Chapter 1
Introduction
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1.2
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1.1 1.2
Overview of the Internet
Network Models & Protocol Layering
OSI
TCP/IP
Circuit & Packet switching
Chapter 1: Outline
1.3
1.4 Standards and Administration
Internet History
Chapter 1: Objective
1.3
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We introduce local area networks (LANs) and wide area networks (WANs) and show that an intranet or the Internet is a combination of these networks.
We introduce the concept of protocol layering to show how the task to be done by the Internet is divided into smaller tasks. We also discuss communication models OSI & TCP/IP protocol suite and show the duty of each layer.
Packet Switching & Circuit Switching
We give a brief history of the Internet.
We introduce the administration of the Internet and define the standards and their lifetime.
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1 OVERVIEW OF THE INTERNET
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• •
• •
First defining a network
Connect networks to create small internetworks.
Structure of the Internet and
Study the Internet in the coming lectures
1.1.1 Networks
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A network is the interconnection of a set of devices capable of communication.
A device can be a host such as a large computer, desktop, laptop, workstation, cellular phone, or security system.
A device in this definition can also be a connecting device such as a router, a switch, a modem that changes the form of data, and so on.
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Types of Communication Networks
Traditional
Traditional local area network (LAN) Traditional wide area network (WAN)
Higher-speed
High-speed local area network (LAN)
Metropolitan area network (MAN)
High-speed wide area network (WAN) Point-to-Point WANs
Switched WANs
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Characteristics of WANs
Covers large geographical areas
Circuits provided by a common carrier Consists of interconnected switching nodes
Traditional WANs provide modest capacity
64000 bps
Business subscribers using T-1 service – 1.544 Mbps
Higher-speed WANs use optical fiber and transmission technique known as asynchronous transfer mode (ATM)
10’s and 100’s of Gbps common
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Characteristics of LANs
Like WAN, LAN interconnects a variety of devices and provides a means for information exchange among them
Traditional LANs
Provide data rates of 1 to 20 Mbps
High-speed LANS
Provide data rates of 100 Mbps to 10 Gbps
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Differences between LANs and WANs
Scope of a LAN is smaller
LAN interconnects devices within a single
building or cluster of buildings
LAN usually owned by organization that owns the attached devices
For WANs, most of network assets are not owned by same organization
Internal data rate of LAN is much greater
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The Need for MANs
Traditional point-to-point and switched network techniques used in WANs are inadequate for growing needs of organizations
Need for high capacity and low costs over large area MAN provides:
Service to customers in metropolitan areas Required capacity
Lower cost and greater efficiency than equivalent service from telephone company
Figure 1.1: An Isolated LAN in the past and today
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Figure 1.2: A Point-to-Point WAN
Figure 1.3: A Switched WAN
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Figure 1.4: An internetwork made of two LANs and one WAN
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Figure 1.5: A heterogeneous network made of WANs and LANs
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1.1.2 Switching
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An internet is a switched network in which a switch connects at least two links together. A switch needs to forward data from a link to another link when required.
Circuit-Switched Network Packet-Switched Network
Figure 1.6: A circuit-switched network
Figure 1.7: A packet-switched network
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Switching Terms
Switching Nodes:
– Intermediate switching device that moves data – Not concerned with content of data
Stations:
– End devices that wish to communicate
– Each station is connected to a switching node
Communications Network:
– A collection of switching nodes
Communication Networks
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Techniques Used in Switched Networks
Circuit switching
– Dedicated communications path between stations
– e.g. public telephone network
Packet switching
– Message is broken into a series of packets
– Each node determines next leg of transmission for each packet
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Circuit Switching – connect phases
Circuit establishment
– An end to end circuit is established through switching nodes
– Dedicated path for duration of connection, even when no data is being transmitted ! ?
Information Transfer
– Information transmitted through the network
– Data may be analog voice, digitized voice, or binary data
Circuit disconnect
– Circuit is terminated (‘teardown’)
– Each node de-allocates dedicated resources
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Characteristics of Circuit Switching
Can be inefficient
– Channel capacity dedicated for duration of connection – Utilization not 100%
– Delay prior to signal transfer for establishment
Once established, network is transparent to users
Information transmitted at fixed data rate with only propagation delay
Communication Networks
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How Packet Switching Works
Data is transmitted in blocks, called packets
Before sending, the message is broken into a series of
packets
– Typical packet length is 1000 octets (bytes)
– Packets consists of a portion of data plus a packet header that includes control information
At each node en route, packet is received, stored briefly and passed to the next node
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Packet Switching
• No call setup or teardown
• Packets may take different paths
• Packets may arrive in different sequence or be lost in transit…
• New datagrams admitted even when network is congested!
