Chapter 3: Introduction to Data Networks
Overview
This
chapter addresses primarily the Internet. Initial information is given as to
the history of the Internet, the Internet structure, various types of data networks,
quality of service issues, why convergence is important, and the standards that
relate. As for the history of the Internet, it is the data counterpart of the
Public Switching Telephone Network. The Internet was developed by the Department
of Defense (DoD) in the 1960s as a means to communicate with manufacturers and
research agencies. It was decided by DoD to turn the operation of the Internet
over the private business in 1994.
·
Two
classes of device: hosts and gateways
·
Expanded
device: router, used as a gateway
·
Three
major applications: telnet, file transfer protocol, simple mail transfer protocol
·
Hypertext
transfer protocol and World Wide Web created Internet boom
·
Web
browsers and graphical user interfaces brought millions of users into the Internet
·
Access
through Internet service providers (ISP)
·
Telephone
and modem access an ISP
·
Asymmetric
digital subscriber line and cable modems increase the bandwidth
·
Data
networks information sliced into bit-size fragments
·
Asymmetric
transfer mode (ATM) 53 bytes long known as cells
·
Other
networks data is sliced into longer fragments known as frames or packet
Structure Of The Internet
Voice
and data networks were previously based on voice circuits. This session identifies
the key differences of the structure between voice and data sessions.
·
Voice
session switched through PSTN or PBX
·
Most
data sessions switched through voice networks
·
Voice
session characteristics different than data sessions
·
Bandwidth
optimized for voice channel
·
Extra
bandwidth adds no value to voice sessions
·
Extra
bandwidth provides shorter transportation time for data sessions
·
Voice
calls have single worldwide addressing scheme
·
Basic
pattern in voice sessions is geographically oriented
·
Voice
call address 540-710-000
·
Data
addresses does not use geographical orientation
·
Network
interface card (NIC) contains permanently assigned address
·
Data
addresses use host computers all over the world
·
Computer
found by identifying domain
·
Registration
of domain with the Internet Network Information Center
·
Network
equipped with logic to find domain name server (DNS)
·
DNSs
installed within networks and Internet service providers’ (ISP) networks
·
First
three digits identify owner organization
·
Internet
protocol (IP) addresses are four-part addresses
·
Data
session IP address is 123.45.67.89
·
IP
address can be located anywhere in the world
·
PSTN
primarily designed for voice traffic
·
Internet
designed for data
·
Either
network can carry both types of traffic
·
Internet
is connectionless
·
Voice
network has connection from originating to terminating parties
Types of Data Networks
Different
types of networks have emerged because voice network is no longer satisfactory
due to the fixed bandwidth. In addition, data applications require no bandwidth
at all for a period of time. This section contains information on the architecture
of data networks as well as a brief description of different data networks.
·
PSTN
operates with switched circuits
·
Data
networks operate with virtual circuits
·
Virtual
circuit is defined in network software
·
Virtual
circuit does not occupy fixed hardware circuit
·
Virtual
circuit is not restricted to fixed bandwidth
·
Internet
is data equivalent to the public switched telephone network
·
ISP
is Internet’s equivalent of the LEC in the PSTN
·
Networks
can be interconnected with telephone circuits
·
Packet
network known as X.25 networks
·
Public
data networks popular middle-to-late 1980s
·
Fiber
optics installed throughout industrialized world by late 1980s
·
Frame
relay emerged to take advantage of high-quality fiber-optic
·
Frame
relay consists of nodes/high-speed interconnecting trunks
·
Frame
relay network does not guarantee packet deliveries
·
Frame
relay network offers greater bandwidth than X.25
·
IP
network is used for routing PDUs between nodes
·
IP
network composed of private or public facilities as designer choices
·
Virtual
private networks can be deployed over IP networks
·
Asynchronous
transfer mode (ATM) developed to carry voice and video
·
PDU
of ATM is cell with 48-byte payload and 5-byte header to steer through network
Quality Of Service
In
order for one to understand convergence of voice and data, one must first know
the quality of service requirements.
·
Major
voice connection concerns: how easy is it to set up the call/how intelligible
is the call
·
Fast
call setup is desirable
·
In
data terms, fast call set-up to a human is unacceptable
·
Data
transfer could be complete in the amount of time it takes for call set-up time
·
Voice
quality-of-service (QoS) is 80% of users satisfied with noise/volume levels
·
Adequate
connection for voice is unusable for data
·
Short
static pops do not affect voice
·
Short
static pops render data connection unusable
·
User
sees slower response time if data network discards or delays packets
·
Voice
and video severely impaired by end-to-end delays
·
End-to-end
delay called latency
·
Variations
in packet arrival known as jitter
·
Quality
service over computer network must identify time-sensitive packets
·
Quality
service over voice and video must be given priority
·
Undesirable
voice signals delay 500 milliseconds
·
Voice
delay >800 milliseconds circuit unusable
·
Data
less sensitive to delay and jitter than voice
·
Voice
is insensitive to small amounts of noise
·
Voice
has no advantage from bandwidth
·
Data
requires bandwidth in bursts
Why Convergence?
The
answer to the question is, with the thousands of Internet users downloading
data from the World Wide Web, the limitations of voice circuits became obvious.
·
Immediate
solution is asymmetrical digital subscriber line (ADSL)
·
ADSL
a form of voice and data integration on a local loop
·
Connections
made on dedicated and dial-up sessions over PSTN/Internet/Public networks
·
Business
forms exchanged over electronic data interchanges (EDI)
·
Electronic
commerce over the Web growing rapidly
·
Interconnected
island of local networks have voice and data requirements
·
Cost
is fixed, incremental capacity added at little cost
·
IP
network takes advantage of voice characteristic, half-duplex
·
Half-duplex,
use of one half of the bandwidth at any instant
·
Typical
voice conversation has silent periods
·
Silent
period allows for squeezing in several data packets
·
IP
equipment makes more efficient use of bandwidth than circuit switching
·
Digitized
voice is compressed to 8 kb/s/one-fourth of the bandwidth of a digital channel
·
One
application of Voice over IP application is an affinity network
·
Affinity
network allows calls into gateway over local PSTN/Internet/terminates at distant
PSTN
·
Quality
less, cost reduced
·
Another
application of Voice over IP is Web-enabled call center
·
Customers
linked to live agent via company’s Web page
·
Visual
display accompanies voice
·
Most
appealing application of Voice over IP is branch office
·
Branch
office connected to headquarters
·
Frame
relay most cost effective connection
·
Tunneling
through Internet connection
·
Security
issues with tunneling connection
·
Router
with voice over IP provides for voice and data paths
Standards
ITU
is the authority for promulgation for voice and data standards. This session
discusses, in detail, a group of individuals known as the Internet Architecture
Board (IAB) and their responsibilities for developing and approving Internet
standards.
·
Individuals
propose standards and improvements through a request for comments (RFC)
·
IAB
appoints one member as RFC editor
·
Editor
publishes the RFC
·
Standards
reviewed by Internet Engineering Steering Group (IESG)
·
IESG
has appeal to IAB
·
RFC
editor prepares and organizes the standard in final form
·
Internet
Engineering Task Force (IETF) opens international consortium
·
IETF
interest in evolution and operation of the Internet