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An Introduction
to
Computer Networks
Lecture 5: Direct Link Networks
University of Tehran
Dept. of EE and Computer Engineering
By:
Dr. Nasser Yazdani
1
Outline

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Issues
ALOHA Network
Ethernet
Token Ring
Wireless
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Main Issues
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Local Area Network (LAN) : Three or more
machines are physically connected and
communicating.
Problems:

How to connect them? Topology
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How to address each machine? Addressing
How to regulate accessing to the media?
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Sharing links
MAC (Media Access method or protocol)
Different technology address each problem in
different way.
Problems are not independent
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LAN Technologies.
Session
Transport
Network
Link
Physical
The 7-layer OSI Model
TCP
TFTP
HTTP
NNTP
Presentation
Telnet
FTP
SMTP
Application
UDP
IP
LAN-LINK
The 4-layer Internet Model
Link layer can have two types of technologies;
• Point to point link like PPP where there are only 2 nodes.
• Broadcast link like Ethernet when there are more than 2
nodes.
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Data link sublayers
Our focus will be on
MAC sublayer.
MAC = “Medium Access Control”
Multiplexing
Media Access (MAC)
Error Detection
Framing
• The link is shared among different sender and receivers.
• Since every frame is simultaneously accessed by different nodes;
•They are called multiaccess links.
•They are called broadcast links. (important)
•LAN because of limited area.
• We need some type of medium access rules to avoid collision.
• Multicast capability of LANs.
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Simple
Random
Examples of MAC Protocols
Packet-Switched Radio Network
Aloha
Carrier Sense Multiple Access/Collision Detection
Complex
Deterministic
Ethernet (IEEE 802.3)
Token Passing
Token Ring (IEEE 802.5)
Fiber Distributed Data Interface (FDDI)
Wireless
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Goals of MAC Protocols
MAC Protocols arbitrate access to a
common shared channel among a
population of nodes
Goals:
1. Fair among users
2. High efficiency
3. Low delay
4. Fault tolerant
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Aloha Protocol
All nodes transmit on one freq.
Central Node
Central node relays packets on
the other frequency
f0
f1
If more than one node transmit at the same time
Collision!
If there is a collision, both nodes re-transmit packets
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CSMA/CD Protocol
All nodes transmit & receive on one channel
Packets are of variable size.
1. Carrier Sense: Check if the line is idle before transmitting.
2. Collision Detection: If more than one node transmit.
Collision!
All nodes detect collision, wait for random delay. Goto 1.
binary exponential backoff
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CSMA/CD Network Size Restriction
Node must be able to hear that there is
a collision before its packet is transmitted completely.
i.e. Packet Transmission Time > Round trip propagation time
i.e. TRANSP > 2.PROP
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Performance of CSMA/CD
Assume time-slotted channel
1. Find  ( p): Probability that exactly one node transmits in
a given slot, where:
p = Prob{a node tries to transmit a packet in a time slot},
N = number of nodes
N
 ( p)    p(1  p) N 1
1 
d
 N (1  p ) N 1  pN ( N  1)(1  p ) N  2
dp
 max  36%  40%
when : p  1 / N
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Ethernet Overview
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History
 developed by Xerox PARC in mid-1970s
 roots in Aloha packet-radio network
 standardized by Xerox, DEC, and Intel in 1978
 similar to IEEE 802.3 standard
Uses CSMA/CD technique for Media access.
Uses 10Mbps physical link originally and now
extended to 100Mbps, Fast Ethernet, and recently to
1000Mbps, Gigabit Ethernet.
Uses variable frame length, 64-1500 bytes.
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The Original Ethernet
Repeaters
every
500m
10Mb/s
l  1500m
PROPmax  l / c  1500 / 2.5 108  6s
Thick copper
coaxial cable
TRANSP  2 PROP  TRANSP  12s
 Packetsize  12s 10Mb / s  120bits
In practice, minimum packet size = 512 bits.
• allows for extra time to detect collisions.
• allows for “repeaters” that can boost signal.
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The Original Ethernet
Original picture drawn by Bob Metcalfe,
inventor of Ethernet (1972 – Xerox PARC)
The Ethernet protocol is implemented in Contoroler (Adaptor)
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Ethernet Frame Format
Bytes:
7
1
6
6
Preamble SFD
DA
SA
2
Type
0-1500
Data
0-46 4
Pad CRC
1. Preamble: trains clock-recovery circuits
2. Start of Frame Delimiter: indicates start of frame
3. Destination Address: 48-bit globally unique address
assigned by manufacturer.
1b: unicast/multicast
1b: local/global address
4. Type: Indicates protocol of encapsulated data (e.g. IP
= 0x0800)
5. Pad: Zeroes used to ensure minimum frame length
6. Cyclic Redundancy Check: check sequence to detect
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bitUniv.
errors.
Ethernet Addresses
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Unique, 6 bytes or 48-bit address assigned to each
adapter by manufacturer.
It is read in : notation, for example: 8:0:e4:b1:2
An address with all 1s is a broadcast address.
multicast: first bit is 1
In order to make the address unique, first 24 bits
are assigned to manufacturers and the last 24 bits
are assigned locally.
Each adaptor accept the packet if the destination
address is its own address, broadcast address or
multicast to which this adaptor belongs.
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The 10Mb/s Ethernet Standard
IEEE 802.3
Ethernet MAC Protocol
10Base-5
Different
physical layer
options
10Base-2
10: 10Mbs
10Base-T
10Base-F
Base: baseband 5: 500 Meter
10Base-5: Original Ethernet: large thick coaxial cable.
10Base-2: Thin coaxial cable version.
10Base-T: Voice-grade unshielded twisted-pair
Category-3 telephone cable.
10Base-F: Two optical fibers in a single cable.
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10Base-T
“Twisted pair Ethernet”
Repeater
“Hub”
100m max cable length
Router
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Designed to run over existing voice-grade “Category3” twisted pair telephone wire.
Centralized management (“managed hubs”) lead to
more reliability.
Created a huge increase in Ethernet usage.
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Transmit Algorithm
If line is idle…
send immediately
upper bound message size of 1500 bytes
must wait 9.6us between back-to-back frames
If line is busy…
wait until idle and transmit immediately
called 1-persistent (special case of p-persistent)
(sending with probability of p)
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Algorithm (cont)
If collision…
jam for 32 bits, then stop transmitting frame
(minimum frame is 64 bytes (header + 46 bytes of data))
delay and try again
1st time: 0 or 51.2us
2nd time: 0, 51.2, or 102.4us
3rd time51.2, 102.4, or 153.6us
nth time: k x 51.2us, for randomly selected k=0..2n - 1
give up after several tries (usually 16)
exponential backoff
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Increasing the data rate
Increasing the data rate create the following
Problem:
 E.g. CSMA/CD at 100Mb/s over 1500m of
TRANSP  2PROP
cable:
8
PROP  1500 / 2.5 10  6s
TRANSP  12s  Packetsize  1200bits

