Local & Metropolitan
Area Networks
ACOE322
Lecture 3
LAN types
Dr. L. Christofi
1
0. LAN types
•
The types of LANs we will examine in this section
are as follows:
1. Ethernet (IEEE 802.3)
2. Token Ring (IEEE 802.5)
3. Fiber Distributed Data Interface (FDDI)
4. Wireless LANs (IEEE 802.11)
Dr. L. Christofi
2
1. Ethernet LANs
1.1 Traditional Ethernet (CSMA/CD)
1.2 10Mbps Ethernet
1.3 100Mbps Ethernet (Fast Ethernet - FE)
1.4 1000Mbps Ethernet (Gigabit Ethernet - GE)
Dr. L. Christofi
3
1.1 Ethernet (CSMA/CD)
• The most widely used LANs today are based on Ethernet
and have been standardized by the IEEE 802.3 standards
committee
• The Ethernet uses Carrier Sense Multiple Access with
Collision Detection (CSMA/CD) as the access method
• Recall:
— Layers specified by 802.3:
• Ethernet Physical Layer
• Ethernet Medium Access (MAC) Sublayer
• Possible Topologies:
— Bus
— Branching non-rooted tree for large Ethernets
Dr. L. Christofi
4
Network Interface Card (NIC)
• Each station on an Ethernet network (such as a PC,
workstation or printer) has its own NIC.
• The NIC fits inside the station and it is the interface
between the station and the network.
• In most desktop computers, the NIC is an Ethernet card
(10, 100 or 1000 Mbps) that is plugged into a slot on the
computer motherboard.
Dr. L. Christofi
5
How does Ethernet work?
• The NIC provides the station with a 6-byte
physical address, normally written in hex
notation. This is the MAC address.
• Using MAC addresses to distinguish between
machines, Ethernet transmits frames of data
across baseband cables using CSMA/CD (IEEE
802.3)
Dr. L. Christofi
6
What is a MAC Address?
• Media Access Control (MAC) Address is the
physical address of any device, e.g. a NIC in a
computer on the network.
• The MAC address is 6-bytes long and has two
parts of 3 bytes long.
Example:
06
7A
D3
The first 3 bytes
specify the company
that made the NIC
Dr. L. Christofi
01
BF
2C
the second 3 bytes
are the serial
number of the NIC
7
Minimal Bus Configuration
Coaxial Cable
Transceiver
Terminator
Transceiver
Cable
Host
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8
Typical Large-Scale
Configuration
Repeater
Host
Ethernet
segment
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Ethernet Configuration
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Ethernet Physical Layer
• Transceiver
• Transceiver Cable
—4 Twisted Pairs
—15 Pin Connectors
• Channel Logic
—Manchester Phase Encoding
—64-bit preamble for synchronization
Dr. L. Christofi
11
Ethernet Physical Configuration
(for thick coaxial cable)
• Segments of 500 meters maximum
• Maximum total cable length of 1500 meters
between any two transceivers
• Maximum of 2 repeaters in any path
• Maximum of 100 transceivers per segment
• Transceivers placed only at 2.5 meter marks on
cable
Dr. L. Christofi
12
Manchester Encoding
Ethernet uses Manchester Encoding scheme
Data stream
1
0
1
1
0
0
Encoded
bit pattern
Recall:
• 1 bit = high->low voltage signal
• 0 bit = low->high voltage signal
Dr. L. Christofi
13
Ethernet Synchronization
• 64-bit frame preamble used to synchronize
reception
• 7 bytes of 10101010 followed by a byte
containing 10101011
• Manchester encoded, the preamble appears like a
sine wave
Dr. L. Christofi
14
Ethernet Cabling Options
•
•
•
•
•
10Base5: Thick Coax
10Base2: Thin Coax (“cheapernet”)
10Base-T: Twisted Pair
10Base-F: Fiber optic
Each cabling option carries with it a different set
of physical layer constraints (e.g., max. segment
size, nodes/segment, etc.)
