Topic 5:
Local Area Network
Technology
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Local Area Networks
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•
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Small geographical area
High reliability
High Data Rate
Privately owened
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Parameters that characterizes
a LAN
• Topology
• Transmission media
• Medium access control techniques
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LAN Topology
• Arrangement of workstations in a shared
medium environment
• Logical arrangement (data flow)
• Physical arrangement (cabling scheme)
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LAN Topologies: Bus
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Multipoint medium
Stations attach to linear medium (bus) using tap
Full-duplex between station and tap
Transmission from any stations travels entire
medium (both directions)
• Termination required at ends of bus
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Bus LAN Diagram
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LAN Topologies: Tree
• Generalization of bus topology
• Branching cable with no closed loops
• Cable(s) begin at headend, travel to branches
which may have branches of their own
• Each transmission propagates through network,
can be received by any station
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Tree LAN Diagram
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Bus/Tree Topology Problems
• How do you identify who the transmission is
intended for?
—Data transmitted in frames
—Each frame has header with addressing info
• How do you regulate access?
—Stations take turns sending, by monitoring control
information in frames
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LAN Topologies: Ring
• Repeaters are joined by unidirectional point-topoint links in a ring
• As a frame circulates past a receiver, the
receiver checks its address, and copies those
intended for it into a local buffer
• Frame circulates until it returns to source, which
removes it from network
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Ring LAN Diagram
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LAN Topologies: Star
• Each station connected directly to central node,
usually with two unidirectional links
• Central node can broadcast info, or can switch
frames among stations
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Star LAN Diagram
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Choosing a Topology
• Factors to consider include
—reliability, flexibility/expandability, and performance
—Needs considering in context of:
• Medium
• Wiring layout
• Access control
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Bus/tree is most flexible
Tree topology easy to lay out
Ring provides high throughput, but reliability problems
Star can be high speed for short distances, but has
limited expandability
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Bus LAN
Transmission Media (1)
• Twisted pair
—Early LANs used voice grade cable
—Didn’t scale for fast LANs
—Not used in bus LANs now
• Baseband coaxial cable
—Uses digital signalling
—Original Ethernet
Bus LAN
Transmission Media (2)
• Broadband coaxial cable
—
—
—
—
As in cable TV systems
Analog signals is used at radio frequencies
Expensive, hard to install and maintain
No longer used in LANs
• Optical fiber
— Expensive taps
— Better alternatives available
— Not used in bus LANs
• All hard to work with compared with star topology twisted pair
• Coaxial baseband still used but not often in new
installations
Medium and Topology
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Medium Access Control
Provides a means of controlling access to a shared medium
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Medium Access Control
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Medium Access Protocols
In broadcast network to make the medium
available to all the nodes a scheme must be
introduced and this scheme is called Medium
Access protocols
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Types Multiple Access
Protocols
• Fixed assignment /channel Partitioning
• Random assignment
• Demand assignment/Taking turns
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Fixed Assignment technique
• Allocate the channel in a static manner. This
allocation scheme doesn’t depend on the
demands of the nodes
• Types
—Frequency Division Multiplexing(FDMA)
—Time Division Multiplexing(TDMA)
—Code division Multiplexing (CDMA)
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TDMA
TDMA: time division multiple access
access
to channel in "rounds"
each station gets fixed length slot (length =
pkt trans time) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have pkt, slots
2,5,6 idle
6-slot
frame
6-slot
frame
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3
4
1
3
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Link Layer
523
FDMA
FDMA: frequency division multiple access



channel spectrum divided into frequency bands each
station assigned fixed frequency band
unused transmission time in frequency bands go idle
example: 6-station LAN, 1,3,4 have pkt, frequency
bands 2,5,6 idle
FDM cable
frequency bands

Link Layer
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Random Assignment Technique
• Random access or contention methods, no
station is superior to another station and none is
assigned control over another. At each instance,
a station that has data to send uses a procedure
defined by the protocol to make a decision or
whether or not to send. This decision depends
on the state of the medium (idle or busy)
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Random Assignment Technique
• Types
—AlOHA
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Pure ALOHA
Sloted ALOHA
CSMA
CSMA/CD
CSMA/CA
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ALOHA
• was developed at the University of Hawaii in
early 1970.
• Designed for a radio (wireless) LAN.
• The medium is shared between the stations.
When a station sends data, another station may
attempt to do so at the same time. The data
from the two stations collide and become
garbled.
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Pure (unslotted) ALOHA
• unslotted Aloha: simpler, no synchronization
• when frame first arrives
—transmit immediately
• collision probability: frame sent at t0 collides
with other frames sent in [t0-1,t0 ]
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Procedure for Pure ALOHA
Protocol
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Slotted ALOHA
assumptions:
• all frames same size
• time divided into equal
size slots (time to
transmit 1 frame)
• nodes are synchronized
• nodes start to transmit
only at slot-beginning
• if 2 or more nodes
transmit in same slot, all
nodes detect collision
operation:
• when node obtains fresh
frame, transmits in next
slot
• if no collision: node can
send new frame in next
slot
• if collision: node
retransmits frame in each
subsequent slot with
prob= p until success
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Frame in Pure ALOHA Network
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Pros:
• single active node can
continuously transmit
at full rate of channel
• highly decentralized:
only slots in nodes
need to be in sync
• simple
Cons:
• collisions, wasting
slots
• idle slots
• nodes may be able
to detect collision in
less than time to
transmit packet
• clock synchronization
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CSMA (carrier sense multiple
access)
• CSMA: listen before transmit:
• if channel sensed idle: transmit entire frame
• if channel sensed busy, defer transmission
human analogy: don’t interrupt others!
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• collisions can still occur:
propagation delay means
two nodes may not hear
each others transmission
• collision: entire packet
transmission time wasted
• distance & propagation
delay play role in
determining collision
probability
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CSMA/CD (collision detection)
• carrier sensing & transmission deferral as in
CSMA
• collisions detected within short time
• colliding transmissions aborted, reducing
channel wastage
• collision detection:
— easy in wired LANs: measure signal strengths,
compare transmitted, received signals
— difficult in wireless LANs: received signal strength
overwhelmed by local transmission strength
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Collision of the first bits in
CSMA/CD
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CSMA/CA
• Carrier sense multiple access with collision
avoidance (CSMA/CA).
• Was invented for wireless network.
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Controlled-access protocols
• In controlled access, the stations consult on
another to find which station has the right to
send.
• A station cannot send unless it has been
authorized by other stations.
Types
• Reservation
• Polling
• Token passing
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Reservation
• Time is divided into intervals.
• A station needs to make a reservation before
sending data.
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“Taking turns” protocols
channel partitioning MAC protocols:
share channel efficiently & fairly at high load
inefficient at low load as 1/N of bandwidth
allocated even if only one active node!
random access MAC protocols
efficient at low load: single node can fully
utilize channel
Inefficient at high load due to high collision
overhead
“taking turns” protocols
look for best of both worlds!
Link Layer 540
“Taking turns” rotocols
polling:



master node
“invites” slave
nodes to transmit in
turn
typically used with
“dumb” slave
devices
concerns:
polling overhead
latency
single point of
failure (master)
data
poll
master
data
slaves
Link Layer 541
“Taking turns” protocols
token passing:



control token passed
from one node to
next sequentially.
token message
concerns:
token overhead
latency
single point of
failure (token)
T
(nothing
to send)
T
data
Link Layer 542
LAN Standards (IEEE802.x)
• Advantages of standards
—Assure sufficient volume to keep costs down
—Enable equipment from various sources to
interconnect
• IEEE 802 committee developed, revises, and
extends standards
• Use a three-layer protocol hierarchy: physical,
medium access control (MAC), and logical link
control (LLC)
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IEEE LAN Standards
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IEEE 802.2: Logic link control (LLC) layer of data link layer
IEEE 802.3: Ethernet
IEEE 802.4: Token bus, an old protocol
IEEE 802.5: Token ring
IEEE 802.6: Distributed queue dual bus (DQDB) protocol, similar to
FDDI
IEEE 802.9: Integrated voice and data networking, including ISDN,
Iso-ethernet
IEEE 802.11: Wireless LAN
IEEE 802.12: 100Base-VG
IEEE 802.13: 100Base-X
IEEE 802.14: Cable modem
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*Logical Link Control
• Specifies method of addressing and controls exchange
of data
• Independent of topology, medium, and medium access
control
• Unacknowledged connectionless service (higher layers
handle error/flow control, or simple apps)
• Connection-mode service (devices without higher-level
software)
• Acknowledged connectionless service (no prior
connection necessary)
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*Medium Access Control
• Provides a means of
controlling access to
a shared medium
• Two techniques in
wide use
—CSMA/CD
—Token passing
• LLC frames data,
passes it to MAC
which frames it
again
—MAC control
(e.g. priority level)
—Destination physical
address
—Source physical
address
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Ethernet (IEEE 802.3)
The Ethernet LAN standard was originally
developed by DEC, Xerox, and Intel, but has
since become a formalized standard by the
Institute of Electrical and Electronics Engineers
as IEEE 802.3
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Ethernet Topology
Ethernet uses a bus topology (a high speed circuit and a
limited distance between the computers, such as within
one building).
From the outside, an ethernet LAN appears to be a star,
because all cables connect to the central hub.
Most ethernet LANs span sufficient distance to require
several hubs, but some ethernet LANs are build without
the use of hubs (coax bus).
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Ethernet Media Access
Control
Carrier Sense Multiple Access with Collision
Detection (CSMA/CD)
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Wait until the bus is free and then transmit.
If no collision, transmission is completed.
If the collision is detected, send a jamming signal.
Wait a random amount of time, then re-broadcast.
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Token Ring (IEEE 802.5)
Token Ring was originally developed by
IBM, and have since been
standardized by IEEE as IEEE 802.5.
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Topology
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Token Ring Media Access
Control
• Token ring uses a controlled-access technique called token passing.
• The “token” is a series of bits, travels between the computers in a
predetermined sequence.
• A computer with a message waits to transmit until it receives a free
token. The computer changes the free token to a busy token and
attaches its message to it. Then it retransmits it on the circuit to
the next computer in the sequence.
• The computer receiving the message, changes the
acknowledgement to ACK (or NAK) and sends the message back to
the sender, who creates a new free token.
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Token Ring Media Access
Control
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Token Ring Media Access
Control
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Token Ring Media Access
Control
Token loss:
• The token crashes before being transmitted - lost a free token
• A computer in the ring crashes - lost a busy token
• A token is always busy.
A solution for the “lost” token problem:
• Designate one computer to be the token monitor and another
computer to be a backup token monitor.
• If no token circulated through the network for a certain length of
time or if a busy token circulates too often, the token monitor will
create a new free token (and destroy the busy token if necessary.)
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