To overcome this two techniques used:
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Cable length limited to 100m:
PROP  200 / 2.5 108  Packetsize  160bits
Use “Ethernet Switching” to prevent collisions.
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Ethernet Switching
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An Ethernet switch is the same as an Ethernet
Bridge.
A Bridge:
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Examines the header of each arriving frame.
If the DA is in its table, it forwards the frame to the
correct output port.
If the DA is not in its table, it broadcasts the frame
to all ports (except the one through which it
arrived).
The table is learned by examining the SA of arriving
packets.
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Ethernet Switch
Ethernet
Switch/Bridge
Router
• If
only one computer per port, no collisions can take
place (each cable is now a self-contained point-to-point
Ethernet link).
• Capacity is increased: the switch can forward multiple
frames to different computers at the same time.
• An Ethernet switch must contain buffers to hold
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frames
during times of
congestion.
Ethernet Switch
Ethernet
Switch/Bridge
Router
Ethernet
Hub
• If only one computer per port, no collisions can take place (each cable
is now a self-contained point-to-point Ethernet link).
• Capacity is increased: the switch can forward multiple frames to
different computers at the same time.
• An Ethernet switch must contain buffers to hold frames during times
of congestion.
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Token Ring
Listen:
Talk:
Data
l4
l1
l3
l2
Token/Data
PROP  i li / c  TRTmin
TRT=Token Rotation Time
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Token Ring (cont)
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It is like people talking in a ring in the round
robin manner.
Common features.
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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
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immediate release
delayed release
remove your frame when it comes back around
stations get round-robin service
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Timed Token Algorithm
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Token Holding Time (THT)
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Token Rotation Time (TRT)
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how long it takes the token to traverse the ring.
ActiveNodes x THT + RingLatency  TRT
Target Token Rotation Time (TTRT) (FDDI)
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upper limit on how long a station can hold the token
agreed-upon upper bound on TRT
Each node measures TRT between successive
tokens
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if measured-TRT > TTRT: token is late so don’t send
if measured-TRT < TTRT: token is early so OK to send
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Token Maintenance
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Lost Token
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no token when initializing ring
bit error corrupts token pattern
node holding token crashes
Solution- have a monitor
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Monitor role in the link
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Generating Tokens
Announces its presence periodically
Check for the corrupt or orphaned frames and
remove them from the ring. Orphaned frame are
those whose sending station have died. Sets the
monitor bit to 0 in sending and to 1 when pass
the monitor.
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Token Maintenance (cont)
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How about when the monitor dies? Or the network
just powered up?
Any station tries to become monitor
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send a claim frame that includes the node’s TTRT bid
if your claim frame makes it all the way around the ring:
 Everyone has accepted you as the monitor
 everyone knows TTRT
 you insert new token
How about receiving another monitor claim at the
same time? Break the tie with
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The lowest TTRT bid wins.
The highest address wins.
Etc.
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Maintenance (cont)
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Monitoring for a Valid Token
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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
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Release After Reception (RAR)
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Computer captures token, transmits data, waits for
data to successfully travel around ring, then releases
token again.
Allows computer to detect errored frames and
retransmit them.
Example time evolution in which host 1 and host 3 have packets to transmit:
TRANSP
PROP
TRANST
Data
Token
l1/c l2/c
Token arrives
at host 1
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TRANST
lN/c
TRANSP
Data
Token
l1/c
l2/c
Token departs Token arrives
at host 3
from host 1
Token arrives
at host 2
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l3/c
time
31
Release After Transmission
(RAT)
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Computer captures token, transmits data, then
releases token again.
FDDI uses this technique.
Example time evolution in which host 1 and host 3 have packets to
transmit:
TRANSP
Data
TRANST TRANST
Token
l1/c
Token arrives
at host 1
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Token departs
from host 1
Token
TRANSP
Data
l2/c
Token arrives
at host 3
Token
time
Token arrives
at host 2
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Frame format
Bytes 1
Start
Delimt.
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1
Access
control
1
Frame
control
6
6
Dest.
Addr
Src.
Addr
Variable
Body
4
Checks
um
1
1
End
Delimt.
Frame
status
Access control is for priority.
Frame control is a Demux key for the higher layer
protocol.
Addresses are like Ethernet. They can also be 16 bit.
Frame Status include two A and C bits.
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A, Active bit, is set by the receiver indicating the station is alive
and has seen the frame.
C, Copy bit, is also set by the receiver indicating the frame has
been copied.
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FDDI: Fiber Distributed
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It is a Dual counter-rotating ring for fault
tolerance. It can be also Single Attachment
(SAS- Single Attachment Station).
100 Mbps on optical fibers
Up to 500 nodes
Total length less than or equal to 200 km
Uses 4B/5B encoding.
Modulation: non-return to zero with inversion
(NRZI)
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FDDI Timed Token Rotation
Protocol
1. All hosts agree on a common Target Token Rotation
Time (TTRT). They will aim to make the token rotate
around the network at least once per TTRT. Hence,
they can each expect to see the token once
TTRT.
2. Each host on the network maintains a timed token
Rotation (TRT) timer, that indicates when the token is
next expected to arrive.
3. If the token arrives before TRT expires, we say it is
“Early”. If the token arrives after TRT expires, we say
it is “Late”.
4. A host can only transmit if it receives the token, AND
the token is Early.
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Wireless LANs
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It is shared media like Ethernet
The standard is IEEE 802.11
Bandwidth: 1 or 2 Mbps
Physical Media
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spread spectrum radio (Up to 2.4GHz)spread signal over a wider frequency band
diffused infrared (10m), sender and receiver
do not have to aimed at each (up to 10m,
bluetooth tech).
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Spread Spectrum
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Idea is to spread signal over wider frequency
band than required
Frequency Hopping
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transmit over random sequence of frequencies
sender and receiver share…
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pseudorandom number generator
seed
802.11 uses 79 x 1MHz-wide frequency bands
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Spread Spectrum (cont)