Dr. L. Christofi
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Ethernet: MAC Layer
• Data encapsulation
—Frame Format
—Addressing
—Error Detection
• Link Management
—CSMA/CD
—Backoff Algorithm
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Ethernet Frame Format
Multicast bit
Destination
(6 bytes)
Source
(6 bytes)
Length
(2 bytes)
Data
(46-1500 bytes)
Pad
FCS
(4 bytes)
64 Bytes < Ethernet frame size < 1518 Bytes
Dr. L. Christofi
17
Ethernet MAC Frame
Address Field
• Destination and Source Addresses:
— 6 bytes each
• Two types of destination addresses
— Physical address: Unique for each user
— Multicast address: Group of users
— First bit of address determines which type of address is
being used
0 = physical address
1 = multicast address
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Communicating Within the LAN
Unicast
Broadcast
Multicast
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IEEE 802.3 frame format (1)
64 Bytes* < Ethernet frame size < 1518 Bytes*
* Excluding Preamble and SFD
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IEEE 802.3 frame format (2)
• Preamble: a 7-byte pattern of alternating 1s and 0s to establish
bit synchronization
• Start frame delimiter (SFD):The sequence 10101011 indicates
the actual start of the frame
• Destination address (DA): specifies the station(s) for which the
frame is intended
• Source address (SA): specifies the station that sent the frame
• Length/type: length of LLC data field in bytes or Ethernet type
field
• LLC data: data unit supplied by LLC
• Pad: bytes added to ensure that the frame is long enough for
proper CD operation. Filled when Length < 46.
• Frame check sequence (FCS): a 4-byte CRC check, based on all
fields except preamble, SFD and FCS.
Dr. L. Christofi
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CSMA/CD revisited
• Recall:
—CSMA/CD is a “carrier sense” protocol.
• If channel is idle, transmit immediately
• If busy, wait until the channel becomes idle
—CSMA/CD can detect collections.
• Abort transmission immediately if there is a collision
• Try again later according to a backoff algorithm
• Carrier sense reduces the number of collisions
• Collision detection reduces the impact of collisions
Dr. L. Christofi
22
CSMA/CD inefficiency
•
Inefficiency of CSMA/CD
— When two frames collide, the medium remains unusable
for the duration of transmission of both damaged
frames
— For long frames, compared to propagation time, the
amount of wasted capacity can be considerable.
— This waste can be reduced if a station continues to
listen to the medium while transmitting
Dr. L. Christofi
23
CSMA/CD and Ethernet
• Ethernet:
—Short end-to-end propagation delay
—Broadcast channel
• Ethernet access protocol:
—CSMA/CD with Binary Exponential Backoff Algorithm
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Ethernet Backoff Algorithm:
Binary Exponential Backoff
• If collision,
— Choose one slot randomly from 2k slots, where k is the
number of collisions the frame has suffered.
— One contention slot length = 2 x end-to-end
propagation delay
This algorithm can adapt to
changes in network load.
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Binary Exponential Backoff
(cont’d)
slot length = 2 x end-to-end delay = 15 ms
A
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B
@ t=0ms:
Assume A and B collide (kA = kB = 1)
A, B choose randomly from 21 slots: [0,1]
Assume A chooses 1, B chooses 1
@ t=30ms:
A and B collide (kA = kB = 2)
A, B choose randomly from 22 slots: [0,3]
Assume A chooses 2, B chooses 0
@ t=45ms:
B transmits successfully
@ t=75ms:
A transmits successfully
26
Binary Exponential Backoff (cont’d)
• In Ethernet,
—Binary exponential backoff will allow a maximum of 15
retransmission attempts
—If 16 backoffs occur, the transmission of the frame is
considered a failure.
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Ethernet Performance
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Ethernet Features and
Advantages
•
Passive interface:
— No active element
•
Broadcast:
—
•
All users can listen
Distributed control:
— Each user makes own decision
Simple
Reliable
Easy to reconfigure
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Ethernet Disadvantages
• Lack of priority levels
• Cannot perform real-time communication
• Security issues
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Ethernet Switching
• Recent development: Connect many Ethernet
segments or subnets through an “Ethernet
switch”
Dr. L. Christofi
to segment 4
to segment 1
to segment 3
to segment 2
31
Why Ethernet switching?
