1. Prof. Athirai Irissappane
CSS432 Link Level Protocols (Shared Access Networks) Textbook Ch
Prof. Athirai Irissappane
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2. Ethernet local area networking (LAN) technology of last 20 years.
Uses CSMA/CD technology
Carrier Sense Multiple Access with Collision Detection.
Carrier sense means that all nodes can distinguish between an idle and a busy link.
Collision detection means that a node listens as it transmits and can therefore detect when a frame it is transmitting has collided with a frame transmitted by another node.
Ethernet transceiver (sends and receives signal) and adaptor
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3. Ethernet
Ethernet segment is implemented on a coaxial cable of up to 500 m
Bandwidth: 10Mbps (10Base2=Thin Coax 200m, 10B5=Yellow Thick Coax 500m, 10BT=Twisted pair 100m), 100Mbps(10BaseT), 1Gbps
Broadcasts Data, Drawback: Contention for the same Ethernet link
Multiple Ethernet segments joined by repeaters.
A repeater forwards digital signals (only 4 allowed)
Hub: multiway repeater,repeats data it hears on one port to all others
Instead of using coax cable, an Ethernet can be constructed from a thinner cable known as 10Base2 (the original was 10Base5)
10 means the network operates at 10 Mbps
Base means the cable is used in a baseband system
2 means that a given segment can be no longer than 200 m
Another cable technology is 10BaseT
T stands for twisted pair
Limited to 100 m in length
With 10BaseT, the common configuration is to have several point to point segments coming out of a multiway repeater, called Hub
Ethernet repeater (l = 500 * 4 seg = 2500 m)
Ethernet Hub
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4. Access Protocol for Ethernet
The University of Adelaide, School of Computer Science
14 April 2018
Access Protocol for Ethernet
Multiple access network, access control to the shared Ethernet link
Media Access Control (MAC).
implemented in Hardware on the network adaptor.
Frame format (Similar to HDLC framing protocol)
Preamble (64bit): allows the receiver to synchronize with the signal (sequence of alternating 0s and 1s).
Host and Destination Address (48bit each).
Packet type (16bit): demux key to identify the higher level protocol where message should be delivered
Data (up to 1500 bytes)
Minimally a frame must contain at least 46 bytes of data.
Frame must be long enough to detect collision.
CRC (32bit)
Inter-frame gap
bytes
Used by layer 3
IP: 0x0800
ARP: 0x0806
IPv6: 0x86DD
Dest
addr 8 6 4
CRC
Preamble
Src
Type
Body 2
Next
frame
46 ~ 1500
Min: 64bytes (512bits) ~ Max: 1518bytes 12
Chapter 2 — Instructions: Language of the Computer

5. Ethernet (MAC) Address
Each host on an Ethernet has a unique Address.
The (unicast) address belongs to the adaptor, not the host.
MAC Address
sequence of six numbers separated by colons
each number corresponds to 1 byte of the 6 byte (48 bit) address and is given by a pair of hexadecimal digits, one for each of the 4-bit nibbles in the byte
Leading 0s are dropped.
E.g., 8:0:2b:e4:b1:2 =
address consisting of all 1s a broadcast address.
All adaptors pass frames addressed to the broadcast address up to the host.
an address with first bit set to 1 but is not the broadcast address is called a multicast address.
host can program its adaptor to accept multicast addresses.
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6. Ethernet Transmit Algorithm
If line is idle…
Adapter sends frame immediately
Upper bound message size of 1500 bytes: MTU (Maximum Transmission Unit)
Must wait 96bits between back-to-back frames
If line is busy…
Adapter waits until idle and transmit immediately
Called 1-persistent
Transmit a packet with probability 1.
(special case of p-persistent: transmitting a packet with P percent, where 0 < p ≤ 1)
Collision of frames
2 or more two (or more) adaptors to begin transmitting at the same time
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7. Transmit Algorithm (cont)
If collision…
Ethernet supports collision detection, each sender is able to determine that a collision is in progress
Send a 32-bit jamming sequence, and then stop transmitting frame (64bit preamble + 32bit jam = 96bits)
Minimum frame is 64 bytes (header + 46 bytes data + 4 bytes CRC) = 512bits
The farther the nodes, the longer the time it takes for a frame sent by one to reach the other, and the network is vulnerable to collision during this time
10Mbps means 10bits/usec.