Direct Sequence
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for each bit, send XOR of that bit and n random
bits
random sequence known to both sender and
receiver
The sent code is called n-bit chipping code
802.11 defines an 11-bit chipping code
1
0
Data stream: 1010
1
0
Random sequence: 0100101101011001
1
0
XOR of the two: 1011101110101001
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Collisions Avoidance
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It is similar to Ethernet, but there is mobility
problem here and also
Problem: hidden and exposed nodes
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B can exchange data with A and C, but not D.,
How A and C send to B, they are not aware, this is the
hidden node problem.
If B send to A, C can send to D, exposed problem.
A
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MACA
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MACA- Multiple Access with Collision
Avoidance
The idea is the sender and receiver to
exchange control information.
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MACA (cont)
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Sender transmits RequestToSend (RTS) frame
Receiver replies with ClearToSend (CTS) frame
Neighbors…
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Receiver sends ACK when has the frame
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see CTS: keep quiet
see RTS but not CTS: OK to transmit
neighbors silent until see this ACK
Collisions if two sender transmit RTS at the same
time.
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no collisions detection
known when don’t receive CTS
exponential backoff
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Supporting Mobility
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Case 1: ad hoc networking when node may or
may not be able to communicate.
Case 2: access points (AP) connected node to
wire
 each mobile node associates with an AP
Distribution system
AP-1
AP-3
F
AP-2
A
B
G
H
C
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Mobility (cont)
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Scanning (selecting an AP)
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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 network
When
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active: when join or move
passive: AP periodically sends Beacon frame
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Network Adaptors

All functionalities are implemented in
Adaptors or network cards.
Each vender has it own adaptor.
Host I/o bus

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Bus
interface
Link
Interface
Network
Adaptor
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Network Adaptors (cont)
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Adaptor, like other devices, are programmed
by CPU.
Adaptor has a Control Status Register
(CSR), usaully located in the memory.
CPU communicate with Adaptor through
CSR.
Two methods for Communication, polling
and interrupt.
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Network Adaptors (cont)
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How to transfer data? Direct memory access
(DMA) and programmed I/O (PIO)
Device drivers are routines to connect OS
with the network hardware.
Memory is bottleneck. Each frame might be
written/read several times from the memory.

It has limited Bandwidth, usually 32 bit x 100
MHz, (around 3.2 Gbps), however, each packet
goes at least two time and there are overheads.
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