Stallings – Fig 3.7 Packet switching: Datagram approach Communication Networks
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Packet Switching Advantages Line efficiency is greater
– Many packets over time can dynamically share the same node to node link
Packet-switching networks can carry out data-rate conversion
– Two stations with different data rates can exchange information
Unlike circuit-switching networks that block calls when traffic is heavy, packet- switching still accepts packets, but with increased delivery delay
Priorities can be used
Communication Networks
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Disadvantages of Packet Switching Each packet switching node introduces a
delay
Overall packet delay can vary substantially
– This is referred to as jitter
– Caused by differing packet sizes, routes taken and varying delay in the switches
Each packet requires overhead information
– Includes destination and sequencing information
– Reduces communication capacity
More processing required at each node
Communication Networks
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Packet Switching Networks – Datagram
Each packet treated independently, without reference to previous packets
Each node chooses next node on packet’s path
Packets don’t necessarily follow same route and may arrive out of sequence
Exit node usually restores packets to original order
Responsibility of exit node or destination to detect loss of packet and take recover action (if any!)
Communication Networks
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Packet Switching Networks – Datagram
Advantages:
– Call setup phase is avoided
– Because it’s more primitive, it’s more flexible
– Datagram delivery is more
reliable
Communication Networks
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Packet using Circuit switching
Preplanned route established before packets sent – All packets between source and destination
follow the same established route
Routing decision not required by nodes for each packet
Emulates a circuit in a circuit switching network but is not a dedicated path
– Packets still buffered at each node and queued for output over a line
– Only buffered, no routing..
Communication Networks
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Packet using Circuit switching
Advantages:
– Packets arrive in original order – Packets arrive correctly
– Packets transmitted more rapidly without routing decisions made at each node
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Communication Networks 2/28/2021 29
Figure 1.8: The Internet today
The most notable internet is called the Internet and is composed of thousands of inter-connected networks. Figure1.8 shows a conceptual (not geographical) view of the Internet.
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1.1.4 Accessing the Internet
Todays Internet is an internetwork that allows any user to become part of it.
The user, however, needs to be physically connected to an ISP.
The physical connection is normally done through a point-to-point WAN.
In this section, we briefly describe how this can happen, but we postpone the technical details of the connection until Chapters 6 and 7.
Using Telephone Networks Dial-up Service
DSL
Using Cable Networks Using Wireless Networks Direct Connection
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1.1.5 Hardware and Software
• We have given the overview of the Internet structure.
• For communication to happen, we need both hardware and software.
• This is similar to a complex computation in which we need both a computer and a program.
• In the next section, we show how these combinations of hardware and software are coordinated with each other using protocol layering.
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2 PROTOCOL LAYERING
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A word we hear all the time when we talk about the Internet is protocol.
• A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively.
• When communication is simple, we may need only one simple protocol;
• when the communication is complex, we need a protocol at each layer, or protocol layering.
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1.2.1 Scenarios
Let us develop two simple scenarios to better
understand the need for protocol layering.
First Scenario (Figure 1.9) Second Scenario (Figure 1.10) Principle of Protocol Layering
Logical Connections
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Figure 1.9: A single-layer protocol
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Figure 1.10: A three-layer protocol
Postal carrier facility
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Figure 1.11: Logical connection between peer layers
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1.2.2 TCP/IP Protocol Suite
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• TCP/IP is a protocol suite used in the Internet today
• It is a hierarchical protocol made up of interactive
modules.
• Each of which provides a specific functionality.
• The term hierarchical means that each upper level protocol is supported by the services provided by one or more lower level protocols.
• The original TCP/IP protocol suite was defined as four software layers built upon the hardware.
• Today, however, TCP/IP is thought of as a five-layer model.
1.2.2 TCP/IP Protocol Suite (continued)
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Layered Architecture
Layered in the Suite
Description of Each Layer
Application Layer Transport Layer Network Layer
Data-link Layer
Physical Layer
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1.2.2 TCP/IP Protocol Suite (continued)
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Encapsulation and Decapsulation
Encapsulation at the Source Host
Decapsulation and Encapsulation at Router
Decapsulation at the Destination Host Addressing
Multiplexing and Demultiplexing
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Figure 1.12: Layers in the TCP/IP protocol suite
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Figure 1.13: Communication through an internet
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Figure 1.14: Logical connections between layers in TCP/IP
Logical connections
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Figure 1.15: Identical objects in the TCP/IP protocol suite
Identical objects (messages) Identical objects (segment or user datagram)
Identical objects (datagram) Identical objects (frame) Identical objects (frame)
Identical objects (datagram)
Identical objects (bits)
Identical objects (bits)
Figure 1.16: Encapsulation / Decapsulation
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Figure 1.17: Addressing in the TCP/IP protocol suite
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Figure 1.18: Multiplexing and demultiplexing
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1.2.2 The OSI Model
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• OSI model established in 1947
• ISO is a multinational body dedicated to worldwide
agreement on international standards.
