CSC 335 Data Communications and Networking

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CSC 335 Data Communications and Networking

• Lecture 7 Local Area Networking
• Dr. Cheer-Sun Yang
• Fall 2000

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Motivation

• Up to this point, weve talked about
  point-to-point communication.
• We many need to connect many computers together.
• Local Area Network(LAN) if they are located in a
  relatively close geographic area.
• Metropolitan Area Network (MAN) extends over
  entire city
• Wide Area Network (WAN) extends across public
  switching network.

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LAN Applications (1)

• Personal computer LANs
• Low cost
• Limited data rate
• Back end networks and storage area networks
• Interconnecting large systems (mainframes and
  large storage devices)
• High data rate
• High speed interface
• Distributed access
• Limited distance
• Limited number of devices

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LAN Applications (2)

• High speed office networks
• Desktop image processing
• High capacity local storage
• Backbone LANs
• Interconnect low speed local LANs
• Reliability
• Capacity
• Cost

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LAN Architecture

• Topologies
• Protocol architecture
• Physical Layer
• Media access control
• Logical Link Control

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Topologies

• Bus A single communication line, typically a
  twisted pair, coaxial cable, or optical fiber,
  represents the primary medium.
• Ring packets can only be passed from one node to
  its neighbor.
• Star A hub or a computer is used to connect to
  all other computers.
• Tree no loop exists (logical connection).

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LAN Topologies
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Frame Transmission - Bus LAN
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Ring Topology

• Repeaters joined by point to point links in
  closed loop
• Receive data on one link and retransmit on
  another
• Links unidirectional
• Stations attach to repeaters
• Data in frames
• Circulate past all stations
• Destination recognizes address and copies frame
• Frame circulates back to source where it is
  removed
• Media access control determines when station can
  insert frame

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Frame TransmissionRing LAN
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Star Topology

• Each station connected directly to central node
• Usually via two point to point links
• Central node can broadcast
• Physical star, logical bus
• Only one station can transmit at a time
• Central node can act as frame switch

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Bus and Tree

• Multipoint medium
• Transmission propagates throughout medium
• Heard by all stations
• Need to identify target station
• Each station has unique address
• Full duplex connection between station and tap
• Allows for transmission and reception
• Need to regulate transmission
• To avoid collisions
• To avoid hogging
• Data in small blocks - frames
• Terminator absorbs frames at end of medium

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Protocol Architecture

• Protocol layering (IEEE 802.X)
• Physical Layer
• Media access control (MAC) Sublayer
• Logical link control (LLC)

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IEEE 802.X

• IEEE 802.3 Ethernet LAN
• IEEE 802.4 Token Bus LAN
• IEEE 802.5 Token Ring LAN
• Other Ring Networks FDDI, Slotted Rings.
• IEEE 802.6 Distributed Queue Dual Bus (DQDB)
  MAN standard.

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IEEE 802 vs. OSI
Fig 6.4
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LAN Protocols in Context
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802 Physical Layer Design Issues

• Encoding/decoding
• Preamble generation/removal
• Bit transmission/reception
• Transmission medium and topology

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802 Physical Layer

• Required hardware for connecting a PC to Ethernet
  directly
• Transceiver
• Attachment Unit Interface (AUI) cable
• Network Interface Card (NIC) also known as
  Network Adapter
• Required hardware for connecting a PC to a remote
  computer modem (with the help of PPP)

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Bus LANs

• Signal balancing
• Signal must be strong enough to meet receivers
  minimum signal strength requirements
• Give adequate signal to noise ration
• Not so strong that it overloads transmitter
• Must satisfy these for all combinations of
  sending and receiving station on bus
• Usual to divide network into small segments
• Link segments with amplifies or repeaters

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Transmission Media

• Twisted pair
• Not practical in shared bus at higher data rates
• Baseband coaxial cable
• Used by Ethernet
• Broadband coaxial cable
• Included in 802.3 specification but no longer
  made
• Optical fiber
• Expensive
• Difficulty with availability
• Not used
• Few new installations
• Replaced by star based twisted pair and optical
  fiber

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Baseband Coaxial Cable

• Uses digital signaling
• Manchester or Differential Manchester encoding
• Entire frequency spectrum of cable used
• Single channel on cable
• Bi-directional
• Few kilometer range
• Ethernet (basis for 802.3) at 10Mbps
• 50 ohm cable

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10Base5

• Ethernet and 802.3 originally used 0.4 inch
  diameter cable at 10Mbps
• Max cable length 500m
• Distance between taps a multiple of 2.5m
• Ensures that reflections from taps do not add in
  phase
• Max 100 taps
• 10Base5

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10Base2

• Cheapernet
• 0.25 inch cable
• More flexible
• Easier to bring to workstation
• Cheaper electronics
• Greater attenuation
• Lower noise resistance
• Fewer taps (30)
• Shorter distance (200m)

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Cable Specifications for 802.3

