Introduction to Computer Network 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization

1. Computer Networks
Chapter 1
Introduction

2. Chap. 1 - Introduction
Prepared by Rutwik
Chapter Overview
1.1 Introduction
1.2 Network Hardware
1.3 Network Software
1.4 Reference Models
1.5 Example Networks
1.6 Example Data Communication Services
1.7 Network Standardization

3. Chap. 1 - Introduction
Prepared by Rutwik
What You Will Learn
1. Terminology
2. Communication basics
 Media and signals
 Binary numbering system
 Data transmission characteristics
asynchronous and synchronous communication
serial and parallel transmission
bandwidth, throughput and noise
multiplexing

4. Chap. 1 - Introduction
Prepared by Rutwik
What You Will Learn
3. Networking and Network Technologies
 Packet Switching, Circuit Switching
 Protocols and Layering
 Network Addressing
 Interconnection (bridges, switches, routers)
 Topologies (star, ring, bus, mesh)
 Routing
 Flow, Error and Congestion Control

5. Chap. 1 - Introduction
Prepared by Rutwik
Introduction
The Big Picture: Where are
We Now?
Some of the overall issues we’ll be dealing
with in this course.
1.1 Introduction
1.2 Network Hardware
1.3 Network Software
1.4 Reference Models
1.5 Example Networks
1.6 Example Data
Communication Services
1.7 Network Standardization

6. Chap. 1 - Introduction
Prepared by Rutwik
• Information Access
• Sharing of Resources
• Facilitate Communications
Introduction
Motivation for
Networks
What A Network
Includes
• Transmission hardware
• Special-purpose hardware devices
– interconnect transmission media
– control transmission
– run protocol software
• Protocol software
– encodes and formats data
– detects and corrects problems

7. Chap. 1 - Introduction
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• Provides communication that is
– Reliable
– Fair
– Efficient
– From one application to another
• Automatically detects and corrects
– Data corruption
– Data loss
– Duplication
– Out-of-order delivery
• Automatically finds optimal path from source to destination
Introduction
What A Network
Does

8. Chap. 1 - Introduction
Prepared by Rutwik
Where are we going: This chapter is basically an overview of the course. Before we
get mired down in details, we want to get a general idea of what this whole course is
about. Here are the kind of issues we’ll be dealing with.
Broadcast Networks -
Single channel with multiple machines connected to it.
Point-to-point -
Individual connections between pairs of machines.
Mostly EE issues.
Packets -
Messages - the "chunk" of data transmitted from one machine to the next.
Addressing -
+ One to one: Packet contains specific target address.
+ Broadcasting: All machines on the network receive and
process the packet.
+ Multicasting: A subset of machines receive and process the packet.
Introduction Lingo

9. Chap. 1 - Introduction
Prepared by Rutwik
Introduction
Can also view messaging in terms of the distance of transmission:
0.1 meter Single processor
1 meter Multiprocessor
10 meter Room
100 meter Building -- Local area network
1 kilometer Campus
10 kilometer City -- Metropolitan area network
100 kilometer Country
1K kilometer Continent -- Wide area network
10K kilometer Planet -- Internet
Serial:
Parallel:
Lingo

10. Chap. 1 - Introduction
Prepared by Rutwik
Introduction
Circuit Switching
• Dedicated
– fixed bandwidth
– route fixed at setup
– idle capacity wasted
– network state
Lingo
Packet Switching
• Best Effort
– end-to-end control
– multiplexing technique
– re-route capability
– congestion problems

11. Chap. 1 - Introduction
Prepared by Rutwik
Network Hardware
The Big Picture: Where are
We Now?
The various possible hardware structures
that are possible. LANs, MANs, WANs
and all that.
1.1 Introduction
1.2 Network Hardware
1.3 Network Software
1.4 Reference Models
1.5 Example Networks
1.6 Example Data
Communication Services
1.7 Network Standardization

12. Chap. 1 - Introduction
Prepared by Rutwik
LOCAL AREA
NETWORKS
LANs are:
Privately owned. Can be up to several kilometers long;
Restricted so worst case transmission time can be contained.
Run at speeds of 10, 100, or more Mbps.
Low delay. High reliability.
Requires collision arbitration.
Ethernet:
IEEE 802.3.
Bus based broadcast network with decentralized control at 10 or 100 Mbps.
Token Ring:
IEEE 802.5
Ring based broadcast network with token arbitration at 4 or 16 Mbps.
Network
Hardware

