAC Address of receiving host
• MAC Address of sender base station
• Fragment number, sequence number
• MAC Address of receiver base station
• Frame data
• CRC for frame header and data

Frame Control (2 octets)
Source Address (6)
Destination Address (6)
Receiving Station Address (6)
Sequence Control (2)
Transmitting Station Address (6)
Data (0-2312)
Frame Check Sequence (2)
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802.11 Frame Header Frame Control
Protocol Version (2 bits)

• Flags
• Management, control or data frame
• Type of management or control frame
• Sent to an access point?
• Sent by an access point?

Type (2)
Subtype (4)
To AP (1)
From AP (1)
More Fragments (1)
Are there more fragments from this frame? Is this
a retransmission of a previous frame? Power state
of sender after transmission Is there more data
to come? Has WEP encryption been applied to
frame? Are the packets strictly ordered?
Retry (1)
Power Management (1)
More Data (1)
WEP (1)
Order (1)
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Wireless Access Points
WAP2
WAP1
WAP3
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Ad Hoc Networks

• In ad hoc networks, stations directly transmit to
  one another
• Hosts are responsible for routing, addressing,
  name translation, security, etc.
• Two ad hoc networks using the same frequency,
  within range of one another will cause conflicts
• Thus, different frequencies should be used

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Handoffs in WAPs

• For WLANs with WAPs, roaming hosts must be
  considered
• If a host moves into the range of another WAP,
  then out of range of their current WAP, a handoff
  takes place
• A handoff is when one WAP gives the
  responsibility for a particular host to one of
  its neighbouring WAPs
• The two WAPs must communicate for this to happen,
  and thus neighbouring WAPs must be within each
  others transmission range

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Wireless LAN Standards

• Some of the main standardized WLANs
• 802.11a, 802.11g 54Mbps, comparable with
  100BaseT Ethernet, under 100M range
• 802.11b 11Mbps, comparable to 10BaseT Ethernet,
  under 100M range
• These technologies are intended for LANs within
  the same small to medium-sized building
• BlueTooth/802.15 721 kbps, under 10M range
• This technology is intended for communicate
  within one room or vehicle