• 10BaseT 10 Mbps, baseband, unshield twisted
• 10Base2 10Mbps, Cat. 2 coaxial
• 10Base5 10 Mbps, Cat. 5, Cat. 5e coaxial
• 100BaseTX 100 Mbps, twisted cable (Fast
  Ethernet)
• 10Broad36 maximum segment length 3600 meters

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Gigabit Ethernet

• 1000Base-SX
• Short wavelength, multimode fiber
• 1000Base-LX
• Long wavelength, Multi or single mode fiber
• 1000Base-CX
• Copper jumpers lt25m, shielded twisted pair
• 1000Base-T
• 4 pairs, cat 5 UTP
• Signaling - 8B/10B

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Connectors

• T-connector used to form a bus topology
• RJ-45 connectors for connecting a PC to another
  PC, Ethernet, or hub.
• Cross-over a direct connection to another PC
• Straight-through connection with the Ethernet
  jack or hub.

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Repeaters

• Transmits in both directions
• Joins two segments of cable
• No buffering
• No logical isolation of segments
• If two stations on different segments send at the
  same time, packets will collide
• Only one path of segments and repeaters between
  any two stations

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Media Access Control Sublayer

• Assembly of data into frame with address and
  error detection fields
• Disassembly of frame
• Address recognition
• Error detection
• Govern access to transmission medium
• Not found in traditional layer 2 data link
  control
• Also known as Contention protocols (section 3.4)

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Collision vs. Contention

• When the communication link is used by one
  station to transmit a frame, another station
  connecting to the same link tries to send a
  packet collision
• Contention accessing the medium with the
  consideration that a collision may occur.
• Contention Protocols the protocol is designed to
  deal with collision using contention.
• Collision-free Protocols the protocol is
  designed so that collision will not occur.

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Contention Protocols

• Pure ALOHA
• Slotted ALOHA
• Carrier Sense Multiple Access (CSMA)
• Persistent and non-persistent CSMA
• CSMA with Collision Detection (CSMA/CD)

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Collision-Free Protocols

• A Bit-Map Protocol reservation protocol
• Binary Countdown

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Pure Aloha

• Packet Radio
• When station has frame, it sends
• Station listens (for max round trip time)plus
  small increment
• If ACK, fine. If not, retransmit
• If no ACK after repeated transmissions, give up
• Frame check sequence (as in HDLC)

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Pure Aloha(contd)

• If frame OK and address matches receiver, send
  ACK
• Frame may be damaged by noise or by another
  station transmitting at the same time (collision)
• Any overlap of frames causes collision
• Max utilization 18 (WHY?)

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The Efficiency of Pure Aloha
G the traffic measured as the average number of
frames generated per slot
S the success rate, success frame / slot
Prk frames are generated G k e G / k !
This is called a probability distribution
function(pdf) for Poisson distribution. (e
2.7818)
S Prno frame is generated e -G
G e 2G (pure Aloha)
S G e G (slotted Aloha)
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The Efficiency of Pure Aloha
If there is no negative acknowledgement frame
received after sending out one frame, the
transmission is successful. So P0 Prno frames
are generated in 2 time slots e -G e
G e 2G
S G P 0
G e 2G (pure Aloha)
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The Efficiency of Pure Aloha

S G P 0 G e 2G (pure Aloha)
We need to find the value of G such that S is
maximized. S G (-2) e 2G e 2G (1 2G)
e 2G
Let S 0 gt G ½ When G ½, S 1/ 2e
0.184 18
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Slotted ALOHA

• A computer is not allowed to send until the
  beginning of the next slot.
• Time in uniform slots equal to frame transmission
  time
• When a frame is allowed to be transmitted, there
  is no collision.
• Need central clock (or other sync mechanism)
• Transmission begins at slot boundary
• Max utilization 37 (WHY?)

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The Efficiency of Slotted Aloha
If there is no other frame received after sending
out one frame, the transmission is successful.
So P0 Prno frames are generated in one time
slots e -G
S G P 0
G e G (slotted Aloha)
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The Efficiency of Slotted Aloha

S G P 0 G e G (slotted Aloha)
We need to find the value of G such that S is
maximized. S G (-1) e G e G (1 G) e
G
Let S 0 gt G 1 When G 1, S 1/ e
0.368 37
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Carrier Sense Multiple Access (CSMA) Protocols

• Protocols in which stations listen for a carrier
  (i.e., a transmission) and act accordingly are
  called carrier sense protocols.
• 1-persistent CSMA
• Non-persistent CSMA
• p-persistent CSMA

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CSMA

• Propagation time is much less than transmission
  time
• All stations know that a transmission has started
  almost immediately
• First listen for clear medium (carrier sense)

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If Busy?

• If medium is idle, transmit
• If busy, listen for idle then transmit
  immediately
• No ACK then retransmit
• If two stations are waiting, it is called a
  collision.