• An ISO standard that covers all aspects of network
communications is the Open Systems Interconnection
(OSI) model.
• It was first introduced in the late 1970s.
OSI versus TCP/IP
Lack of OSI Model’s Success
Figure 1.19: The OSI model
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Layer 1: Physical Layer
Modulation, electrical, optical, radio-frequency and physical definition of the host to network interface.
Modulation converts the digital data representation into a specific signal format imposed on the electrical, optical or radio-frequency channel.
Contention between nodes for the transmission medium is usually handled in the physical layer.
The definition of cables, connectors, and other physical components is part of the physical layer.
The physical later determines most of the characteristics of point-to-point links used in a network.
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Physical Layer – Examples
10-Base-5(Thick-Ethernet)OR10-Base-2Thin-Ethernet–usingBNCs.
10-Base-T Ethernet – Twisted pair using RJ connectors.
100-Base-T Ethernet – Twisted pair using RJ connectors.
1000-Base-T or 1-Gigabit Ethernet – Twisted pair using RJ connectors.
10-Gigibit Ethernet – Twisted pair using RJ connectors.
RS-232C – EIA +/- 12V signalling using DB-9, DB-25.
USBbus–TTLsignallingusingUSBconnectors.
IEEE1394Firewire–usingFirewireconnectors.
FDDI-Opticalsignalling
ADSL – electrical over telephone cables
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Layer 2: Data Link Layer
Provides a functional / procedural way of transferring data between network nodes and detecting (and often correcting errors Physical layer transmission errors).
An addressing scheme is used, which is typically hardcoded or preprogrammed in the network interface.
In Ethernet (802.3) networks, the DLL is further subdivided into the Medium Access Control (MAC) layer and the IEEE 802.2 Logical Link Control (LLC) layer.
Transmission protocols designed for DLL are typically strongly optimised for the specific physical layer they operate with.
The DLL thus includes packet structures, and protocols for exchanging packets.
Switches are layer 2 devices.
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Layer 3: Network Layer
The Network layer will provide:
A mechanism for transporting data payloads across one
or more datalink layer networks.
A mechanism for Quality of Service as required by upper layers.
Fragmentation and defragmentation mechanisms to accommodate different Layer 2 network packet sizes.
Route discovery and route maintenance mechanisms for the network.
Error messaging or delivery management.
Routers are layer 3 devices.
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Layer 4: Transport Layer
The Transport layer will provide:
A mechanism for the transparent transfer of data
between end users.
A mechanism for reliable/unreliable data transfer.
In reliable protocols, error control (retransmission management) and flow control mechanisms.
In reliable protocols, management of state information for virtual circuits across the network.
Examples of Layer 4 protocols are
the Transmission Control Protocol (TCP) which is reliable, and the User Datagram Protocol (UDP), which is unreliable.
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Layer 5: Session Layer
The Session layer will provide:
Control of dialogues or sessions between network hosts.
Establishment, management and termination of connections between local and remote applications.
Possibly duplex or half-duplex operation.
Establishment of checkpointing, adjournment,
termination, and restart procedures.
Checkpointing and recovery of sessions is often not implemented.
In the TCP/IP suite, session close functions are performed in TCP.
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Layer 6: Presentation Layer
The Presentation layer will provide:
Transformation of data formats intended to provide a standard or
common interface for the Application layer.
This is necessary to ensure that applications can handle data which may otherwise be in incompatible formats.
Examples include the:
NFS (Network File System)
XDR ( eXternal Data Representation) protocol which handles byte ordering and word sizes,
emailMIMEencoding,
data compression, Data encoding / decoding
data encryption techniques, IBM EBCDIC to ASCII conversion and vice versa.
Presentation layer functions are often embedded in applications, such as browsers or mailers.
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Layer 7: Application Layer
The Application layer will provide:
A mechanism for accessing information on the network via an
application.
The primary interface between user applications.
Examples include Telnet for remote logins, File Transfer Protocol (FTP) for file transfers, Simple Mail Transfer Protocol (SMTP) for email and Hypertext Transfer Protocol (HTTP) for web browsing.
It is important to distinguish the protocol from the application.