13. Chap. 1 - Introduction
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METROPOLITAN AREA
NETWORKS:
MANS are:
Larger version of LAN ("city" wide).
Public or private / data or voice.
Broadcast - no switches.
Can be distinguished from LANs based on wiring mechanism.
Network
Hardware

14. Chap. 1 - Introduction
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WIDE AREA NETWORKS
WANS are: Networks spanning large distances.
Hosts or End Systems -
Machines running user applications.
(Communication) Subnet -
Connections between hosts - transmission lines + switches.
A "locality" understanding each other's addresses.
Circuits/Channels/Trunks -
Transmission lines move the bits.
Packet switching nodes/Intermediate systems/Data switching exchanges
Specialized computers moving data between several inputs
to several outputs.
Network
Hardware

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WIDE AREA NETWORKS
WANS are: Networks spanning large distances.
Router - Generic term for switching computers.
Point-to-point/Store-and-forward/Packet-switched -
Moving through a series of routers, packets are received at a router,
stored there, then forwarded to the next router.
Topologies -
Network
Hardware

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Prepared by Rutwik
Media
Wireline
String
Garden Hose
Copper
Twisted Pair
Coax
Optical Fiber
Network
Hardware
Wireless
Sound
Light and mirrors
Infrared
RF
Microwave

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WIRELESS NETWORKS
What are they: Used where computer is mobile or far away from wires.
Only 1 - 2 Mbps / higher error rates / interference.
Wireless Mobile Applications
NO NO Stationary workstations in offices.
NO YES Using portable in hotel.
YES NO LANs in unwired buildings.
YES YES Portable office; PDA.
INTERNETWORKS:
( or internet )
• Collection of interconnected networks.
• internet ( lower case i ) is generic term.
• Internet ( upper case I ) is worldwide connection to all kinds of machines.
Network
Hardware

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Network Software
The Big Picture: Where are
We Now?
This covers the way the software is put
together. It talks about the philosophy of
connecting together two entities.
“Layering” is the key word.
1.1 Introduction
1.2 Network Hardware
1.3 Network Software
1.4 Reference Models
1.5 Example Networks
1.6 Example Data
Communication Services
1.7 Network Standardization

19. Chap. 1 - Introduction
Prepared by Rutwik
Network Software
Layers -
The concept that network software is organized functionally into levels. A level on
one host talks to the same level on another host (its peer).
Protocol -
The protocol is the convention or standard that a layer uses to talk to the other
layer. An agreement or standard on the conversation.
Physical Medium -
Underneath the layers is the wire or
fiber or whatever.
Interface -
Defines the services that one layer
offers another (either up or down.)
Important to keep this simple and clean.
Important that each layer perform
specific actions.
PROTOCOL
HIERARCHIES

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Network Software
Network architecture -
• A set of layers and protocols. It contains details on what happens in the layer and
what the layers says to its peer.
• Functional interfaces and implementation details are not part of the spec, since
that's not visible outside the machine.
PROTOCOL
HIERARCHIES
Protocol stack -
• A list of protocols used by a
system, one protocol per layer.
Information flow -
• "Send_to_peer" rather than
"call_next_layer_down".

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Network Software
Both Directions Simultaneous
Simplex No No
Half duplex Yes No
Full duplex Yes Yes
o Number of logical channels per connection (for priority purposes)
o Error control. (garbled or missing.)
o Preservation of message ordering.
o Flow control.
o Breaking up messages into a smaller chunks (and reassembly.)
o Multiplexing messages on same connection.
o Routing - how to get from one host to another.
DESIGN ISSUES
FOR THE LAYERS
That word “Multiplexing”:

22. Chap. 1 - Introduction
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INTERFACES AND
SERVICES
Purpose of each layer is to provide services to the layer above it.
Entities / Peer entities -
Active element in each layer (process/IO chip).
Peer entity = layer N entity <--> layer N entity.
Layer N entity provides service for layer N + 1.
Service providers and users -
Layer N is a provider for user N + 1.
SAPs (Service Access Points) -
Entry points in N that layer N + 1 can access.
Has an address that uniquely identifies it.
IDUs (Interface Data Unit) -
The information from N + 1 provided at the SAP.
Made up of SDU + control information.
Network Software

