Transcript Basic Concepts

Technology Focus: Network and
Communications Technology
Major Network Categories

Major Network Types

The Global Internet

Internal Corporate Networks

The Worldwide Telephone System
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Major Network Categories

Telecommunications Spans Two Concerns

Voice and Video Communication vs

Data Communication
 At least one party is a computer

The two are converging
Telecommunications
Voice and
Video
Data
Communication
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Network

A network is a system of hardware, software
and transmission components that collectively
allow two application programs on two
different stations connected to the network to
communicate well.
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Elements of a Network
Server
Station
Client
Station
Mobile Client
Station
Stations are computers
and other devices
such as cellphones and PDAs
Server
Station
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Elements of a Network
Frame
Server
Station
Client
Station
Mobile Client
Station
Stations communicate by
sending messages called
Frames
Server
Station
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Elements of a Network
Switch
Switch
Switch
Switch
Frames may pass
through multiple switches
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Elements of a Network
Access
Link
Access
Link
Access
Link
Access links connect
stations to the
first switch
Access
Link
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Elements of a Network
Trunk
Link
Trunk
Link
Trunk Link
Trunk links
connect switches
Trunk
Link
Trunk
Link
Higher capacity
than access links
Often optical
fiber
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Client/Server Architecture
Server
Client PC
Service
Network
Clients
Receive
Services
Servers
Provide
Services
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Clients and Servers

Network Applications - File sharing, Network printing, Email, etc.

Clients – Access the server to perform an application


Servers – Host one or more applications; Store application files;
Respond to client requests


Most common desktop operating systems – Windows 95/98/XP (desktop)
Common network operating systems: Microsoft Windows Server, Novell
Network, UNIX, LINUX
A server is not a type of computer. Any computer can be changed
from a workstation to a server. It is dependent on the operating
system and application programs loaded onto the computer.

Since servers need to support simultaneous access from multiple clients
and be operational 24x7, they tend to have greater power, storage
capacity, and stability. Hence, computer companies label computers
“servers”.
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Packets and Frames

Messages in Single Networks are Called
Frames

Messages traveling across multiple networks
(in Internets) are Called Packets
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Packet vs. Circuit Switching

Circuit switching: dedicated path between two nodes. Entire
circuit must be available during the transmission. (Example: local
telephone network.)

Relatively inefficient for data transmission. (Why?)

Message switching: store-and-forward process. (Postal mail
analogy)

Packet switching: also store-and forward, this time messages
divided into packets of uniform length:


Improved line utilization over message switching.
Key differences between circuit switching and packet switching:



sharing of links,
storing and forwarding,
graceful degradation
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Packet Switching
1. Message Broken into
Smaller packets
Original Message
Packet
Switch
Computer X
A
Packet
B
C
Switching
Decision
E
Computer Y
F
D
2. Packets Routed individually
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Packet Switching

Packet Switching Reduces Trunk Line Costs

Packets from several conversations are multiplexed
on trunk lines
Trunk Link
Packet from A to B
Packet from C to D
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Packet Switching
Packet
Trailer
Data Field
Header
Other
Header
Field
Address
Field
Message Structure
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Quality of Service

It is not enough that networks work


They must work well
Quality of Service (QoS) defines quantitative
measures of service quality

Speed and Delay (Latency)

Reliability

Security (not a QoS measure but crucial)

Service Level Agreements (SLAs)

Guarantees for various service parameters

Network provider pays performance penalties if guarantees
are not met
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Quality of Service (QoS)

Data Transmission Rate




Bits per second (bps)
Multiples of 1,000 (not 1,024): kbps, Mbps, Gbps, Tbps
Congestion and Latency

Congestion because traffic chronically or momentarily exceeds capacity

Latency (delay measured in milliseconds)

Especially bad for some services such as voice communication or highly interactive
applications
Reliability

Availability
 Percentage of time the network is available to users for transmission and
reception
 Want 24 x 7 x 365 availability
 Telephone network: Five 9s (99.999%)

Error Rate
 % Lost or damaged messages or bits
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Geographic Scope

Local Area Network (LAN)

Limited geographical distance: home, office,
building, campus, industrial part

Customer premises operation
 User firm chooses technology
 User firm needs to manage on ongoing basis

Low cost per bit transmitted
 Companies can afford high speed
 100 Mbps to the desktop is typical
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Geographic Scope

Wide Area Network (WAN)
WAN

To link sites
 Long distances

Requires the use of carriers to provide service
 Limited and complex choices but carrier manages

High cost per bit transmitted
 Companies cannot afford high transmission rates




Charges are distance dependent
Typically transmit 1.544 Mbps or less
Large organizations, like GMU, have bigger access links (Up
to 45 Mbps)
Carriers have very large trunk links (155 Mbps – 10 Gbps)
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Internet

An internet is a group of networks linked
together with routers in a way that allows an
application program on any station on any
network in the internet to be able to
communicate with an application program on
another station on any other network.
Network 1
Network 2
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An Internet
Multiple Networks
Connected by Routers
Path of a Packet is its Route
Network 3
Routers
Packet
Network 2
Network 1
Route
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An Internet
Single Networks Have Switches
Switches Connect Station-to-Router or Router-to-Router
Network X
Switches
Network Y
Network Z
Routers
Switches
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Frames and Packets

Within a single network, the packet is carried
in the data field of that network’s frame,
probably across multiple switches
Packet
Frame
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Frames and Packets

Router removes packet from source network’s frame,
sends back out in receiving network’s frame format
Same
Packet
Frame
Different Frame Format
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Frames and Packets

Router removes packet from source network’s frame,
sends back out in receiving network’s frame format
Same
Packet
3d Frame
Format
Different Frame Format
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Frames and Packets

Like passing a shipment (the packet) from a truck
(frame) to an airplane (frame) at an airport.
Shipper
Same
Shipment
Truck
Airport
Receiver
Airport
Truck
Airplane
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The Internet: Universal Addressing
Scheme
Browser
Webserver
Software
Network
Packet
Router
Packet
Route
Router
Router
Packet
User PC
(Host)
IP Address=128.150.50.9
Webserver
(Host)
IP Address=128.171.17.13
Host name=voyager.cba.hawaii.edu
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Hosts

All computers connected to the Internet are
hosts

Server hosts (webservers, FTP servers, etc.)