• LANs may grow very large
—The switch has a very fast backplane
—It can forward frames very quickly from one segment to
another
• Cheaper than upgrading all host interfaces to use
a faster network
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32
1.2 10Mbps Ethernet
• Physical configurations
— 10BASE5
— 10BASE2
— 10BASE-T
— 10BASE-F
Dr. L. Christofi
Note:
10 refers to data rate in Mbps,
BASE=Baseband
5= 50Ω coaxial cable
2= thinner 50Ω coaxial cable
T= Unshielded twisted pair cable
F = Fiber Optic cable
33
10BASE-T medium specification
• Unshielded twisted pair (UTP) cables are found prewired in office
buildings as excess telephone cable and can be used for LANs
• 10BASE-T defines a star topology
• Stations attach to a multiport repeater via a point-to-point link
• The link consists of two UTPs.
• The data rate is 10Mbps using Manchester encoding
• Length of link is limited to 100m
• If an optical cable is used, the maximum length is 500m.
• A 10BASE-T system can be mixed with 10BASE5 and 10BASE2
systems via repeaters
• Maximum transmission path between any two stations is 5
segments and 4 repeater sets.
— A segment is a point-to-point link or a coaxial cable
— The maximum number of coaxial cable segments in a path is 3.
Dr. L. Christofi
34
10BASE-F medium specification
• Added to IEEE 802.3 in 1993
• Advantages of the distance and transmission on optical fiber
• 10BASE-FP (passive): passive-star topology for
stations/repeaters with up to 1 km per segment; makes
synchronous retransmission
• 10BASE-FL (link): a point-to-point link for connecting
stations/repeaters at up to 2 km; asynchronous signaling;
any timing distortions are propagated through a series of
repeaters
• 10BASE-FB (backbone): a point-to-point link for
connecting repeaters at up to 2 km; a cascade up to 15
repeaters in sequence to activate greater length.
Dr. L. Christofi
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High speed LANs
Standard Ethernet LANs and MANs (up to 10Mbps) are based on
one copper wire.
This is OK for low speed and short distances but not suitable for
high speed and longer distances -> use fiber instead.
 Fast and Gigabit Ethernet
 FDDI (Fiber Distributed Data Interface)
 HIPPI (High Performance Parallel Interface)
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1.3 Fast Ethernet
• Operates at 100 Mbps
• Standardized in IEEE 802.3 as 100BASE-T and 100BASE-F
• Basic idea behind Fast Ethernet (FE) was simple: keep all the
old packet format, interfaces, procedural rules and cables, but just
reduce the bit time from 100ns to 10ns.
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IEEE 802.3 100BASE-T Physical
Layer Medium Alternatives
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Configuration and operation
• In its simplest form, a 100BASE-T network is
configured in a star-wire topology, with all
stations connected directly to a central point
(multiport repeater).
• The repeater is responsible for detecting
collisions, not the attached devices. Its functions
are:
—A valid signal appearing on any input is repeated on all
output links
—If two inputs occur at the same time, a jam signal is
transmitted on all links
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39
Collision domain
• Used to define a single CSMA/CD network
• This means that if two stations transmit at the same time, a
collision will occur.
— Stations separated by a simple multiport repeater are within
the same collision domain
— Stations separated by a bridge are in different collision domains
Bridged Ethernet
Dr. L. Christofi
Separates collision domains
40
Example: 100-Mbps Ethernet
Backbone Strategy
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1.4 Gigabit Ethernet (GE)
• Operates at 1Gbps
• Same strategy as FE
• CSMA/CD protocol and frame format, same as in 10Mbps and
100Mbps Ethernet
• Compatible with 100BASE-T and 10BASE-T
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Protocol architecture
• The MAC layer is an enhanced version of the basic 802.3
MAC algorithm
• A separate Gigabit Medium-Independent Interface (GMII) is
optional except for the UTP
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GE Physical layer
• 1000BASE-LX
— Long-wavelength option
— Supports duplex links up up to 550m for multimode fiber or up
to 5km for single mode fiber
• 1000BASE-SX
— Short-wavelength
— Supports duplex links up to 550m for multimode fiber
• 1000BASE-CX
— Supports 1Gbps among devices located within a single room
using copper jumpers (STP cable up to 25m long)
• 1000BASE-T
— Makes use of 4 pairs CAT5 UTP cables to support devices over
a range of up to 100m
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GE Medium Access Layer (1)
• Same CSMA/CD frame format and MAC protocol as in
10Mbps Ethernet and FE.