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INTERFACES AND
SERVICES
Purpose of each layer is to provide services to the layer above it.
SDUs (Service Data Unit) -
The portion of the IDU that will be passed up to the peer entity.
PDUs (Protocol Data Unit) -
The SDU may be broken up into PDUs, that being the chunk size for
further transmission.
Rolled up in
this figure:
Network Software

24. Chap. 1 - Introduction
Prepared by Rutwik
CONNECTION-
ORIENTED /
CONNECTIONLESS
SERVICES:
Connection oriented service -
Like the phone system. The system establishes a connection, uses it,
and closes it. Acts like a tube. Data comes out the other end in the same
order as it goes in.
Connection Setup
Data Transfer
Connection Termination
Connectionless service -
Like the post office. Each message has the entire address on it. Each
message may follow a different route to its destination. Ordering not
maintained.
Data Transfer
Network Software

25. Chap. 1 - Introduction
Prepared by Rutwik
CONNECTION-
ORIENTED /
CONNECTIONLESS
SERVICES:
Quality of service -
Will the message arrive??
A reliable connection-oriented service guarantees success.
o Message sequence - message boundaries and order are maintained.
o Byte streams - messages are broken up or combined; flow is bytes.
Can pair mechanism with upper-layer requirements.
Network Software

26. Chap. 1 - Introduction
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CONNECTION-ORIENTED /
CONNECTIONLESS
SERVICES:
Datagram Service -
Like junk mail. It's not worth the cost to determine if it actually arrived. Needs a
high probability of arrival, but 100% not required. Connectionless, no
acknowledgment.
Acknowledged datagram service -
As above, but improved reliability via acknowledgment.
Request-reply service -
Acknowledgment is in the form of a reply.
Summarized in this Table.
Network Software

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SERVICE PRIMATIVES:
Primitives are:
The operations available to an entity. Possibilities include:
Request -- An entity want some work done.
Indication -- An entity is told about an event.
Response -- An entity wants to respond to an event.
Confirm -- Response to earlier request has come back.
Example of Connectionless Protocol:
Unitdata.request |--------->| Unitdata.indication
Unitdata.indication |<---------| Unitdata.request
Example of Connection-oriented Protocol:
Connect.request |--------->| Connect.indication
Connect.confirm |<---------|
Data.request |--------->| Data.indication
Data.indication |<---------| Data.request
Disconnect.request |--------->| Disconnect.indication
Disconnect.confirm |<---------|
Network Software

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SERVICE PRIMATIVES
THE RELATIONSHIP OF SERVICES TO PROTOCOLS:
Services are primitives that a layer provides for the layer above it.
Protocols are rules governing the meaning of frames/packets/messages
exchanged with the peer entity.
Network Software

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Reference Models
The Big Picture: Where are
We Now?
There are two competing models for how the
software is layered. These are the OSI
and the TCP models. We talk about each
of these here.
1.1 Introduction
1.2 Network Hardware
1.3 Network Software
1.4 Reference Models
1.5 Example Networks
1.6 Example Data
Communication Services
1.7 Network Standardization

30. Chap. 1 - Introduction
Prepared by Rutwik
Reference Models Layering

31. Chap. 1 - Introduction
Prepared by Rutwik
Reference Models Headers, Data, and
Trailers
Encapsulation

32. Chap. 1 - Introduction
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Reference Models
OSI == Open Systems Interconnection
Developed by ISO == International
Standards Organization
Principles used to develop OSI Layering:
1. Need a layer for each different level of
abstraction.
2. Each layer performs a well defined
function.
3. Each layer should be standard-izable.
4. Layer boundaries should minimize data
flow across those boundaries.
5. The right number of layers - don't put too
many functions together, but not too
many layers either.
THE OSI
REFERENCE MODEL

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Reference Models
Physical Layer -
Purpose -- Transmits raw bits across a medium.
Electrical -- Concerns are voltage, timing, duplexing, connectors, etc.
Where Taught -- Differentiates an EE course from a CS course.
Data Link Layer -
Framing -- Breaks apart messages into frames. Reassembles frames into
messages.
Error handling -- solves damaged, lost, and duplicate frames.
Flow control -- keeps a fast transmitter from flooding a slow receiver.
Gaining Access -- if many hosts have usage of the medium, how is access
arbitrated.
THE OSI
REFERENCE MODEL