Also client PCs at home, at school, and at work

Also PDAs and Internet-enabled cellphones

The Internet treats all hosts of all sizes as equals

Only application software distinguishes between
them (browsers for client PCs, webserver
applications for server hosts, etc.)
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Host Addresses and Host Names

Host IP Address

Official address of host on the Internet

Every host must have an IP address, including
client PCs and PDAs

32 bits long

Often expressed in dotted Decimal Notation for
human reading (e.g., 128.171.17.13)

Hosts and routers work with 32-bit binary form
IP Address: 128.171.17.13
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Internet Service Providers (ISPs) and
Internet Backbone Carriers
Webserver
User PC
Internet Backbone
(Multiple Carriers)
Router
ISP
ISP
Internet
Service Provider
Internet
Service Provider
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Wide Area Networks (WANs)

Organizations connect their computing resources locally through
so-called LAN’s (Local Area Newtorks)

LAN’s are typically considered a local IT resource, internally
owned and operated

So called WAN’s , wide area networks are needed to interconnect
geographically distant computing resources (branches of a bank,
factories and warehouses, production and R&D facilities etc) of a
single organization

Businesses WAN connectivity options include:



Connecting their corporate servers (and/or LAN’ s) together using
leased lines (i.e., building private WANs)
Using the existing infrastructure owned by specialized providers:
 Public switched data networks
Using the Internet infrastructure (Virtual Private Network technology)
(VPN)
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Trunk-Line Based Leased Line
T1 Trunk Line (1.544 Mbps)
Access
Line
Computer
Telephone
Switch
Trunk
Line
Telephone
Switch
Telephone
Switch
Server
T1 Leased Line (1.544 Mbps)
End-to-End Circuit with Trunk Line Speed
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Leased Line Networks

Leased Line (Private Line or Dedicated Line)

Point-to-point connection

Always on (no dialing or other “call setup” procedures)

If used extensively, lower cost per minute than dial-up service

Must be provisioned (set up) ahead of time (instantaneous
provisioning not possible)

T1s have historically been very expensive to lease. Lower-cost
alternatives include:
Fractional T1
 Lease a portion of the T1 (in DSO (64 Kbps) or 2-DSO (128
Kbps) increments). Common increments:




64 Kbps (Called a DS-0, not Fractional T1)
128 bps, 256 kbps, 384 kbps, 512 kbps, 768 kbps
Essentially, a leased line is a circuit switched service
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Alternative #1: Leased Line Corporate Data
Network
Site A
Multisite Leased Line Mesh Network
Site B
OC3 Leased Line
T3 Lease
Line
56 kbps
Leased
Line
T1 Leased
Line
Site C
T1 Leased
Line
56 kbps
Leased
Line
56 kbps
Leased
Line
Site D
Site E
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Leased Line Trade-Offs

Leased Line Network

Many leased lines, often span long distances
 Pricing is distance-sensitive

Company must design/install the network, buy the switching
equipment (e.g., Routers), order many leased lines, and
maintain the network.

Pros: Control, Security, Predictable Performance

Cons:
 More expensive,
 more management and administration responsibilities
 scalability issues for large number of nodes
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Alternative #2: Corporate Use of a Public
Switched Data Networks
Alternative: Use a public Packet-Switched Network
Site B
Site A
POP
POP
Packet-Switched Network
(Based on Frame Relay, SONET,
or other WAN technology)
Point of Presence
POP
Site D
POP
Site C
Site E
One leased line per site. Typically in form of a “service” (not a ‘leased line’)
that includes the physical layer protocol.
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Public Switched Data Network Trade-Offs

Public Packet-Switched Network

Owned and operated by the telecommunications Companies
(AT&T, Sprint UUNet etc.)

Only need one leased line from each site to a POP
 Short distances to the first available POP (intra-LATA)

Pricing is typically not distance sensitive
 Mostly it is based on requested data rate parameters

Business still needs one router per site and some maintenance,
but bulk of the design, installation, equipment, and maintenance
falls to the Communications Company.
 Pros: Less expensive, less administration and management
 Cons: Less control, potential security issues, less
performance predictability
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Alternative #3: Corporate Customer orders
Internet VPN services
VPN connection
The Internet
(A collection of networks owned by
Communications Companies).
Customer connects to its ISP’s POPs. The ISP
Sends traffic over its own TCP/IP network or
to other TCP/IP networks, as needed.
VPN connection
LAN(s)
Ethernet
Switch(es)
Customer Premises B
VPN connection
Customer
Premises C
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What is ordered?

The customer orders

(1) either:
 Leased Lines from the telephone company to
connect its sites to the ISP POPs; or
 The physical line connection comes bundled
with the ISP’s Internet VPN services

(2) VPN services

(3) A Service Level Agreement (SLA)
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