1. Overview Yesterday we looked at Ethernet: CSMA/CD network
Today we will look at wireless networks: CSMA/CA

2. Challenges Limited wireless transmission range
Broadcast nature of the wireless medium
Hidden terminal problem
Packet losses due to transmission errors
Mobility-induced route changes
Mobility-induced packet losses
Battery constraints
Potentially frequent network partitions
Ease of snooping on wireless transmissions (security hazard)
Nitin UIUC

3. EM Spectrum    Pravin Bhagwat @ AT&T Labs ISM band
902 – 928 Mhz
2.4 – Ghz
5.725 – Ghz
ISM band
AM radio
S/W radio
FM radio TV TV
cellular LF HF
VHF
UHF
SHF
EHF MF  
30kHz
300kHz
3MHz
30MHz
300MHz
30GHz
300GHz
10km
1km
100m
10m 1m
10cm
1cm
100mm
3GHz
X rays
Gamma rays 
infrared
visible UV
1 kHz
1 MHz
1 GHz
1 THz
1 PHz
1 EHz
Propagation characteristics are different in each frequency band
Pravin AT&T Labs

4. Unlicensed Radio Spectrum
33cm
12cm
5cm
26 Mhz
83.5 Mhz
125 Mhz
902 Mhz
2.4 Ghz
5.725 Ghz
928 Mhz
Ghz
5.785 Ghz
cordless phones
baby monitors
Wireless LANs
802.11
Bluetooth
Microwave oven
802.11a
Pravin AT&T Labs

5. Spread Spectrum: resilient transmission
Idea
spread signal over wider frequency band than required
originally designed to thwart jamming
Frequency Hopping
transmit over random sequence of frequencies
sender and receiver share…
pseudorandom number generator
seed
uses 79 x 1MHz-wide frequency bands

6. Spread Spectrum (cont)
Direct Sequence
for each bit, send XOR of that bit and n random bits
random sequence known to both sender and receiver
called n-bit chipping code
defines an 11-bit chipping code 1
Data stream: 1010 1
Random sequence: 1
XOR of the two:

7. Collisions Avoidance Similar to Ethernet
Problem: hidden and exposed nodes

8. Hidden Terminal Problem
Node B can communicate with A and C both
A and C cannot hear each other
When A transmits to B, C cannot detect the transmission using the carrier sense mechanism
If C transmits, collision will occur at node B A B C
Nitin UIUC

9. Solution: MACAW Sender transmits RequestToSend (RTS) frame
Receiver replies with ClearToSend (CTS) frame
Neighbors…
see CTS: keep quiet
see RTS but not CTS: ok to transmit
Receive sends ACK when has frame
neighbors silent until see ACK
Collisions
no collisions detection
known when don’t receive CTS
exponential backoff

10. RTS/CTS Handshake Nitin Vaidya @ UIUC Sender sends Ready-to-Send (RTS)
Receiver responds with Clear-to-Send (CTS)
RTS and CTS announce the duration of the transfer
Nodes overhearing RTS/CTS keep quiet for that duration
RTS/CTS used in IEEE C
CTS (10) 10
RTS (10)
RTS/CTS serves two purposes
quicker retransmissions if there is a collision because there is no collision detection
avoids collisions caused by hidden terminals A B D
Nitin UIUC 10

11. Supporting Mobility Case 1: ad hoc networking
Case 2: access points (AP)
tethered
each mobile node associates with an AP
Distribution system
AP-1
AP-3
AP-2 F A B G H C E D

12. Mobility (cont) Scanning (selecting an AP) When node sends Probe frame
all AP’s w/in reach reply with ProbeResponse frame
node selects one AP; sends it AssociateRequest frame
AP replies with AssociationResponse frame
new AP informs old AP via tethered network
When
active: when join or move
passive: AP periodically sends Beacon frame

13. Shared Access Networks
The next technology that we will look at tries to guarantee who gets access to the network
Token Ring
ATM
Fibre channel
Myrinet

14. Token Ring Overview Examples
16Mbps IEEE (based on earlier IBM ring)
100Mbps Fiber Distributed Data Interface (FDDI)
4B/5B encoding

15. Token Ring (cont) Idea Frame Format
Frames flow in one direction: upstream to downstream
special bit pattern (token) rotates around ring
must capture token before transmitting
release token after done transmitting
immediate release
delayed release
remove your frame when it comes back around
stations get round-robin service
Frame Format 8 8 48 48 32 8 24
Start of
Dest
Src
End of
Control
Body
CRC
Status
frame
addr
addr
frame