96bits needs 9.6usec
… 64bits
bits
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8. Collisions 500m x 5 = 2500m (with 4 repeaters) Time t Time t + td
latency A
A sends a frame at time t B
Time t A B
Ethernet
Arrives at B at time t + td
Time t + td
(td = 25.6us: approx. 0.01us/m)
Token Ring A B
B sends frame at t + td
A collision occurred
30%
Network load
Jam seq A B
Time t + 2td
(2td = 51.2us)
Jamming seq arrives at t + 2td
Bandwidth
10Mbps
100Mbps
1Gbps
Bits/usec
10bits/usec
100bits/usec
1000bits/usec
Jamming sequence
96bits => 9.6usec
96bits => 0.96usec
96bits => 0.096usec (96nsec)
Max delay (RTT = 2td)
512bits => 51.2usec
512bits => 5.12usec
512bits => 0.512usec
Speed
51.2usec/5000m = 10.2nsec/m
51.2usec/200~400m = 12.8nsec/m
0.512usec/25m (cupper) = 20.5nsec/m
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9. Collisions RTT = 51.2 us for 2*2500=5000m Transmit Time >= RTT
Size/Bandwidth >=RTT
Size >= RTT * Bandwidth
Size >= 51.2us * 10Mbps
Size >=512 bits; min bits to be transmitted = 512 bits
If RTT > 51.2 us , i.e., cable length > 2500m, min number of bits > 512 for 10Mbps ethernet; Limited Cable Length!

10. Quiz: First to solve get 0.02 points extra added to discussion score
The speed of transmission: ns/m, i.e, time/distance
Min frame size = 512 bits
Calculate the maximum length of the ethernet cable for a 100Mbps network
The higher network bandwidth, the more sensitive the NICs should be to detect a collision.
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11. Collisions On collision detection (Exponential Backoff)
adapter stops transmission
Sends jamming sequence (total of 96 bits)
delays and tries again.
(Exponential Backoff)
Doubles delay interval between retransmission
1st time: waits for 0 or 51.2us (selected at random)
2nd time: 0, 51.2, 102.4us or 153.6us (random)
3rd time: waits for k * 51.2, k = 0…23 – 1 (random)
nth time: wait for k x 51.2us, for randomly selected k=0..2n – 1
give up after several tries (usually 16)
Wait 96 bit time ,i.e., 9.6us for 10Mbps; 960ns for 100Mbps before transmitting back to back frames
Recovery time in case of collisions, clock recovery

12. Wireless Network No physical medium for transmission
Transmission through electromagnetic signals
Radio, microwave, infrared
Wireless links (WIFI, Bluetooth, FM radio) all share the same medium (air/free space)
Share medium efficiently without interference
Dividing the medium using frequency and space dimensions
Allocations of bands (frequency) ranges determined by government agencies such as FCC in USA
Bands for government use, AM radio, FM radio, televisions, satellite communications, cell phones,
Specific frequencies within these bands are then allocated to individual organizations for use within certain geographical areas.
Several bands set aside for which a license is not needed
FCC (Federal Communications Commission) ;
AM radio: 740 kilohertz,
PCS mobile broadband: Mhz
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13. Wireless Network
Devices that use license-exempt frequencies are still subject to certain restrictions
Limitation on transmission power: limits the range of signal, making it less likely to interfere with another signal
For example, a cordless phone might have a range of about 100 feet.
Baby monitors, home security systems, even WIFI..
Use Spread Spectrum Technique:
Sharing spectrum without interference
While using the same constant frequency for transmission results in interference from other signals, easy to intercept (not secure)
Bandwidth: width of the frequency band: range 3300 Hz – 300 Hz = 3000 Hz (range of signals that can be accommodated)
No.of bits transmitted per second that can be transmitted on the link (bandwidth / data rate)
Frequency: number of occurences of a repeating event per unit time; number of cycles per uit time
WIFI –licese exempt 2.5 GHz frequency band
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14. Wireless Network
Spread Spectrum technique: spread the signal over a wider frequency band
Frequency hopping: Transmit signal over a random set of frequencies
Transmit at one frequency, then a second, then a third…
Sequence of frequencies is not truly random, instead computed algorithmically by a pseudorandom number generator
Receiver uses the same algorithm
Hop frequencies in sync with the transmitter to correctly receive the frame
Direct sequence: Represents each bit in the frame by multiple bits in the transmitted signal.