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Reference Models
Network Layer -
Routing -- What path is followed by packets from source to destination. Can
be based on a static table, when the connection is created, or
when each packet is sent.
Congestion -- Controls the number packets in the subnet.
Accounting -- Counts packets/bytes for billing purposes.
Heterogeneity -- Interfacing so one type of network can talk to another.
THE OSI
REFERENCE MODEL

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Reference Models
Transport Layer -
Reliability -- Ensures that packets arrive at their destination. Reassembles out
of order messages.
Hides network -- Allows details of the network to be hidden from higher level
layers.
Service Decisions -- What type of service to provide; error-free point to point,
datagram, etc.
Mapping -- Determines which messages belong to which connections.
Naming -- "Send to node xyzzy" must be translated into an internal address
and route.
Flow control -- keeps a fast transmitter from flooding a slow receiver.
THE OSI
REFERENCE MODEL

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Reference Models
Session Layer -
Sessions -- Provides services that span a particular message. For instance, a
login session could be logged.
Synchronization-- Provide way to subdivide a long mechanism for reliability.
Presentation Layer -
Prettiness -- Syntax and semantics of information transmitted. Understands the
nature of the data being transmitted. Converts ASCII/EBCDIC, big
endian/little endian
Application Layer -
Interfacing -- Terminal type translation.
File transfer -- Programs able to understand directory structures and naming
conventions and map them onto various systems.
THE OSI
REFERENCE MODEL

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Reference Models
Data Transmission in the OSI Model -
THE OSI
REFERENCE MODEL

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Reference Models
Used in the Arpanet and in the Internet. Common mechanism that is gaining
on/surpassing the OSI Model.
THE TCP/IP
REFERENCE MODEL
Host to Network Layer -
This lowest level is not defined in
this model. Various
mechanisms abound.
Internet Layer -
Connector -- Provides packet
switched connectionless
service.
Routing -- The IP (Internet
Protocol) does delivery and
congestion control.

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Reference Models
Transport Layer -
Allows peer entities to communicate.
TCP -- Transmission Control Protocol provides a reliable connection
oriented protocol that delivers a byte stream from one node to
another. Guarantees delivery and provides flow control.
UDP -- User Datagram Protocol provides an unreliable connection-less
protocol for applications that provide their own.
Application Layer -
Terminal -- Telnet
File transfer -- FTP
The Web -- HTTP
THE TCP/IP
REFERENCE MODEL

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Reference Models
OSI has good definition of service, interface, and protocol as discussed before.
Fits well with object oriented programming concepts. Protocols are better
hidden.
With TCP, the protocols came first; model was just a description of the
protocols. But then the model isn't good for any other protocols.
LAYERS
Transport Network
OSI Connectionless
Connection-Oriented Connection-Oriented
TCP Connectionless Connectionless
Connection-Oriented
Allows connectionless applications!
COMPARISON OF
REFERENCE MODELS

41. Chap. 1 - Introduction
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Reference Models
A CRITIQUE OF OSI:
Bad Timing -
• TCP already in use by the time OSI came along.
Bad Technology -
• Layers don't match reality . Chosen because IBM's SNA has seven layers.
• Dominated by phone company mentality.
Bad Implementation -
• Huge, unwieldy, slow.
A CRITIQUE OF TCP/IP:
• Doesn't separate spec from implementation.
• Model is only good for describing TCP.
• Doesn't specify physical and data link layers.
THE HYBRID REFERENCE MODEL:
5 Application
4 Transport
3 Network
2 Data Link
1 Physical
THE TCP/IP
REFERENCE MODEL

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Example Networks
The Big Picture: Where are
We Now?
This section talks about some real networks.
1.1 Introduction
1.2 Network Hardware
1.3 Network Software
1.4 Reference Models
1.5 Example Networks
1.6 Example Data
Communication Services
1.7 Network Standardization

43. Chap. 1 - Introduction
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Example Networks
NOVELL NETWARE:
Heavily used in PC world.
• Proprietary protocol stack
Network layer -- IPX - unreliable connectionless
Transport layer -- NCP (Network Core Protocol)
- connection oriented
- other services
• Naming and Addressing --
SAP (Service Advertising Protocol) - Servers advertise their address to router
machines. Clients, when booted, ask for location of nearest server.
THE ARPANET:
1968 Originally intended as reliable network, with multiple routing.
Used TCP/IP precursor, which got built into early UNIX.

