Local Area Networks Yerel Alan Aglari

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Local Area Networks(Yerel Alan Aglari)
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Network classification

• Local Area Networks (Yerel Alan Aglari)
• Room, building, campus
• Metropolitan Area Networks (Metropol Alan Aglari)
• City cable TV
• Wide Area Networks (Genis Alan Aglari)
• Country, continent

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Network topologies

• Mesh (Örgü)

• Star (Yildiz)

Fully connected mesh topology contains N(N-1)/2
connections between nodes
All communication passes through Central node
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Network Topologies

• Ring (Halka)

• Tree (Agaç)

One path between two nodes
Whose turn is it to transmit?
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Network Topologies

• Bus (Veriyolu)

Broadcast
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Multiple Access Protocols

• ALOHA
• Carrier Sense Multiple Access Protocols
• Collision-Free Protocols
• Limited-Contention Protocols
• Wavelength Division Multiple Access Protocols
• Wireless LAN Protocols

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Pure ALOHA

• In pure ALOHA, frames are transmitted at
  completely arbitrary times.

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Pure ALOHA (2)

• Vulnerable period for the shaded frame.

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Pure ALOHA (3)

• Throughput versus offered traffic for ALOHA
  systems.

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Persistent and Nonpersistent CSMA

• Comparison of the channel utilization versus load
  for various random access protocols.

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Ethernet (IEEE 802.3)

• Ethernet Cabling
• Manchester Encoding
• The Ethernet MAC Sublayer Protocol
• The Binary Exponential Backoff Algorithm
• Ethernet Performance
• Switched Ethernet
• Fast Ethernet
• Gigabit Ethernet
• IEEE 802.2 Logical Link Control
• Retrospective on Ethernet

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Ethernet Cabling

• The most common kinds of Ethernet cabling.

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Ethernet Cabling (2)

• Three kinds of Ethernet cabling.
• (a) 10Base5, (b) 10Base2, (c) 10Base-T.

10Base5 Vampire tap
10Base2 Passive T junction
10BaseT Hub (kablo göbegi)
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Ethernet Cabling (3)

• Cable topologies. (a) Linear, (b) Spine, (c)
  Tree, (d) Segmented.

Single pathgtNo interference
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Encoding types

• Unipolar
• One nonzero voltage level for 1 bit
• Polar
• One positive, one negative voltage level
• NRZ (Non return to zero)
• NRZ-L ve stands for 0 bit, -ve for 1 bit
• NRZ-I Voltage inversion indicates 1 bit
• RZ (Return to zero)
• ve to zero transition indicates 1 bit, -ve to
  zero indicates 0 bit
• Manchester high to low transition indicates 1
  bit, low to high indicates 0 bit - Enables better
  synchronization
• Differential Manchester transition indicates 0
  bit, lack of transition indicates 1 bit Better
  noise immunity
• Bipolar
• Zero voltage for 0 bit, alternating ve and ve
  voltages for 1 bits

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Ethernet Cabling (4)

• (a) Binary encoding, (b) Manchester encoding,
  (c) Differential Manchester encoding.

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Ethernet MAC Sublayer Protocol

• Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.

Preamble (64 bits) 10101010....10101010 for
synchronization Destination address 1 in MSB
indicates multicast, all 1s indicates
broadcast Half of the addresses are local, the
other half are global
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Ethernet MAC Sublayer Protocol (2)
Minimum frame length should be So that
transmitter has not finished transmission by the
time noise burst generated by B arrives at A
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CSMA/CD

• At time 0 A senses the channel as empty and sends
  out a frame
• Packet propagates
• Before As packet arrives at B, B senses the
  channel as empty and sends out its own frame
• Collision occurs
• B receives more power than it sent out, transmits
  a noise burst to jam channel
• A detects collision after it sent out
  first bit and aborts transmission.
• Both A and B apply binary exponential backoff to
  repeat transmission

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Typical calculation

• 500mx5segments2500 meters distance between two
  ends
• 50 roundtrip propagation time
• 10Mbps Ethernet gt frame sizegt 500bits
• Rounded to 512bits64bytes
• 1Gbps Ethernet,2.5kmgtframe sizegt6400bytes
• 1Gbps Ethernet,0.25kmgtframe sizegt640bytes

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CSMA with Collision Detection

• CSMA/CD can be in one of three states
  contention, transmission, or idle.

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Binary exponential backoff(Ikili üssel geri
çekilme)

• After a collision divide time into slots of
  length
• After i consecutive collisions wait a random
  number of slots.
• After 16 collisions inform the higher layer
  (network layer) process
• When number of users is large choose a large
  randomization interval
• When number of users is small choose a small
  randomization interval

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Switched Ethernet

• A simple example of switched Ethernet with 4
  plug-in cards.

Either each card has its own collision domain or
each port has its own collision domain.
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Fast Ethernet (IEEE 802.3u)

• Fast Ethernet is faster, but backward compatible.

Hubs and switches, but no vampire taps or BNC
connectors 100Base-T4 25Mhz signalling in 4
twisted pairs one upstream, three
downstream downstream total log2(33)4bits/cycle
100Base-TX 125Mhz signalling in 2 twisted
pairs One upstream, one downstream 4B/5B coding
yields 100Mbps for data 100Mbps for control
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Gigabit Ethernet (IEEE 802.3z)
Also backward compatible

• (a) A two-station Ethernet. (b) A multistation
  Ethernet.

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Gigabit Ethernet

• Modes of operation
• Full duplex Computer to switch, no contention,
  all lines buffered
• Half duplex Computer to hub, contention,
  CSMA/CD. Distance gt25m.
• Carrier extension Pad to 512 bytes by hardware
• Frame bursting Concatenate multiple frames in a
  single transmission
• Flow control to prevent fast transmission from
  overflowing buffers
• PAUSE frames

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Gigabit Ethernet (2)

• Gigabit Ethernet cabling.