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GE Medium Access Layer (2)
• For hub operation (half duplex) there are two
enhancements to the basic CSMA/CD scheme
—Carrier extension
• Appends a set of special symbols to the end of short MAC
frames so that the resulting block is at least 4096 bit times
in duration, up from the minimum 512 bit times imposed at
standard Ethernet and FE.
—Frame bursting
• Allows for multiple short frames to be transmitted
consequentially without releasing control for CSMA/CD
between frames
• Avoid the overhead of carrier extension when a single
station has a number of small frames ready to send
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2. Token Ring
• Widely used although not as popular as Ethernet
• Based on IEEE 802.5 standard
• We will now examine the Medium Access Layer
and then the Physical layer aspects of this
specification
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47
IEEE 802.5 Token Ring
Medium Access Control
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Token ring operation
• Based on the use of a small frame, called a token,
that circulates when all stations are idle
• Whenever a station wishes to send a frame, it
first waits for the token
• On receipt of the token, it initiates transmission of
the frame, which includes the address of the
intended recipient at its header
• The frame is repeated by all stations in the ring,
until it circulates back to the initiating station,
where it is removed
• The intended recipient also retains a copy of the
frame
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49
Token Passing (1)
Token is traveling along the ring
Dr. L. Christofi
Station A captures the token and
sends its data to D
50
Token Passing (2)
Station D copies the frame and
sends the data back to the ring
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Station A receives the frame and
releases the token
51
MAC frame (1)
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MAC frame (2)
• The MAC frame consists of the following fields:
— Starting Delimiter (SD): Indicates start of frame
— Access Control (AC): Contains info for priority, reservation
variables and monitor bit
— Frame Control (FC): indicates if this is an LLC data frame
— Destination address (DA): As with 802.3
— Source address (SA): As with 802.3
— Data Unit (DU): Contains LLC data unit
— Frame Check Sequence (FCS): As with 802.3
— End Delimiter (ED): Contains the error detection bit (E),
which is set if any repeater detects an error and the
Intermediate (I) bit, which is used to indicate that this is a
frame other than the final one of a multiple frame transmission
— Frame Status (FS): Contains the address recognized (A) and
frame Copied (C) bits.
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53
Token maintenance
• To overcome error conditions, one station is designated as
the active monitor
— Periodically issues an Active-Monitor-Present control frame to
assure other stations that there is an active monitor on the ring
— To detect a lost-token condition, the monitor uses a valid-frame
timer that is greater than the time required to traverse the
ring.
— The timer is reset every valid token or data frame
— If the time expires, the monitor issues a priority 0 token
— If it sees a persistently circulating data frame with the monitor
bit set, it absorbs it and transmits a priority 0 token
— No token should circulate completely around the ring at a
constant nonzero priority level
— If the active monitor detects another active monitor, it
immediately goes into standby status.
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Dedicated Token Ring (DTR)
• A Ring can be configured in a star topology by use of a hub,
or concentrator.
• The token-passing algorithm can be used so that the ring
capacity is still shared and access control is determined by
the token.
• It is also possible to have the central hub function as a
switch with the connection between each station as a fullduplex point-to-point link.
• The DTR specification defines the use of stations and
concentrators in the switched mode.
• The DTR concentrator acts as a frame-level relay rather
than a bit-level repeater: each link from concentrator to
station is a dedicated full-duplex link with immediate access
possible (token passing is not used).
