Layer 2 Technologies At layer 2 we create and transmit frames over communications channels Format of...
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![Page 1: Layer 2 Technologies At layer 2 we create and transmit frames over communications channels Format of frames and layer 2 transmission protocols are dependent.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649eb65503460f94bc00c1/html5/thumbnails/1.jpg)
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
Star Ring
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LAN/WAN Topology
Bus Tree
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LAN/WAN Topologies
Full Mesh Partial Mesh
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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 committee802.1802.2 Logical Link (inactive)802.3 Ethernet and variants802.4 Token Bus802.5 Token Ring802.6 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.
LANEthernet and IEEE 802.3
Wireless LANS and IEEE802.11
WANPoint 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
ComputingCenter
Campus A Campus B
413 MHz 413 MHz
407 MHz 407 MHz
Channels 9600 bps
Frame 704 bits
What is maximum channel utilization ?
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t = time to transmit frame
vulnerable
t0 t0 + t t0 + 2t
t
t0 + 3t
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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 < S < 1
• Let G = retransmissions + new packets (S) G ≥ S
• It is assumed that G and S follow Poisson Distribution
!)(
k
ekf
k
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Aloha Throughput
Based on this we can show
GGeS 2
G
S
.5
at G = .5
S = .5(e-1) = 1/(2e)
S = approx .18
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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 toGGeS 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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CSMANon 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