Introduction to Computer Networks

1

• Introduction to Computer Networks

2
The Purpose of Networking

• Goals Remote data exchange and remote process
  control.
• A few desirable properties
• Interoperability,
• Flexibility,
• Geographical range,
• Scalability,
• Privacy and security.

3
Range of Coverage
We can classify computer networks according
to their geographical coverage
LAN local area network WLAN wireless local
area network MAN metropolitan area
network WAN wide area network (long haul
network)
Most commonly, were interested in the seamless
integration of all these levels (as in the
Internet).
Note Different levels use very different
technologies.
4
Matters of Protocol
Everything in networking happens through
protocols

• A protocol determines how hosts share and access
  the medium,
• A protocol determines how hosts deal with the
  media bandwidth, errors, flow control, etc,
• A protocol determines how connections between
  hosts are established and maintained,
• A protocol determines how information is routed
  across short and long distances.
• Etc, etc, etc

Question Ok, but what is protocol?
5
The ISO/OSI Reference Model Source Computer
Networks, Andrew Tanenbaum
ISO International Standards Organization OSI
Open Systems Interconnection
Application
The protocol stack
Presentation
Session
The idea behind the model Break up the design to
make implementation simpler. Each layer has a
well-defined function. Layers pass to one
another only the information that is relevant at
each level. Communication happens only
between adjacent layers.
Transport
Network
Data link
Physical
6
The Layers in the ISO/OSI RF Model
Physical Transmit raw bits over the
medium. Data Link Implements the abstraction of
an error free medium (handle losses, duplication,
errors, flow control). Network
Routing. Transport Break up data into chunks,
send them down the protocol stack, receive
chunks, put them in the right order, pass them
up. Session Establish connections between
different users and different hosts. Presentation
Handle syntax and semantics of the info
examples encoding, encrypting. Application
Protocols commonly needed by applications (cddb,
http, ftp, telnet, etc).
7
Communication Between Layers within the Same Host
Its important to specify the services offered to
higher layers in the hierarchy. What they are
how to use them interface API.
Layer n1

SAP
SAP
Layer n
SAPs (service access points) Note This is ISO
terminology.

SAP
SAP
Layer n-1
8
Communication Between Layers across Different
Hosts
receiver
sender
data
data
Application
Application
AH
data
PH
Presentation
Presentation
data
Session
Session
SH
data
TH
Transport
Transport
data
Network
Network
NH
data
Data link
Data link
DH DT
data
BITS
Physical
Physical
9
The Layers in the TCP/IP Protocol Suite Source
The TCP/IP Protocol Suite, Behrouz A. Forouzan
Application
FTP
HTTP
DNS
NFS

Presentation
Session
Transport
TCP
UDP
IP
ICMP
IGMP
Network
ARP
RARP
Data link
Physical
10
Design Alternatives
Point-to-point channels physical links (as in
wiring) connect every two communicating parties
with a private channel.
Broadcast channels communicating parties are
connected by a shared medium hosts can hear
transmissions not necessarily addressed to them.
11
Network Topology
We can classify computer networks according
to their topology
bus
star
mesh
hypercube
ring
12
Technology
Network Interface Card (NIC) I/O device in the
computer system that allows it to join a network.
The NIC works with a specific medium
(twisted-pair, coaxial cable, optic fiber, etc).
As long as its bus allows, a host can have
multiple NICs.
Host
NIC

NIC
To the host, the NIC is just another I/O device,
which has its own address. A protocol determines
how the NIC accesses the medium.
13
Ethernet
Ethernet has a bus topology.
Bus Arbitration by Collision Detection Carrier
Sense Multiple Access with Collision Detection
(CSMA/CD)
Host A listens and finds the bus idle.
Host A starts TX.
Host B starts TX.
Host A detects collision.
Host B completes TX.
Host A backs off.
COLLISION!
Host B backs off.
time
Host B listens and finds the bus idle.
Host B detects collision.
Host B listens and finds the bus idle.
Host B starts TX.
14
COTS Ethernet
Ethernet has a bus topology
The medium could be anything that allows for a
bus implementation. (some options are easier to
work with than others)
Host
NIC
Host
NIC

Hub An out-of-the box bus
Host
NIC
15
Switched LANs
The bus bandwidth is limited, so go for a
switched architecture.
Host
NIC
Host
NIC

Switch allows for more than one pair to talk at
the same time.
Host
NIC
16
Token Ring
Hosts connect to a switch which closes the ring.
Medium access is done by passing around a token.
A
C
B
D
F
E
17
Network Architecture for Performance
The keyword is hierarchy
Subnet 1
hub
Subnet 2
hub

Switch
Subnet N
hub
18
Network Architecture for Performance and Coverage
Again, the keyword is hierarchy
Subnet 1
Subnet 1