16. Timed Token Algorithm Token Holding Time (THT)
upper limit on how long a station can hold the token
Token Rotation Time (TRT)
how long it takes the token to traverse the ring.
TRT <= ActiveNodes x THT + RingLatency
Target Token Rotation Time (TTRT)
agreed-upon upper bound on TRT

17. Algorithm (cont) Each node measures TRT between successive tokens
if measured-TRT > TTRT: token is late so don’t send
if measured-TRT < TTRT: token is early so OK to send
Two classes of traffic
synchronous: can always send
asynchronous: can send only if token is early
Worse case: 2xTTRT between seeing token

18. Token Maintenance Lost Token Generating a Token (and agreeing on TTRT)
no token when initializing ring
bit error corrupts token pattern
node holding token crashes
Generating a Token (and agreeing on TTRT)
execute when join ring or suspect a failure
send a claim frame that includes the node’s TTRT bid
when receive claim frame, update the bid and forward
if your claim frame makes it all the way around the ring:
your bid was the lowest
everyone knows TTRT
you insert new token

19. Maintenance (cont) Monitoring for a Valid Token
should periodically see valid transmission (frame or token)
maximum gap = ring latency + max frame < = 2.5ms
set timer at 2.5ms and send claim frame if it fires

20. FDDI (Fiber Distributed Data Interface)
The late-80’s version of token ring (100Mbps, fiber-based)
Dual-ring (two fibers): 2nd ring used for fault recovery
Can handle single point failures

21. ATM Technology (courtesy: ATM Forum)
Negotiated Service Contract
Connection Oriented - virtual circuit
End-to-End Quality of Service
Cell Switching
53 Byte Cell
48 Byte Payload, 5 Byte Header
Fixed Size
Header contains virtual circuit information
Payload can be voice, video or other data types
ATM Technology is based on powerful, yet flexible concepts.
1. When information needs to be communicated, the sender NEGOTIATES a “requested path” with the network for a connection to the destination. When setting up this connection, the sender specifies the type, speed and other attributes of the call, which determine the end-to-end quality of service. An analogy for this negotiation of qualities would be similar to determining a method of delivery using US mail. One can choose to send 1st class, overnight, 2 day delivery, etc. and can ask for certified mail.
2. Another key concept is that ATM is a switched based technology. By providing connectivity through a switch (instead of a shared bus) several benefits are provided:
* Dedicated bandwidth per connection
* Higher aggregate bandwidth
* Well-defined connection procedures
* Flexible access speeds
Using ATM, information to be sent is segmented into fixed length cell,
transported to and re-assembled at the destination. The ATM cell has a
fixed length of 53 bytes. The fixed length allows the information to be
transported in a predictable manner. This predictability accommodates
different traffic types on the same network.
The cell is broken into two main sections, the header and the payload. The
payload (48 bytes) is the portion which carries the actual information-either
voice, data, or video. The header (5 bytes) contains information used for routing. 3

22. ATM System Architecture
End Station
Switch
End Station
Voice
Voice
AAL
ATM
PHY
PHY
ATM
PHY
PHY
ATM
AAL
Data
Data
Video
Video
Cells
Adaptation Layer (AAL): Inserts/extracts information into 48 byte payload
ATM Layer: Adds/removes 5 byte header to payload
Physical Layer: Converts to appropriate electrical or optical format
ATM is a layered architecture allowing multiple services like voice, data and video, to be mixed over the network. Three lower level layers have been defined to implement the features of ATM.
The Adaptation Layer assures the appropriate service characteristics and divides all types of data into the 48 byte payload that will make up the ATM cell.
The ATM Layer takes the data to be sent and adds the 5 byte header information that assures the cell is sent on the right connection.
The Physical Layer defines the electrical characteristics and network interfaces. This layer puts the “bits” on the transport media. ATM is not tied to a specific type of physical transport. 4

23. Fibre Channel Connect servers, workstations, disk storage etc.
Optical or electrical media
133 Mbps to 1062 Mbps
10 km
point-to-point links or loop or connect to a switch
IP, SCSI etc.

24. Myrinet 2 GB full duplex high speed network interface

25. Myrinet
Few sec latency