We might say ‘use telnet to log in remotely’ but this really means ‘use an application program which can communicate using telnet protocol to log in remotely’.
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Figure 1.20: TCP/IP and OSI model
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3 INTERNET HISTORY
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• Now that we have given an overview of the Internet and its protocol.
• Let us give a brief history of the Internet.
• This brief history makes it clear how the Internet has evolved from a private network to a global one in less than forty years.
1.3.2 Early History
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There were some communication networks, such as telegraph and telephone networks, before 1960.
These networks were suitable for constant-rate communication at that time, which means that after a connection was made between two users, the encoded message (telegraphy) or voice (telephony) could be exchanged.
Birth of Packet-Switched Networks ARPANET
1.3.3 Birth of the Internet
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• In 1972, Vint Cerf and Bob Kahn, both of whom were part of the core ARPANET group, collaborated on what they called the Inter-netting Project.
• They wanted to link dissimilar networks so that a host on one network could communicate with a host on another.
• There were many problems to overcome: diverse packet sizes,
diverse interfaces,
diverse transmission rates, as well as differing reliability requirements.
1.3.3 Birth of the Internet (continued)
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TCP/IP – Transmission Control Protocol/Internet Protocol MILNET – Military Network or Military Net
CSNET – CSNET=Computer Science Network (CSNET)
NSFNET – NSFNET=National Science Foundation Network
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1.3.3 Internet Today
• Today, we witness a rapid growth both in the infrastructure and new applications.
• •
The Internet today is a set of pier networks that provide services to the whole world.
What has made the Internet so popular is the invention of new applications.
World Wide Web Multimedia
Peer-to-Peer Applications
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4 STANDARDS AND ADMINISTRATION
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•
In the discussion of the Internet and its protocol, we often see a reference to a standard or an administration entity.
In this section, we introduce these standards and administration entities to familiarize
•
1.4.1 Internet Standards
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• An Internet standard is a thoroughly tested specification.
• That is useful to and adhered to by those who work with the Internet.
• It is a formalized regulation that must be followed.
• There is a strict procedure by which a specification
attains Internet standard status.
• A specification that begins as a Internet draft.
1.4.1 Internet Standards (Continued)
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Maturity Levels
Proposed Standard
Draft Standard Internet Standard Historic
Experimental
Informational
Requirement Levels Required
Recommended
Elective
Limited Use
Not Recommended
Figure 1.21: Maturity levels of an RFC
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1.4.2 Internet Administration
• The Internet, with its roots primarily in the research domain, has evolved and gained a broader user base with significant commercial activity.
•
Various groups that coordinate Internet issues have guided this growth and development.
1.4.2 Internet Administration (continued
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ISOC
IAB
IETF
IANA and ICANN
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)
Chapter 1: Summary
A network is a set of devices connected by communication links.
The Internet today is made up of many WANs and LANs networks
joined by connecting devices and switching stations.
Most end users who want Internet connection use the
services of Internet service providers (ISPs).
There are backbone ISPs, regional ISPs, and local ISPs.
A protocol is a set of rules that governs communication. In protocol layering, we need to follow two principles to provide bidirectional communication.
OSI 7 – layer communication model
TCP/IP is a hierarchical protocol suite made of five layers:
application, transport, network, data-link, and physical.
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Chapter 1: Summary (continued)
The Internet history of internetworking started
with ARPA in the mid-1960s.
The birth of the Internet can be associated with the work of Cerf and Kahn and the invention of a gateway to connect networks.
The Internet administration has evolved with the Internet.
We discussed ISOC, IAB, IETF, IRTF, ICANN, and NIC.
An Internet standard is a thoroughly tested specification.
An Internet draft is a working document with no official status
and a six-month lifetime.
A draft may be published as a Request for Comment (RFC). RFCs go through maturity levels and are categorized according to their requirement level.
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References
Computer Networks A Top-Down Approach by Behrouz A. Forouzan & Firouz Mosharraf by Mc Graw Hill 1st edition 2012
Open Systems Interconnection Reference Model (OSI) – http://en.wikipedia.org/wiki/OSI_model
Kopp C., An introduction to cable modems, Cable Modem Technology, Asia/Pacific Open Systems Review, Sydney Australia, August 1996, 60-63, 4pp.
KoppC.,Digitalsubscriberlinetechnologies-Thetechnicalissues, Asia/Pacific Open Systems Review, Sydney Australia, September 1996, 50-51, 2pp.
Kopp C., Networking – A perspective, PPP Protocol, Asia/Pacific Open Systems Review, Sydney Australia, November 1996, 24-28, 5pp.
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