For each bit, the sender sends the XOR of that bit and n random bits
random bit generator known to both the sender and the receiver.
The transmitted values, known as an n-bit chipping code, spread the signal across a frequency band that is n times wider
Bandwidth: width of the frequency band: range 3300 Hz – 300 Hz = 3000 Hz (range of signals that can be accommodated)
No.of bits transmitted per second that can be transmitted on the link (bandwidth / data rate)
Frequency: number of occurences of a repeating event per unit time; number of cycles per uit time
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15. Wireless Network
A second spread spectrum technique called Direct sequence
Represents each bit in the frame by multiple bits in the transmitted signal.
For each bit the sender wants to transmit
It actually sends the exclusive OR of that bit and n random bits
The sequence of random bits is generated by a pseudorandom number generator known to both the sender and the receiver.
The transmitted values, known as an n-bit chipping code, spread the signal across a frequency band that is n times wider
The first computer to come online is assigned by the Token Ring system to monitor network activity. The monitoring computer makes sure that frames are being delivered and received correctly. It does this by checking for frames that have circulated the ring more than once and ensuring that only one token is on the network at a time.
The process of monitoring is called beaconing. The active monitor sends out a beacon announcement every seven seconds. The beacon is passed from computer to computer throughout the entire ring. If a station does not receive an expected announcement from its upstream neighbor, it attempts to notify the network of the lack of contact. It sends a message that includes its address, the address of the neighbor that did not announce, and the type of beacon. From this information, the ring attempts to diagnose the problem and make a repair without disrupting the entire network. If it is unable to complete the reconfiguration automatically, manual intervention is required.
THT: copy time: time token in host (fixed) (10 ms)
TRT Token rotation time = Num stations * THT + Ring laterncy
TTRT: upper bound on TRT (to ensure that a node gets the opportunity to transmit within a certain time )
The token holding time (THT) is a counter which is linearly decremented and forces the token to be released at expiration. The token holding timer accepts the remaining time from the TRT and determines how long a station may still send asynchronously
4-bit chipping code
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16. Wireless Network CDMA: Code Division Multiple Access (Cellular-3G)
Multiplexing technique different from Time/Frequency Division multiplexing
Senders send information, share bandwidth, transmit at different frequencies;
Direct Sequence Spread spectrum technique
Differentiated using chipping codes (random number)
Each cell phone sends data with a different (but pre-assigned chipping code.)
A base station distinguish many cell phones using their unique chipping code.
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17. Wireless Network Topology Mesh or Ad-hoc network (no base station)
Wireless Network with Base Station
Mesh or Ad-hoc network
Two end-points are usually different kinds of nodes
One end-point usually has no mobility, but has wired connection to the Internet (known as base station)
The node at the other end of the link is often mobile
Wireless communication supports point-to-multipoint communication
Communication between non-base (client) nodes is routed via the base station
Three levels of mobility for clients
No mobility: the receiver must be in a fix location to receive a directional transmission from the base station (initial version of WiMAX)
Mobility is within the range of a base (Bluetooth)
Mobility between bases (Cell phones and Wi-Fi)
Mesh or Ad-hoc network (no base station)
Nodes are peers
Messages may be forwarded via a chain of peer nodes
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18. Wireless Network
Ad-hoc Network
Base Stations
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19. Wireless Network
Wireless technologies differ in a variety of dimensions
How much bandwidth they provide
How far apart the communication nodes can be
Four prominent wireless technologies
Bluetooth
Wi-Fi (more formally known as )
3G cellular wireless
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20. IEEE 802.11 (WIFI) Wireless Link Technology
Like its Ethernet and token ring siblings, is designed for use in a limited geographical area (homes, office buildings, campuses)
Primary challenge is to mediate access to a shared communication medium – in this case, signals propagating through space
Operates in the license exempt band 2.4 GHz or 5 GHz
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21. Collision
Ethernet:
Every node receives every other node’s transmissions
Every node can transmit and receive at the same time
WIFI
Cannot transmit and receive at the same time on the same frequency, power generated by transmitter is much higher than receiver; WIFI runs at half-duplex
Every node cannot received transmissions from the other node because of the WIFI range, distance between the nodes, blockage between the nodes
Ethernet: Echo can cellation
The method is called echo cancellation, and it requires a bit of signal processing. Basically, the idea is since you know what you're sending out, then you can separate the signal you just sent from what is coming in from the far end of the link. The way the circuitry is set up, the transmit and receive signals are superimposed on top of each other, more or less adding together.