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1-persistent CSMA

• When a station has data to send, it first listens
  to the channel to see if anyone else is
  transmitting at that moment.
• If the channel is busy, the station waits until
  it becomes idle.
• The station retransmits with a probability of 1
  when it finds that the channel is idle.

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Non-persistent CSMA

• When a station has data to send, it first listens
  to the channel to see if anyone else is
  transmitting at that moment.
• If the channel is busy, the station waits until
  it becomes idle.
• The station does not keep trying. It waits for a
  random number of time and retries.

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Non-persistent CSMA

• This applies to slotted channels. When a station
  has data to send, it first listens to the channel
  to see if anyone else is transmitting at that
  moment.
• If the channel is idle, it transmits with a
  probability p. With a probability of 1-p, it
  defers until the next slot.
• If the next slot is also idle, it transmits or
  defers again with probability p and q.

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CSMA

• Max utilization depends on propagation time
  (medium length) and frame length. Longer frame
  and shorter propagation gives better utilization.
• Collisions still can be a problem, especially
  with p-persistent CSMA.
• One way to reduce the frequency of collision with
  CSMA is to lower the probability that a station
  will send when a previous is done. (Fig. 3.26)
• Smaller values of p gt fewer collision.

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Any Other Way?

• Is there another way to improve the successful
  rate?
• Yes if there is a way to detect collision prior
  to transmission.
• Why is this faster?

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Collisions with and without Detection

• Without collision detection, a station must send
  and then wait for 2 time slots before another
  attempt to send.
• With collision detection, a station can stop
  transmission if collision detection requires less
  time than sending a frame.

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Collision Detection

• On baseband bus, collision produces much higher
  signal voltage than signal
• Collision detected if cable signal greater than
  single station signal
• Signal attenuated over distance
• Limit distance to 500m (10Base5) or 200m
  (10Base2)
• For twisted pair (star-topology) activity on more
  than one port is collision
• Special collision presence signal

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CSMA/CD

• With CSMA, collision occupies medium for duration
  of transmission
• Stations listen while transmitting
• If medium idle, transmit
• If busy, listen for idle, then transmit
• If collision detected, jam then ease transmission
• After jam, wait random time then start again
• Binary exponential back off

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CSMA/CDOperation
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Binary Exponential Back Off

• If a stations frame collides for the first time,
  wait 0 or 1 time slot (chosen randomly) before
  trying again.
• If it collides a second time, wait 0, 1, 2, or 3
  slots (again, chosen randomly).
• After a third collision, wait anywhere from 0 to
  2 n 1 slots if n lt 10, if n gt 10, wait between
  0 to 1024 (2 10) slots.
• After 16 collisions, give up. Further recovery is
  up to the upper layer, such as a user.

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IEEE 802.3 Frame Format
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Frame Format (802.3)

• Start of frame delimiter 10101011
• Destination address
• Source address
• Data length field
• Data field
• Pad field the data field must be at least 46
  octets.
• Frame check sequence using 32-bit CRC.

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Efficiency of 802.3(p.375)

P the probability that a frame is sent without
a collision Ps the probability that a station
sends The probability of a collision 1 P. The
probability of a transmission requiring exactly
N attempts N-1 collisions followed by a
success N Ps ( 1- Ps) N - 1
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Efficiency of 802.3 (p.376-377)

We would like to know under what conditions the
largest number of frames are sent successfully.
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Efficiency of 802.3(p.375)

The probability of a transmission requiring
exactly N attempts P N Ps ( 1- Ps) N
1 dP/dPs N(1-Ps)N-1 N Ps (N-1)(1-Ps)N-2
N (1-Ps) N-2 1- Ps Ps (N 1) Let
dP/dPs 0 gt Ps 1/N
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Efficiency of 802.3

How many time slots has passed before a frame is
sent successfully?
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The Efficiency of 802.3
The contention period the number of time slots
passed before a successful transmission

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The Efficiency of 802.3

Assume that the probability of a success in each
attempt p. The probability of a collision 1
p. The probability of a transmission requiring
exactly i1 attempts P(i1) i collisions
followed by a success p (1- p) i
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The Efficiency of 802.3

The contention period (1-p)/p
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The Efficiency of 802.3
The contention period (C) (1/p) 1

0 lt p lt 1 (p the successful rate) If p -gt 1 C
-gt 0 If p -gt 0, C large Weve found that when
Ps1/N, p is maximized. So, C (1-1/N)1-N -1
when p is maximized. If N-gtlarge, C 2.718 1
1.718 (close to 2).
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The Utilization Rate of Ethernet
The percent utilization (U) does not depend on
the number of stations in practice. A station
will try to send regardless of how many other
stations there are. The previous result often is
used as benchmarks against which measures are
made to estimate efficiency.

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The Utilization Rate of Ethernet
The percent utilization (U) is defined as the
amount of time spent on transmitting a frame as a
percentage of the total time spent on contending
and transmitting. Assume R transmission rate F
number of bits in a frame T slot time So
U

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Required Reading

• Section 6.1, 3.4, 6.2