Computer Networks

1. Chapter 5
Link Layer
Computer Networking: A
Top Down Approach
4th edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2007.

2. Ethernet Frame Structure
LAN technology
 First widely used LAN technology
 Invented in 1970s, evolved over the years
 Kept up with speed race: 10 Mbps – 10 Gbps
Sending adapter encapsulates IP datagram in
Ethernet frame

3. Ethernet: Unreliable, connectionless
 Connectionless: No handshaking between sending and
receiving NICs
 Unreliable: receiving NIC doesn’t send acks to
sending NIC



stream of datagrams passed to network layer can have gaps
(missing datagrams)
gaps will be filled if app is using TCP
otherwise, app will see gaps
 Ethernet’s MAC protocol: CSMA/CD

4. Ethernet CSMA/CD algorithm
CSMA/CD does the following;
1. An adapter may begin to transmit at any time; that
is no notion of time slots.
2. An adapter uses carrier sensing.
3. An adapter aborts its transmission as soon as it
detects collision.
4. Before attempting a retransmission, an adapter
waits a random time
If the propagation delay is very small excellent performance.
 2nd and 3rd mechanisms are implemented by measuring voltage
levels before and during transmission.
 Each adapter runs CSMA/CD without explicit coordination with
other adapters.


5. Ethernet CSMA/CD algorithm
1. NIC receives datagram from network layer, creates
frame
2. If NIC senses channel idle (96 bit times), starts
frame transmission. If NIC senses channel busy (plus
96 bit times), waits until channel idle, then transmits
3. If NIC transmits entire frame without detecting
another transmission, NIC is done with frame !
4. If NIC detects another transmission while
transmitting, aborts and sends jam signal
 Send jamming signal (48 bit) after collision is
detected to insure all hosts see collision.
 Interestingly, the actual format of jam signal is
unspecified in the Ethernet specifications.
Most manufacturers have used alternating 1s and 0s as
jam signal.

6. Ethernet CSMA/CD algorithm
5. After aborting, NIC enters exponential backoff
phase: after mth collision, NIC chooses K at
random from {0,1,2,…,2m-1}. NIC waits K·512 bit
times, returns to Step 2.
 Ethernet frames are at least 64 bytes (512 bits)
 At 10Mbps it takes 0.1us to transmit one bit so 512 bits
(64B) take 51.2us to send
 The size of set from which K is selected grows
exponentially with the number of collisions
 1st time: choose K from {0,1} then delay = K *
51.2us
 2nd time: choose K from {0,1,2,3} then delay = K
* 51.2us
 nth time: delay = K x 51.2us, for K=0..2n – 1
• Note max value for k = 1023

Give up after several tries (15)
• Report transmit error to host

7. State Diagram for CSMA/CD
Packet?
No
Sense
Carrier
Send
Detect
Collision
Yes
Discard
Packet
attempts < 16
attempts == 16
Jam channel
b=CalcBackoff();
wait(b);
attempts++;

8. Ethernet’s CSMA/CD (more)
Why uses Exponential Backoff?
 If delay were not random, then there is a chance
that sources would retransmit in lock step
 Why not just choose from small set for K?
 This works fine for a small number of hosts
 Large number of nodes would result in more
collisions
 Does not take into account the collisions that
occurred in the past

9. Ethernet’s CSMA/CD (more)
Is CSMA/CD fair?

10. Introduction to Wireless Networks

11. Ideal Network
HOT SPOTS
WORK
HOME
Public Networks/
Internet
OUTDOORS
Manageable
Faster
RURAL
PUBLIC
VEHICLE
TRANSPORT
More
Secure
Seamless connectivity to all
networks, applications, &
services
Anytime Anywhere
Ease
11
At Home

12. Wired or Wireless: The Future
• Portability
• Mobility
• Ability to be reached regardless of location (Roaming)
• Session continuity while traversing between BS and across
diverse networks (Handover)
• Coverage
• Extend to rural subscribers
• Areas where wired networks not available
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13. Components of Wireless Network
• Wireless hosts
• Laptops, palmtops, PDA, desktop computer, phone
• WiMAX calls wireless hosts as ‘Customer Premises Equipment
(CPE)’
• Access Points
• The base station (BS)
• Responsible for sending and receiving data
• A wireless host is associated with a BS
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14. Components of Wireless Network
• Wireless link
• Communication link between wireless host and access
point/BS and/or among wireless hosts
• The link characteristics depend upon the wireless
technology
• Network Infrastructure
• A larger network with which a wireless host wishes to
communicate
• Service providers e.g. Wi-Tribe, Wateen, Mobilink etc.
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15. Classification of Wireless Networks
• Single Hop / Multiple Hop
•
Number of hops a packet undergoes in the wireless network
• Infrastructure Based / Infrastructure Less
•
•
BS present or absent
Adhoc Networks– no BS
• Examples
•
•
Single Hop Infrastructure based: Wi-Fi in classroom/library/cafe , cellular networks
and Wimax
Single Hop Infrastructure less: Bluetooth
• Fixed / Mobile
•
Nomadicity
•
•
Ability to be reached regardless of location - Roaming
Session continuity while traversing between BS - Handover
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16. Wireless Network Taxonomy
Wireless
Networking
Infrastructure
No
Infrastructure
Single Hop
Multiple Hops
Single Hop
Multiple Hops
WiFi, WiMax
Wireless
Mesh
Networks
Bluetooth
MANET,
VANET