Simple example to give you an idea of how this works: if the transmitter sends
+1, +1, -1, +1
and the local receiver gets
+2, 0, -2, +2
then you can work out that the signal from the other end must have been
+1, -1, -1, +1
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22. Collision Avoidance Hidden Nodes: Exposed Node Problem:
A and C want to exchange frames with B but do not know about each other. A and C are said to hidden nodes
Hidden Node Problem: Signals from A and C can collide at B
Exposed Node Problem:
B sends signal to A; C becomes aware and stops sending to D thinking it will interfere with A
MACA Multiple Access with Collision Avoidance Alg.
Sender and receiver exchange control frames (Request to Send RTS, Clear to Send CTS) with each other before the sender actually transmits any data.
Any node that sees the CTS frame is close to the receiver, therefore cannot transmit for the period of time it takes to send a frame of the specified length
Any node that sees the RTS but not CTS is free to transmit
Receiver sends an ACK to the sender after successfully receiving a frame All nodes must wait for this ACK before trying to transmit
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23. Distribution System 802.11 uses Base Station topology currently
Nodes are free to move around
Access Points / Base stations (routers in WIFI)
some nodes are allowed to roam
some are connected to a wired network infrastructure
they are called Access Points (AP) and they are connected to each other by a so-called distribution system
Extend the WIFI range: Main base station connects to internet, others physically connect to each other
Each of these regions is analogous to a cell in a cellular phone system with the APs playing the same role as a base station
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24. Distribution System Each nodes associates itself with one access point
For node A to communicate with node E, A first sends a frame to its AP-1 which forwards the frame across the distribution system to AP-3, which finally transmits the frame to E
The technique for selecting an AP is called scanning
The node sends a Probe frame
All APs within reach reply with a Probe Response frame
The node selects one of the access points and sends that AP an Association Request frame
The AP replies with an Association Response frame
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25. Bluetooth ( )
Used for very short range communication between mobile phones, PDAs, notebook computers and other personal or peripheral devices
Operates in the license-exempt band at 2.45 GHz
Has a range of only 10 m
Communication devices belong to one person or group, Bluetooth is categorized as Personal Area Network (PAN)
Version 2.0 provides speeds up to 2.1 Mbps
Power consumption is low
master
active
slave
parked 7
255 :
Request
Reponses o e
TDM
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26. Bluetooth ( )
Bluetooth network configuration is called a piconet
Consists of a master device and up to seven slave devices
Any communication is between the master and a slave
The devices can switch roles, by agreement
The slaves do not communicate directly with each other
A slave can be parked: set to an inactive, low-power state (255 parked slaves allowed in a piconet)
Synchronous Time division multiplexing: Master transmits in odd time slot, slave in the even time slot. Slave transmits as a response to request in the previous master slot (avoids contentions between slaves)
master
active
slave
parked 7
255 :
Request
Reponses o e
TDM
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27. Cell Phone Technology Wireless Technology
Use Licensed Spectrum, owned by cellular phone operators (AT & T, Verizon, T-Mobile) – costly
Topology: Relies on a wired network of base stations
Geographic area served by a base station is a cell
Base station can server one or more cells
Cells overlap
A phone is in control of only one base station at a time
When phones moves to an area of overlap, the base station senses the weakening of signal from phone and gives control to another base station which receives the strongest signal.
If the phone is involved in a call, this transfer is called handoff
Base satations 1 2 3 P
Call
Handoff
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28. Reviews Exercises in Chapter 2
Ethernet: k-persistent, exponential back off, and the relationship between the minimum frame size and collisions.
Exercises in Chapter 2
Ex. 42 (Ethernet)
Ex. 46 (Ethernet)
Ex. 53 (Wi-Fi)
Ex. 54 (Wi-Fi)
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