Chapter 1-Computer Networks-Basics chap.pdf

Introduction
Chapter 1
• Uses of Computer Networks
✓ Network Hardware
✓ Network Software
✓ Reference Models
• Example Networks
• Network Standardization
• Metric Units
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Revised: August 2011 and February 2018

Uses of Computer Networks
Computer networks are collections of autonomous
computers interconnected by a single technology.
e.g., the Internet
They have many uses:
• Business Applications »
• Home Applications »
• Mobile Users »
These uses raise:
• Social Issues »
This text covers networks for all of these uses
1.1

request
response
Business Applications
Companies use networks and computers for resource
sharing with the client-server model:
Other popular uses are communication, e.g., email, VoIP,
and e-commerce

Home Applications
Homes contain many networked devices, e.g., computers,
TVs, connected to the Internet by cable, DSL, wireless, etc.
Home users communicate, e.g., social networks, consume
content, e.g., video, and transact, e.g., auctions
Some application use the peer-to-peer model in which
there are no fixed clients and servers:

Mobile Users
Tablets, laptops, and smart phones are popular devices;
WiFi hotspots and 3G cellular provide wireless connectivity.
Mobile users communicate, e.g., voice and texts, consume
content, e.g., video and Web, and use sensors, e.g., GPS.
Wireless and mobile are related but different:

Social Issues
• Network neutrality – no network restrictions
• Content ownership, e.g., DMCA takedowns
• Anonymity and censorship
• Privacy, e.g., Web tracking and profiling
• Theft, e.g., botnets and phishing

Network Hardware
Networks can be classified by their scale:
Scale Type
Vicinity PAN (Personal Area Network) »
Building LAN (Local Area Network) »
City MAN (Metropolitan Area Network) »
Country WAN (Wide Area Network) »
Planet The Internet (network of all networks)
1.2

Personal Area Network
Connect devices over the range of a person
Example of a Bluetooth (wireless) PAN:

Local Area Networks
Connect devices in a home or office building
Called enterprise network in a company
Wireless LAN
with 802.11
Wired LAN with
switched Ethernet

Metropolitan Area Networks
Connect devices over a metropolitan area
Example MAN based on cable TV:

Connect devices over a country
Example WAN connecting three branch offices:
Wide Area Networks (1)

An ISP (Internet Service Provider) network is also a WAN.
Customers buy connectivity from the ISP to use it.

A VPN (Virtual Private Network) is a WAN built from virtual
links that run on top of the Internet.

Network Software
• Protocol layers »
• Design issues for the layers »
• Connection-oriented vs. connectionless service »
• Service primitives »
• Relationship of services to protocols »
1.3

Protocol Layers (1)
Protocol layering is the main structuring method used to
divide up network functionality.
•Each protocol instance
talks virtually to its peer
•Each layer communicates
only by using the one below
•Lower layer services are
accessed by an interface
•At bottom, messages are
carried by the medium

Protocol Layers (2)
Example: the philosopher-translator-secretary architecture
Each protocol at different layers serves a different purpose

Protocol Layers (3)
Each lower layer adds its own header (with control inform-
ation) to the message to transmit and removes it on receive
Layers may also split and join messages, etc.

Design Issues for the Layers
Each layer solves a particular problem but must include
mechanisms to address a set of recurring design issues
Issue Example mechanisms at different layers
Reliability despite
failures
Codes for error detection/correction (§3.2, 3.3)
Routing around failures (§5.2)
Network growth
and evolution
Addressing (§5.6) and naming (§7.1)
Protocol layering (§1.3)
Allocation of resources
like bandwidth
Multiple access (§4.2)
Congestion control (§5.3, 6.3)
Security against
various threats
Confidentiality of messages (§8.2, 8.6)
Authentication of communicating parties (§8.7)

Connection-Oriented vs. Connectionless
Service provided by a layer may be kinds of either:
• Connection-oriented, must be set up for ongoing use
(and torn down after use), e.g., phone call
• Connectionless, messages are handled separately,
e.g., postal delivery

Service Primitives (1)
A service is provided to the layer above as primitives
Hypothetical example of service primitives that may provide
a reliable byte stream (connection-oriented) service:

Service Primitives (2)
Hypothetical example of how these primitives may be used
for a client-server interaction
Client Server
LISTEN (0)
RECEIVE
ACCEPT (2)
SEND (4)
CONNECT (1)
RECEIVE
SEND (3)
DISCONNECT (5)
Connect request
Accept response
Request for data
Reply
Disconnect
DISCONNECT (6)
Disconnect

Relationship of Services to Protocols
Recap:
• A layer provides a service to the one above [vertical]
• A layer talks to its peer using a protocol [horizontal]

Reference Models
Reference models describe the layers in a network
architecture
• OSI reference model »
• TCP/IP reference model »
• Model used for this text »
• Critique of OSI and TCP/IP »
1.4

OSI Reference Model
A principled, international standard, seven layer model to
connect different systems
– Provides functions needed by users
– Converts different representations
– Manages task dialogs
– Provides end-to-end delivery
– Sends packets over multiple links
– Sends frames of information
– Sends bits as signals

OSI Reference Model(Contd..)
The model is called the ISO OSI (Open Systems Interconnection) Reference
Model because it deals with connecting open systems—that is, systems that are
open for communication with other systems.
The principles that were applied to arrive at the seven layers can be briefly
summarized as follows:
1. A layer should be created where a different abstraction is needed.
2. Each layer should perform a well-defined function.
3. The function of each layer should be chosen with an eye toward defining
internationally standardized protocols.
4.The layer boundaries should be chosen to minimize the information flow
across the interfaces.
5. The number of layers should be large enough that distinct functions need not
be thrown together in the same layer out of necessity and small enough that the
architecture does not become unwieldy.