17. Standards of Wireless Technology
WAN
IEEE 802.16d
(Fixed WiMAX)
MAN
LAN
Bluetooth
IEEE 802.16e
(Mobile WiMAX)
PAN
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IEEE 802.11
(Wi-Fi )

18. Wi-Fi: The Predecessor of WiMAX
• Wi-Fi (Wireless Fidelity) is a set of technologies that are
based on the IEEE 802.11a, b, and g standards
• Wi-Fi is considered to be one of the first widely
deployed fixed broadband wireless networks
• The Wi-Fi architecture consists of a base station that
wireless hosts connect to in order to access network
resources
• As long as the users remain within 300 feet of the fixed
wireless access point, they can maintain broadband
wireless connectivity

19. Wi-Fi Standards
Standard
Throughput
Range
(outdoor)
Frequency
802.11a
Up to 54 Mbps
Up to 300 ft
5 Ghz
802.11b
Up to 11 Mbps
Up to 300 ft
2.4 Ghz
802.11g
Up to 54 Mbps
Up to 300 ft
2.4 Ghz
802.11n
100+ Mbps
Up to 500 ft
2.4/5 Ghz

20. Wi-Fi Issues
• Effective range
• Limited level of mobility
• Interference from other sources
• Operate in unlicensed spectrum
• Signal strength decreases with distance
• Multipath propagation
• Wave reflects of ground and objects
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21. What is WiMAX?
• WiMax (Worldwide Interoperability for Microwave
Access) is a standards-based technology enabling the
delivery of last mile wireless broadband access as an
alternative to cable and DSL
• The technology is specified by the Institute of
Electrical and Electronics Engineers Inc., as the IEEE
802.16 standard
• WiMAX eliminates the constraints of Wi-Fi

22. WiMAX Strengths
• Effective range
•
•
2-5 miles
Broad operating range – unlicensed (2.5 GHz, 5 GHz), licensed (3.5 GHz)
• Interference from other sources
•
Operate in licensed spectrum
• Signal strength decreases with distance
• Multipath propagation
•
Wave reflects of ground and objects
Efficient signal processing
schemes, e.g. OFDM, OFDMA
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23. WiMAX Vs Wi-Fi
Parameter
Fixed WiMAX
Mobile WiMAX
Wi-Fi
Standards
IEEE 802.16d2004.
IEEE802.16e-2005.
IEEE 802.11a/b/g
Coverage (Nonline-of-site)
3-5 miles
< 2 miles
<100 ft.
Mobility
Not applicable
Supported
Supported
DL: 6.1 – 9.4 Mbps
DL: 32-46 Mbps
UL: 3.3 – 6.5 Mbps
UL: 4-7 Mbps
Data rate
10 – 66 GHz (Line-of -site)
Frequency Range
2-11 GHz (Non-line-of-site)
IEEE 802.11b: 11
Mbps
IEEE 802.11 a & g:
54 Mbps
IEEE 802.11.b:
2.4GHz
IEEE 802.11.a:
5GHz
IEEE 802.11g:
2.4Ghz
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24. WiMAX Frequency Allocation
Picture Courtesy: Motorola Training Manual CP16
International Frequency Allocation
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25. WiMAX Organisations
 IEEE 802.16 Working Group (WG)
 WiMAX Forum
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26. Responsibilities of IEEE 802.16 WG and
WiMAX Forum
Picture Courtesy: Motorola Training Manual
CP16
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27. Inter Operability Testing (IOT)
 Interoperability testing allows
vendors to bilaterally verify the
standards compliant interfaces
between their network elements in
a controlled environment using
suitable test cases prior to software
release in common operator
networks.
 WiMAX network components are
put to IOT tests to check their
compliancy with WiMAX standards
 We have performed successful
IOT’s for CPE devices manufactured
by various vendors such as
Motorola, Alcatel-Lucent, Huawei,
Gemtech, ZTE, Green Packet etc.
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28. NUST SAMPLE CPE CERTIFICATE
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