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Example DTR configuration (1)
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Example DTR configuration (2)
• A DTR switch consists of C-ports and a data transfer unit
(DTU). A C-port may operate in 2 modes:
— Transmit immediate protocol (TXI)
• Full duplex mode of transmission
• C-port or device does not need the capture of a token to transmit
frames and can transmit frames at any time
— Token-passing protocol (TPP)
• Classic IEEE802.5 protocol, that allows integration of classic tokenpassing stations and ring-concentrators into a DTR configuration
• C-port behaves as if it is part of a ring and passes the token
accordingly
• The DTU is the switching mechanism that connects the Cports within a DTR switch
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57
Token ring trade-offs
• Advantage
—Flexible control over access that it provides
• Disadvantage
—The requirement for token maintenance
• Loss of token prevents further utilization of the ring
• Duplication of the token cal also disrupt ring operation
• One station must be selected as monitor to ensure that
exactly one token is on the ring and to reinsert a free token
if necessary
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58
IEEE 802.5 Token Ring
Physical layer
• The standard imposes a maximum frame size of
—4.550 bytes at 4Mbps, and
—18.200 bytes for 16Mbps and 100Mbps
—This compares with a maximum frame size of 1518 bytes
for IEEE 802.3 LANs
• At 4Mbps and 16Mbps, token-passing access
control or switched DTR technique can be used
• At 100Mbps, the DTR technique is mandatory.
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59
IEEE 802.5 physical layer
medium alternatives
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60
3. FDDI (Fiber Distributed
Data Interface)
•
•
•
•
•
This is a 100Mbps token Ring LAN/MAN specification
Uses Optical Fiber cabling
High reliability (dual rings)
 Distances of up to 200 km
Immune to interference
 Up to 1000 hosts attached
Standardized by ANSI
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61
Fiber Distributed Data Interface
(FDDI) Standards
• ANSI ASC X3T9.5 Standard Committee
• ISO 9314 Standard Series
• FDDI serves both LAN and MAN
• FDDI – a token ring scheme, similar to the IEEE 802.5
specification
• FDDI was designed to accommodate better the rate of 100
Mbps
• Some differences are at the MAC layer and at the physical
layer.
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62
FDDI operation
• FDDI protocols are closely modelled to the 802.5 protocols.
– To transmit data, a station must first capture the token.
– Then it transmits a frame and removes it when it comes
around again.
• The difference between FDDI and 802.5 is that in 802.5 a
station may not generate a new token until its frame has
gone all the way around and come back.
– With FDDI, the amount of time wasted waiting for the
frame is substantial due to the large distances and large
number of station involved. For this reason, a station is
allowed to put a new token back onto the ring as soon
as it has finished transmitting its frames.
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63
FDDI MAC frame format
Similar to Token Ring
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FDDI MAC Frame
• Preamble: synchronizes the frame with each station’s clock
• Starting Delimiter (SD): Indicates the start of frame
• Frame Control (FC): Indicates if this is a synchronous or
asynchronous frame
• Destination address (DA): specifies the station(s) for
which the frame is intended
• Source address (SA): specifies the station that sent the
frame
• Information: Contains LLC data unit or control info
• Frame Check Sequence (FCS): 4-byte CRC
• End Delimiter (ED): marks the end of the frame
• Frame Status (FS): Contains the error detected (E),
address recognized (A) and frame copied (F) indicators
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FDDI token frame
• Preamble: synchronizes the frame with each
station’s clock
• Starting Delimiter (SD): Indicates the start of
frame
• Frame Control (FC): has the bit pattern 1000
0000 or 1100 0000 to indicate that this is a token
• End Delimiter (ED): marks the end of the token
frame
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66
Comparison of FDDI and IEEE
802.5 Token Ring (1)
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67
Comparison of FDDI and IEEE
802.5 Token Ring (2)
• Quite similar
• The FDDI frame includes a preamble to aid in
clocking, which is more demanding at the higher
data rate
• Both 2-byte and 4-byte addresses are allowed in
the same network with FDDI
—More flexible than the scheme used on all 802 standards
• Some differences in control bits
—FDDI does not include priority and reservation bits
—Capacity allocation is handled in a different way
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68
FDDI MAC Protocol
• Fundamentally, the same as IEEE 802.5 Token Ring MAC
Protocol but with two differences:
• First Difference
—In FDDI, a station waiting for a token seizes the token by
aborting (failing to repeat) the token transmission as
soon as the token frame is recognized. The station
begins transmitting one or more data frames. No flipping
a bit to convert a token [high data rate].
