Transcript Chapter 2

Chapter 6
High-Speed LANs
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Introduction
Fast Ethernet and Gigabit Ethernet
 High-speed Wireless LANs
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Table 6.1
Characteristics of some High-Speed LANs
Fast
Ethernet
Gigabit
Ethernet
Fibre
Channel
Wireless
LAN
Data Rate
100 Mbps
1 Gbps,
10 Gbps
100 Mbps –
3.2 Gbps
1 Mbps –
54 Mbps
Transmission
Media
UTP,
STP,
optical fibre
UTP,
shielded
cable, optical
fibre
Optical fibre,
coaxial cable,
STP
2.4 GHz,
5 GHz,
microwave
Access
Method
CSMA/CD
CSMA
Switched
CSMA/CA
Polling
Supporting
Standard
IEEE 802.3
IEEE 802.3
Fibre Channel
Association
IEEE 802.11
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Emergence of High-Speed LANs
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2 Significant trends
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Computing power of PCs continues to grow
rapidly
Network computing
Examples of requirements
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Centralized server farms
Power workgroups
High-speed local backbone
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Classical Ethernet
Bus topology LAN
 10 Mbps
 CSMA/CD medium access control
protocol
 2 problems:
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A transmission from any station can be
received by all stations
How to regulate transmission?
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Solution to First Problem
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Data transmitted in blocks called
frames:
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User data
Frame header containing unique address of
destination station
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Figure 6.1
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CSMA/CD
Carrier Sense Multiple Access/ Carrier Detection
1.
2.
3.
4.
If the medium is idle, transmit.
If the medium is busy, continue to listen
until the channel is idle, then transmit
immediately.
If a collision is detected during
transmission, immediately cease
transmitting.
After a collision, wait a random amount of
time, then attempt to transmit again (repeat
from step 1).
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Figure 6.2
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Figure 6.3
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Medium Options at 10Mbps
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<data rate> <signaling method> <max length>
10Base5
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10 Mbps
50-ohm coaxial cable bus
Maximum segment length 500 meters
10Base-T
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Twisted pair, maximum length 100 meters
Star topology (hub or multipoint repeater at central
point)
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Hubs and Switches
Hub
 Transmission from a station received by
central hub and retransmitted on all outgoing
lines
 Only one transmission at a time
Layer 2 Switch
 Incoming frame switched to one outgoing line
 Many transmissions at same time
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Figure 6.5
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Bridge
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Layer 2 Switch
Frame handling
done in software
Analyze and forward
one frame at a time
Store-and-forward
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Frame handling
done in hardware
Multiple data paths
and can handle
multiple frames at a
time
Can do cut-through
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Layer 2 Switches
Flat address space
 Broadcast storm
 Only one path between any 2 devices
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Solution 1: subnetworks connected by
routers
 Solution 2: layer 3 switching, packetforwarding logic in hardware
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Figure 6.6
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Figure 6.7
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Figure 6.8
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Figure 6.9
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Figure 6.10
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Figure 6.11
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Benefits of 10 Gbps Ethernet over
ATM
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No expensive, bandwidth consuming
conversion between Ethernet packets and
ATM cells
Network is Ethernet, end to end
IP plus Ethernet offers QoS and traffic
policing capabilities approach that of ATM
Wide variety of standard optical interfaces for
10 Gbps Ethernet
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Wireless LAN
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Forming LAN without wires has taken off big
time
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Specified as IEEE 802.11
Started with link rate: 1 Mbps  54 Mbps
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License-free operation
Different PHYs available 802.11b, a, g
Quality of service is being introduced thru
802.11e  4 classes
Mesh networks are formed for broader
coverage
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IEEE 802.11 Wireless LAN
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802.11b
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2.4-2.5 GHz
unlicensed radio
spectrum
up to 11 Mbps
widely deployed,
using base stations
802.11a
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5-6 GHz range
up to 54 Mbps
802.11g
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2.4-2.5 GHz range
up to 54 Mbps
All use CSMA/CA for MAC protocol
All have infrastructure and ad-hoc
network versions
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Infrastructure Approach
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McGraw-Hill
Wireless host communicates with an access point
Basic Service Set (BSS) (a.k.a. “cell”) contains:
 wireless stations
 one access point (AP)
BSSs combined to form a distribution system (DS)
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©The McGraw-Hill Companies,
Ad Hoc Approach
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No AP!
Wireless stations communicate with each other
Typical usage:
 “laptop” meeting in conference room, car
 interconnection of “personal” devices
 battlefield
IETF MANET (Mobile Ad hoc
Networks) working group
looks into this approach
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Special needs such wireless routing, security
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IEEE802.11 MAC Layer
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Two medium access control
schemes
 Distributed Coordination Function - DCF
 Point Coordination Function - PCF
Contention Free
Services
Contention Services
Point Coordination
Function
Distributed Coordination
Function
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IEEE 802.11: MAC protocol
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Collision if 2 or more nodes transmit at same
time as the wireless channel is shared
CSMA makes sense:
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get all the bandwidth if you’re the only one
transmitting
shouldn’t cause a collision if you sense another
transmission
Thus, it uses CSMA with collision avoidance
(CSMA/CA)
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Not CD because detecting collision is difficult in
wireless environment
Two-handshaking
Chapter used
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DCF
 Basic
access method
 Contention based, distributed protocol
 DCF uses CSMA/CA and a random
backoff time following a busy medium
condition
 Uses RTS/CTS extension
to combat the hidden terminal problem
 to reduce the BW wastage due to packet
collision
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PCF
 Contention free, centralised access protocol
 Channel access is controlled using polling
 Point coordinator will switch the access mode
between DCF and PCF
 Unpopular with vendors, mostly not used!
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Latest Developments
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802.11 has no QoS support; 802.11e MAC
introduced in 2005
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Supports 4 classes uses soft resource reservation
and backoff manipulation
802.11s taskforce working on extending
802.11 hotspots to larger wireless networks
Courtesy of
Sensors Mag
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Summary
Wired networks has become really fast
 Everything is Ethernet
 End to end is Ethernet
 Wireless networks supplementing wired
nets for mobility
 Next: How to design and test networks
without actually deploying them?
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Simulate!
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