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

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

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

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

LAN/WAN Topologies

Star Ring

LAN/WAN Topology

Bus Tree

LAN/WAN Topologies

Full Mesh Partial Mesh

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

Interconnected LANs

Bridge

Interconnected LANs

Bridge

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)

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)

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

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?

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

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 ?

t = time to transmit frame

vulnerable

t0 t0 + t t0 + 2t

t

t0 + 3t

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

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

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

Aloha

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

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

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

CSMA