Layer 2 Technologies

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Layer 2 Technologies

• At layer 2 we create and transmit frames over
  communications channels
• Format of frames and layer 2 transmission
  protocols are dependent on technology and
  topology
• There are two major classes of networks
• Wide Area Networks (WAN)
• Local Area Networks (LAN)
• Each class may use a variety of topologies

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Layer 2 Technologies

• Point-to-Point
• A station is directly connected to another
• Often used in WANs
• Each station may connect to all other stations
  (fully meshed)
• Or, one station may connect to only a subset of
  stations in a network
• For large networks, fully meshed networks are
  rare due to cabling complexity and costs
• May be wired or wireless

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Layer 2 Technologies

• Shared Channels
• Stations share a communications channel
• Most often used in LANs
• May require some form of addressing
• May allow transmitting to many or all
  stations
• Full broadcast
• Multicast group of stations
• How does a station get its fair share?
• How does a station gain permission to transmit?
• Must implement Media Access Control
• May also be wired or wireless

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Layer 2 Topologies

• Topology is a connection strategy
• May be influenced by the medium
• Dictates in part the path a frame will take
• Many general topologies have been implemented.
  Both for LANs and WANs

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LAN/WAN Topologies
Ring
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LAN/WAN Topology
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LAN/WAN Topologies
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Locality of Reference

• Computers are most often organized in groups and
  locally interconnected as LANs
• Computers often communicate with computers that
  are close by or local.
• Examples are computers communicating with
  departmental servers
• As a result, most traffic is local and never
  leaves the LAN
• Thus, networks are organized in groups, or LANs.
• LANs of any topology can then be interconnected
  to form larger networks

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Interconnected LANs
Bridge
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Interconnected LANs
Bridge
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LAN Standards

• Managed by an IEEE committee called the 802
  committee
• Many subgroups within the 802 committee
• 802.1
• 802.2 Logical Link (inactive)
• 802.3 Ethernet and variants
• 802.4 Token Bus
• 802.5 Token Ring
• 802.6 Metropolitan Area Networks (inactive)
• .
• 802.11 Wireless LANs (WiFi) (a,b,g,e,I,n)
• 802.16 Broadband wireless (WiMAX)

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Network Architectures

• We will discuss two major layer two network
  architectures.
• LAN
• Ethernet and IEEE 802.3
• Wireless LANS and IEEE802.11
• WAN
• Point to Point Protocols (HDLC)

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Broadcast Networks

• Consider N users on a shared channel
• User transmits when a frame is ready to send
• Other users also transmit at will
• If frames from 2 users overlap we have a
  collision
• Collisions yield damaged frames
• Users can hear own transmission and therefore
  can detect collisions

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Broadcast Networks

• Summary
• Transmit at will
• Listen for own frame
• Retransmit if collision
• Collisions are wasted bandwidth
• On average, if we have a shared broadcast channel
  what throughput can we expect?

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Aloha Network

• Early broadcast network developed at University
  of Hawaii 1970
• Designed to interconnect 7 campuses of the U of H
  on 4 islands
• Used radio broadcasts
• Central Computing Center served as hub
• Campuses communicated through this center over
  shared radio frequencies

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Aloha Network
Channels 9600 bps Frame 704 bits
What is maximum channel utilization ?
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t time to transmit frame
vulnerable
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Aloha Throughput

• Let time to transmit a packet to be 1 unit
• Let S Average number of packets per packet time
• 0 lt S lt 1
• Let G retransmissions new packets (S)
• G S
• It is assumed that G and S follow Poisson
  Distribution

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Aloha Throughput

• Based on this we can show

at G .5 S .5(e-1) 1/(2e) S approx
.18
S
G
.5
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Aloha

• This is called Pure Aloha stations transmitted
  at will
• This was later modified that required stations to
  only transmit at predefined times driven by a
  central clock
• Now we need no collision for 1 packet time rather
  than 2
• In this case the utilization improved to

or S 1/e .36
This is called Slotted Aloha
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Aloha
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Carrier Sense Multiple Access

• We can improve on Aloha if we listen before
  transmitting
• A family of protocols called Carrier Sense
  Multiple Access (CSMA) has evolved
• All involved some form of listen before speak
• Variations of when to speak if no transmissions
  (carrier) heard

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CSMA

• Non Persistent
• Listen on channel
• If free, transmit
• If busy, wait random time repeat
• If collision, wait random time repeat
• Persistent (also called 1-persistent)
• Listen on channel
• If free, transmit
• If busy, continue to listen until free transmit
• If collision, wait random time - repeat

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CSMA

• p Persistent
• Compromise
• Listen on channel
• If free, transmit with probability p
• and delay 1 unit time with probability 1-p
• If channel busy wait until free, repeat

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CSMA