1Ghz signalling possible with lasers (no
LEDs) (1.3 microns and 0.85microns
lasers) 1000BaseSX/LX encoding 8B/10B each 8bits
encoded as 10 bits with codeword rules to balance
1s and 0s (ensure 0 DC) 1000Base T encoding
Five voltage levels (2 data bits) for each of 4
twisted pairs at 125Mhz.
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Services and Ethernet

• Ethernet is connectionless (best efforts)
  protocol
• IP is a connectionless protocol, IP packets can
  just be pluged into 802 payload field
• ATM is connection oriented and much less
  interoperable with IP
• For enabling connectionless service at DLL level
  LLC which fits on top of 802 MAC can be used. LLC
  provides flow control.

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IEEE 802.2 Logical Link Control

• (a) Position of LLC. (b) Protocol formats.

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Wireless LANs

• The 802.11 Protocol Stack
• The 802.11 Physical Layer
• The 802.11 MAC Sublayer Protocol
• The 802.11 Frame Structure
• Services

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The 802.11 Protocol Stack

• Part of the 802.11 protocol stack.

11Mbps
54Mbps
54Mbps
1-2Mbps
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The 802.11 Physical Layer

• Infrared Not line of sight, 4bits encoded as 16
  bits codeword or 2 bits encoded as 4 bits
  codeword (Gray code 0001,0010,0100,1000)
• FHSS psedo random hopping from frequency to
  frequency on 79 1Mhz channels in 2.4 Ghz ISM band
• Security and insensitivity to interference
• DSSS Similar to CDMA, 1 Mbaud phase shift
  modulation with 1bit/baud or 2bits/baud
• OFDM Multiple narrow frequency bands in
• 5Ghz ISM band, 802.11a
• 2.4 Ghz ISM band 802.11g
• HR-DSSS 11million chips/sec.
• 1 Mbaud 1 bit/baud, 2 bit/baud
• 1.375 Mbaud 4bit/baud, 8bit/baud

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The 802.11 MAC Sublayer Protocol
Differences from Ethernet

• (a) The hidden station problem. (C transmits to
  B)
• (b) The exposed station problem.(B transmits to
  D)
• (c) Can not transmit and listen at the same time.

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MAC Modes of operation

• DCF (Distributed Coordination Function)
• Mandatory
• CSMA/CA (Collisions can occur)
• Physical channel sensing Similar to Ethernet
• Virtual channel sensing ARQ and Internal NAV
  issued upon hearing RTS/CTS and information it
  contains
• Frame fragmentation to keep frame error rates low
  in a noisy environment
• PCF (Point Coordination Function)
• Polling by base station (No collision occurs)

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The 802.11 MAC Sublayer Protocol (2)

• The use of virtual channel sensing using CSMA/CA.

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The 802.11 MAC Sublayer Protocol (3)

• A fragment burst.

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PCF and DCF coexistence

• Prioritization by time elapse After an ACK for a
  fragment
• Allow the sender to send the next fragment
• if not allow the base station to send its frame
  or fragment sequence
• if not allow a new station to send a new frame
• if not allow an error to be reported

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The 802.11 MAC Sublayer Protocol (4)

• Interframe spacing in 802.11.

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The 802.11 Frame Structure
Version a, b or g Type data, control,
management Subtype RTS/CTS To DS/ From DS Going
to/coming from distribution system type MF More
frames W Encryprion used O Order of
transmission important Duration FrameACK
roundtrip time Addresses Source station,
destination station. Source base
station, destination base station (except
management and control) Sequence Frame no.
0-4095, fragment no. 0-15 (Except control)
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802.11 Services
Distribution Services

• Association(Connection setup upon entry into
  cell, announce id, capabilities
• Disassociation (Either mobile station or base
  station breaks down connection)
• Reassociation (Smooth handover to another base
  station)
• Distribution (Base station to destination)
• Integration (Translation to non 802.11 format)

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802.11 Services
Intracell Services

• Authentication (Check for authorization/possession
  of secret key)
• Deauthentication (Leaving the network)
• Privacy (Encryption)
• Data Delivery(Best effort transmission)

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Token Ring (IBM Zurich-IEEE802.5)

• 4Mbps (1985)-16Mbps (1989)
• Shielded twisted pair
• Differential Manchester encoding
• Stations are connected to the ring via network
  adapters
• Token is a short frame that consists of a unique
  sequence of bits (three bytes)
• Token has two states
• Free no data is attached, frame consists of
  token only
• Busy token is followed by data (payload)

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Token Ring

• Network Adapter

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Token Ring

• A free token may be captured by a station that
  wishes to transmit
• A station that captures the free token attaches
  its data to it and fills the source and
  destination addresses and sends out a busy token
• Busy token circles the ring and data is read off
  by the destination station
• Busy token is converted to free token when it
  arrives back at the sending station
• Upon a transmission error, no token or multiple
  tokens Active Monitor detects and removes or
  reinserts tokens as necessary
• Active Monitor also ensures synchronization of
  signal for stations and sufficient delay for the
  token to sit on the ring.

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Token Ring Free/Busy Token Formats

• Busy
• token
• SD (Start Delimiter)8 bit start of frame marker
  (via code violations)
• AC(Access Control)Specifies priority, busy/free,
  monitored
• FC(Frame Control)Specifies data/control, if
  control type
• DA(Destination Address), SA(Source Address)
• ED(End delimiter) 8bit end of frame marker (via
  code violations)
• FS (Frame Status) Primitive ACK on whether frame
  was correctly received
• Free token
• Abort