hub
hub
Switch
Switch
Subnet 2
Subnet 2
hub
hub


Subnet N
Subnet N
Router
hub
hub
Connection to other networks
19
Security Issues

• Desirable properties

• Availability
• Accessibility
• Non-repudiation
• Flexibility

• Confidentiality
• Authenticity
• Integrity
• Freshness
• Scalability

20
Wireless Networks
21
Reasons to Go Wireless
1) 2) 3)
Challenges in Going Wireless
1) 2) 3)
22
Medium The Radio Spectrum
Wireless communications use the electromagnetic
spectrum, which is regulated by government
institutions such as the Federal Communications
Commission (FCC). Regulations specify what
bands of frequency can be used for different
applications. For instance FM radio has
88-108MHz (200KHz) and AM radio has 540-1600KHz
(10KHz bandwidth). Regulations also specify the
transmission power that can be used in each band.
There are portions of the spectrum that are
UNLICENSED, however. The most popular wireless
networks of today operate in unlicensed bands.
23
Omnidirectional Antenna
Radiates in all azimuth directions.
24
Directional Antenna
Radiates in a cone.
25
Design Alternatives
Point-to-point channels Information flows in
beams that connect communicating parties.
signal received
signal not received
The antennas on the transmitter and receiver need
to be properly aligned for signals to go through.
On the flip-side, directional antennas have great
power efficiency and range.
Directional antennas are a good choice for
systems with fixed infrastructure. They introduce
additional difficulties in infrastructure-less
systems or when transmitters and receivers can
move around, but offer reduced power consumption.
26
Design Alternatives
Broadcast channels Information radiates in all
possible directions from the transmitter.
signal received
Theres no need to align antennas on the
transmitter and receiver. If signals radiate in
all directions, a receiver will the transmitters
independently of their relative positions.
Because power radiates all around,
omnidirectional antennas cant reach as far as
directional antennas. Note also that quite a bit
of power can be wasted.
signal received
This is a good choice for mobile systems. At the
expense of increased power consumption, coverage
reaches 360o.
27
Concepts in Radio Communications
Coverage f(PowerTX)
D
A
E
B

• Interference

C
Multipath
Noise
28
Multiple Access to the Radio Spectrum
Spatial Division Multiple Access (SDMA) each
pair of nodes communicates through a tight beam
that takes a portion of space.
Frequency Division Multiple Access (FDMA) each
pair of nodes uses a distinct subrange of the
total frequency band for the application.
T0
T1
T2
T3
T4
T5
T6
T7
T8
A
B
frequency spectrum
29
Multiple Access to the Radio Spectrum
Time Division Multiple Access (TDMA) each pair
of nodes uses a different time slot to
communicate. During its time, the pair can use
the entire frequency band allocated for the
application.
T0
T2
T3
T4
T4
T0
T0
T1
T1
T1
T2
T3
time
one time unit
one time unit
one time unit
Code Division Multiple Access (CDMA) can be seen
as a combination of FDMA and TDMA.
Frequency Hopping Spread Spectrum (FHSS)
transmitters use each frequency band for a random
time then move to another randomly chosen TX
and RX must agree to a hopping sequence.
T0
time
T1
time
T2
time
30
Remember Ethernet?When Problems Get Worse

• Assume that all wireless devices use the same
  channel. Arbitration of access to the medium
  (Medium Access Control, or MAC, a protocol in the
  Data Link layer) is similar to Ethernets
  CSMA/CD.
• Most radios in wireless networking cant transmit
  and receive at the same time, so we cant detect
  collisions. Instead, well do CSMA/CA (collision
  avoidance).
• Collisions are bad because they reduce the
  effective bandwidth and also because they cause
  waste of power.
• Even when two transmissions do not collide, they
  may still interfere with each other causing bit
  error rates to rise.

31
Types of Wireless Networks
Fixed Infrastructure
wired backbone
BS
BS
BS
Ad Hoc

• Easy to deploy.
• Good in changing environments.
• Allows for node mobility.
• Can be designed for self-configurability.
• Can be designed for scalability.

32
Data Link Layer Medium Access Control
(Coordinated access to a shared resource)

• Power is a scarce resource so is the RF
  spectrum.
• Collisions lead to wasted power and wasted
  spectrum.
• Need to impose some kind of access discipline so
  as to avoid collisions.

33
The MAC Layer Challenge

• Maximize throughput
• Minimize collisions.
• Avoid exposed nodes.
• An interesting option schedule medium access.
• Related challenges
• Clock synchronization.
• Distributed coordination for determining
  schedule.

34
IEEE 802.11 DCF (CSMA/CA)
start
NAV starts with the Duration field value of the
last transmission sensed on the medium and counts
down to zero.
NAV0
NO
YES
Sense Medium
Medium Idle
Random Backoff Time
NO
YES
Transmit Frame
Collision?
YES
NO
35
The Hidden Node Problem

• Station C can sense stations A and B.
• Stations A and C cant sense each other.
• Problem coordinate transmissions from A and C so
  as to avoid collisions.

A
B
C
36
The Hidden Node Problem

• Station C can sense stations A and B.
• Stations A and C cant sense each other.
• Problem coordinate transmissions from A and C so
  as to avoid collisions.

A
B
C

• Solution RTS/CTS/DATA/ACK handshake A sends
  RTS
• to B, B sends CTS to A, C hears CTS and stays
  quiet, A sends DATA
• to B, B replies to A with an ACK.

37
The Exposed Node Problem
A
B
C
D
An exposed node is one that is in range of the
transmitter, but outside range of the receiver.
Problem exposed nodes reduce bandwidth.
38
The Network Layer Challenge

• How do we build routes dynamically?
• Pro-active algorithms.
• Reactive algorithms.
• Will the routing protocol scale up to LARGE
  networks?
• Can routing adapt to changes in network traffic,
  propagation conditions, etc.?
• Packet forwarding costs power. Can we do routing
  in a way that balances power consumption?

39
Security Issues

• Desirable properties

• Availability
• Accessibility
• Self-organization
• Non-repudiation
• Flexibility

• Confidentiality
• Authenticity
• Integrity
• Freshness
• Scalability

• As of today, the network can be vulnerable at
  multiple levels
• PHY radio jamming, physical actions to the node.
• MAC DoS via fake requests or schedules.
• NET fake route advertisements (black hole
  attack).