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Example Networks
NSFNET:
Late 1970s - Many other folks wanted to get on the net, but Arpanet was essentially
limited to military contractors. NSF set up another network to handle this need.
Started at 448 Kbps and by 80's upgraded to 1.5 Mbps.
1990 Formed ANS (Advanced Networks and Services) -- MERIT, MCI, IBM
took over from the government running at 45 Mbps.
1995 ANSNET sold to AOL, who now runs it.
THE INTERNET:
Growing exponentially.
All nodes run TCP/IP. Means that all nodes have an IP address by which they can
be contacted.
Services provided include:
o e-mail o news o remote login o file transfer o the web
GIGABIT TESTBEDS:
Testing higher speed long distance nets. Typically 622 Mbps. While throughput
improves, latency remains limited by the speed of light.

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Example Data
Communications
These are public networks owned by the
phone companies and offered to the
public - thus called public networks.
1.1 Introduction
1.2 Network Hardware
1.3 Network Software
1.4 Reference Models
1.5 Example Networks
1.6 Example Data
Communication Services
1.7 Network Standardization

46. Chap. 1 - Introduction
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Example Data
Communications
These are subnets, often owned by the phone companies, offered to subscribers.
This is a public network.
SMDS - SWITCHED MULTIMEGABIT DATA SERVICE
Interconnects the LANs operating within a single company. Connects diverse
branches.
Public Networks

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Example Data
Communications
X.25 NETWORKS
Developed during 70's. Interface between public packet-switched networks and
customers.
Operate at 64 Kpbs, so are very slow and becoming outdated. However there are
still many of them in operation.
Connection oriented. Uses:
• Switched Virtual Circuit - established when the first packet is sent. Circuit
remains for duration of session providing in-order delivery, and flow control.
• Permanent Virtual Circuit - established by agreement between the customer and
the carrier. Like a leased line.
PAD ( Packet Assembler Disassembler ) allows non-X.25-speaking terminals to
connect to X.25.
Public Networks

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Example Data
Communications
FRAME RELAY:
• Takes advantage of modern high-speed reliable digital phone lines.
• This allows simple protocols with work done by user computers rather than by the
network. Runs at 1.5 Mbps with few features.
• Customer leases a permanent virtual circuit between two points. This "virtual
leased line" means that the wire is shared with other users at a great price
reduction. Each is allowed some bursti-ness, but doesn't get the line all day.
Public Networks

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Example Data
Communications
BROADBAND ISDN AND ATM:
The above networks, together with a multitude of old versions, mean a major
headache. Need a way to provide all of these, and many new services with a
single high speed network.
ISDN (Integrated Services Digital Network) will offer cable, video on demand, e-
mail, etc. etc. etc.
ATM (Asynchronous Transfer Mode) is underlying mechanism. Transmits in small
fixed-size cells.
5 48
+---------+---------------------------------------+
| Header | User Data |
+---------+---------------------------------------+
Public Networks

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Example Data
Communications
ATM (Continued)
Packet (cell) switching is dramatic change for phone companies.
ATM is connection oriented; make connecting request first; then all cells follow the
same path.
Target is 155 Mbps and 622 Mbps. Allows TV transmission.
See the ISDN ATM Reference Model below:
Public Networks

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Example Data
Communications
ATM (Continued)
Comparisons to other models.
Public Networks
ATM promises much but will take a while to deliver.
• Moving from circuit switching will take a long time.
• Requires replacement of existing wires.

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Example Data
Communications
COMPARISONS:
Here’s an overview of the properties of these Public Networks
Public Networks

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Standards
The Big Picture: Where are
We Now?
Networks have more standards than any
other area of the computer industry.
The reason is because hardware/software
from different manufactures must all play
together - and you can’t do that unless
the vendors get together and establish
agreements about the behavior of this
equipment.
1.1 Introduction
1.2 Network Hardware
1.3 Network Software
1.4 Reference Models
1.5 Example Networks
1.6 Example Data
Communication Services
1.7 Network Standardization

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Standards
Issues Include:
de facto and de jure.
Phone companies and the governments that regulate them.
ISO (International Standards Organization)
ANSI (American National Standards Institute)
NIST (National Institute of Standards and Technology)
IEEE (Institute of Electrical and Electronics Engineering)
IAB (Internet Architecture Board)

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Summary
We (and the text) use the Internet and ATM as examples.
Architecture (not hardware) of physical Layer.
Data Link Layer.
MAC (Medium Access Layer) - part of Data Link Layer.
Network Layer
Transport Layer
Application Layer (naming, mail, news, web, etc.)