The Physical Layer
• Transmitting raw bits over a communication channel.
• What electrical signals should be used to represent a 1 and a 0
• How many nanoseconds a bit lasts whether transmission may
proceed simultaneously in both directions.
• How many pins the network connector has, and what each pin is
used for.
• Design issues largely deal with mechanical, electrical,timing
interfaces and physical transmission medium(which lies below
physical layer)

Data Link Layer
• Data link layer task is to transform a raw transmission facility into a
line that appears free of undetected transmission errors.
• It does so by masking the real errors.
• It break up the input data into data frames (typically a few hundred
or a few thousand bytes) and transmit the frames sequentially.
• If the service is reliable, the receiver confirms correct receipt of each
frame by sending back an acknowledgement frame.
• How to keep a fast transmitter from drowning a slow receiver in
data.(flow control)
• Control access to the shared channel

Network Layer
• Network layer controls the operation of the subnet
• Rouring-Determining how packets are routed from source to destination
• Quality of service provided (delay,transit time, jitter, etc.)
• Addressing
• Fragmentation and reassembly

Transport Layer
• Provides end-to-end delivery
• The transport layer is a true end-to-end layer; it carries data all the way
from the source to the destination. In other words, a program on the
source machine carries on a conversation with a similar program on
the destination machine, using the message headers and control
messages.
Session Layer
• Session layer allows users on different machines to establish sessions
between them.
• Sessions offer various services, including
➢ Dialog control (keeping track of whose turn it is to transmit).
➢ Token management (preventing two parties from attempting the
same critical operation simultaneously).
➢ Synchronization (pick up from where they left
off in the event of a crash and subsequent recovery).

Presentation Layer
• Presentation layer is concerned with the syntax and semantics of
the information transmitted
• Data structures to be exchanged can be defined in an abstract way,
along with a standard encoding to be used
• Presentation layer manages these abstract data structures and
allows higher-level data structures (e.g., banking records) to be
defined and exchanged.
Application Layer
• Application layer contains a variety of protocols that are commonly
needed by users. Ex: HTTP,file transfer, Email, Remote login,
Network news

TCP/IP Reference Model
A four layer model derived from experimentation; omits
some OSI layers and uses the IP as the network layer.
IP is the
“narrow waist”
of the Internet
Protocols are shown in their respective layers

Model Used in this Book
It is based on the TCP/IP model but we call out the
physical layer and look beyond Internet protocols.

Critique of OSI & TCP/IP
OSI:
+ Very influential model with clear concepts
– Models, protocols and adoption all bogged down by politics
and complexity
TCP/IP:
+ Very successful protocols that worked well and thrived
– Weak model derived after the fact from protocols
1.4.5
1.4.6

Example Networks
• The Internet »
• 3G mobile phone networks »
• Wireless LANs »
• RFID and sensor networks »
1.5

Internet (1)
Before the Internet was the ARPANET, a decentralized,
packet-switched network based on Baran’s ideas.
Nodes are IMPs,
or early routers,
linked to hosts
56 kbps links
ARPANET topology in Sept 1972.

Internet (2)
The early Internet used NSFNET (1985-1995) as its
backbone; universities connected to get on the Internet
NSFNET topology in 1988
T1 links
(1.5 Mbps)

Internet (3)
The modern Internet is more complex:
• ISP networks serve as the Internet backbone
• ISPs connect or peer to exchange traffic at IXPs
• Within each network routers switch packets
• Between networks, traffic exchange is set by
business agreements
• Customers connect at the edge by many means
– Cable, DSL, Fiber-to-the-Home, 3G/4G wireless, dialup
• Data centers concentrate many servers (“the cloud”)
• Most traffic is content from data centers (esp. video)
• The architecture continues to evolve

Internet (4)
Architecture of the Internet

3G Mobile Phone Networks (1)
3G network is based on spatial cells; each cell provides
wireless service to mobiles within it via a base station
1.5.2

Base stations connect to the core network to find other
mobiles and send data to the phone network and Internet

As mobiles move, base stations hand them off from one
cell to the next, and the network tracks their location
Handover

Wireless LANs (1)
In 802.11, clients communicate via an AP (Access
Point) that is wired to the rest of the network.
1.5.3

Wireless LANs (2)
Signals in the 2.4GHz ISM band vary in strength due to
many effects, such as multipath fading due to reflections
– requires complex transmission schemes, e.g., OFDM

Wireless LANs (3)
Radio broadcasts interfere with each other, and radio
ranges may incompletely overlap
– CSMA (Carrier Sense Multiple Access) designs are used

RFID and Sensor Networks (1)
Passive UHF RFID networks everyday objects:
– Tags (stickers with not even a battery) are placed on objects
– Readers send signals that the tags reflect to communicate
1.5.4

RFID and Sensor Networks (2)
Sensor networks spread small devices over an area:
– Devices send sensed data to collector via wireless hops

Network Standardization
Standards define what is needed for interoperability
Some of the many standards bodies:
Body Area Examples
ITU Telecommunications G.992, ADSL
H.264, MPEG4
IEEE Communications 802.3, Ethernet
802.11, WiFi
IETF Internet RFC 2616, HTTP/1.1
RFC 1034/1035, DNS
W3C Web HTML5 standard
CSS standard
1.6

Metric Units
The main prefixes we use:
• Use powers of 10 for rates, powers of 2 for storage
– E.g., 1 Mbps = 1,000,000 bps, 1 KB = 1024 bytes
• “B” is for bytes, “b” is for bits
Prefix Exp. prefix exp.
K(ilo) 103 m(illi) 10-3
M(ega) 106 μ(micro) 10-6
G(iga) 109 n(ano) 10-9

End
Chapter 1

Chapter 1-Computer Networks-Basics chap.pdf

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