• Second Difference
—A station that has been transmitting data frames releases
a new token as soon as it completes data frame
transmission, even if it has not begun to receive its own
transmission.
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FDDI
Token
Ring
Operation (1)
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FDDI Token Ring Operation (2)
• With reference to the previous slide:
—After station A has seized the token, it transmits frame
F1 and immediately transmits a new token.
—F1 is addressed to station C, which occupies it as it
circulates past.
—The frame eventually returns to A, which is absorbed.
—Meanwhile, B seizes the token issued by A and transmits
F2 followed by a token
—This action could be repeated any number of times, so
that at any one time there may be multiple frames
circulating the ring
—Each station is responsible for absorbing its own frames
based on the source address field
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71
FDDI Dual Ring Operation
(a) FDDI
consists of
2 counterrotating
rings.
Dr. L. Christofi
(b) In the
event of
failure of
both rings at
one point,
the two rings
can be joined
together to
form a single
long ring
72
FDDI Physical Layer
specifications
• The FDDI standard specifies a ring topology operation at
100Mbps
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73
4. Wireless LANs
• Wireless LANs dispense with cables and use radio or
infrared frequencies to transmit signals through the air.
• WLANs are growing in popularity because they eliminate
cabling and facilitate network access from a variety of
locations and for mobile workers.
• The most common wireless networking standard is IEEE
802.11, often called Wireless Ethernet or Wireless LAN.
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Wireless Ethernet (IEEE 802.11)
Key Points
• The principal technologies used for wireless LANS are
— infrared
— spread spectrum
— narrowband microwave.
• The IEEE 802.11 standard defines a set of services and
physical layer options for wireless LANs.
• The IEEE 802.11 services include managing associations,
delivering data, and security.
• The IEEE 802.11 physical layer includes infrared and
spread spectrum and covers a range of data rates.
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Technology
• Infrared (IR) LANs: Individual cell of IR LAN
limited to single room.
— IR light does not penetrate opaque walls.
• Spread spectrum LANs: Mostly operate in ISM
(industrial, scientific, and medical) bands.
— No Federal Communications Commission (FCC)
licensing is required in USA.
• Narrowband microwave: Microwave
frequencies but not use spread spectrum
— Some require FCC licensing
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Infrared Wireless LANs
• Less flexible than IEEE 802.11 WLANs because, as with TV
remote controls that are also infrared based, they require
line of sight to work.
• Infrared Hubs and NICs are usually mounted in fixed
positions to ensure they will hit their targets.
• The main advantage of infrared WLANs is reduced wiring.
• A new version, called diffuse infrared, operates without a
direct line of sight by bouncing the infrared signal off of
walls, but is only able to operate within a single room and at
distances of only about 50-75 feet.
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77
Terminology
• Access Point (AP): Provides access to the distribution
system via the wireless medium.
• Basic Service Set (BSS): A set of stations controlled by
a single coordination function.
• Coordination function: Determines when a station is
operating.
• Distribution System (DS): Interconnection between
BSS and LANs to form an ESS.
• Extended Service Set (ESS): A set of interconnected
BSSs and LANs.
• MPDU / MSDU: MAC Protocol/Service Data unit.
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Wireless LAN Topology
•
WLAN topologies are the same as on Ethernet: physical star,
logical bus.
•
Wireless LAN devices use the same radio frequencies, so they
must take turns using the network.
•
Instead of hubs, WLANs use devices called access points
(AP). Maximum transmission range is about 100-500 feet.
Usually a set of APs are installed making wireless access
possible in several areas in a building or corporate campus.
•
Each WLAN computer uses an NIC that transmits radio signals
to the AP.
•
Because of the ease of access, security is a potential problem,
so IEEE 802.11 uses 40-bit data encryption to prevent
eavesdropping.
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Access Point
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Wireless LAN Applications (1)
• (1) LAN Extension:
— Saves installation of LAN cabling.
— Eases relocation and other modifications to network
structure.
• However, increasing reliance on twisted pair cabling for
LANs as newer buildings are prewired with Cat 5.
• Wireless LAN to replace wired LANs has not happened.
• Buildings with large open areas: role for the wireless LAN
(Airports, Manufacturing plants, stock exchange trading
floors, warehouses).
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Single Cell Wireless LAN
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Multi-Cell Wireless LAN
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Wireless LAN Applications (2)
• (2) Cross-Building interconnect:
— Connect LANs in nearby buildings.
— Point-to-point wireless link.
— Connect bridges or routers.
• (3) Nomadic Access:
— Link between LAN hub and mobile data terminal like a
Laptop or notepad computer.
— Also useful in extended environment such as campus
or cluster of buildings.
— Users move around with portable computers.
— May wish access to servers on wired LAN.
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Infrastructure Wireless LAN
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Wireless LAN Applications (3)
• (4) Ad Hoc Networking (=special purpose
networking)
—Point-to-point wireless link, Peer-to-peer network.
—Set up temporarily to meet some immediate need.
—E.g. group of employees, each with laptop or palmtop,
in business or classroom meeting.
—Network for duration of meeting.
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Ad Hoc Wireless LAN
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Wireless LAN Requirements (1)
• Throughput: MAC should make efficient use of
the wireless medium to maximise capacity.
• Number of nodes: Support hundreds of nodes.
• Connection to backbone LAN: Use control
modules to connect to both types of LANs.
• Service area: Diameter of 100 to 300m
• Battery power consumption: Wireless LAN
have features to reduce power consumption.
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Wireless LAN Requirements (2)
• Transmission robustness and security: The
design must permit reliable transmission in noisy
environment and protection from eavesdropping.
• Collocated network operation: Two or more
wireless LANs in same area.
• License-free operation.
• Handoff/Roaming: Mobile stations can move.
• Dynamic configuration: Addition, deletion,
and relocation of end systems without disruption
to users.
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Advantages / Disadvantages
•
•
•
•
Extremely high data rates.
Use ceiling reflection to cover entire room.
Does not penetrate walls or other opaque objects.
More easily secured against eavesdropping than
microwave.
• Inexpensive and simple
•
•
•
•
Background radiation
Sunlight, indoor lighting
Noise, requiring higher power and limiting range
Power limited by concerns of eye safety and power
consumption.
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MAC Frame
•
•
•
•
•
•
Dr. L. Christofi
Frame Control: Control management or data.
Duration/Connection ID: Time the channel is allocated in μs
for a transmission of a frame.
Addresses: Source, destination, transmitting station and
receiving station.
Sequence control: Contains data fragmentation and reassembly
and the number of frames.
Frame body: Contains an MSDU.
Frame Check Sequence: CRC.
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IEEE 802.11 Services
Service
Provider
Category
Association
Distribution system
MSDU delivery
Authentication
Station
LAN access and security
Deauthentication
Station
LAN access and security
Dissassociation
Distribution system
MSDU delivery
Distribution
Distribution system
MSDU delivery
Integration
Distribution system
MSDU delivery
MSDU delivery
Station
MSDU delivery
Privacy
Station
LAN access and security
Reassocation
Distribution system
MSDU delivery
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IEEE 802.11xx standards
• IEEE 802.11a: 5GHz, Orthogonal FDM, data
rates of 6-54Mbps.
• IEEE 802.11b: 5.5Mbps and 11Mbps.
• IEEE 802.11g: Higher speed extension of
802.11b 20 Mbps.
• IEEE 802.16: Wi-Fi – Starbucks Wi-Fi spot
100 Mbps.
• IEEE 802.21 WiMax, over 100 Mbps, 50km
range, broadband speeds.
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Review Questions
• List and briefly define the four application
areas of wireless LANs.
• List and briefly define the key requirements for
wireless LANs.
• What are some key advantages and
disadvantages of wireless LANs.
• List and define some of the IEEE 802.11
services.
• List and describe the IEEE 802.11xx family.
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References
•
W. Stalling, Local and Metropolitan Area Networks,
6th edition, Prentice Hall, 2000
•
B.A. Forouzan, Data Communications and
Networking, 3rd edition, McGraw-Hill, 2004
•
F. Halsall, Data Communications, Computer
Networks and Open Systems, 4th edition, Addison
Wesley, 1995
•
W. Stallings, Data and Computer Communications,
7th edition, Prentice Hall, 2004
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