TDC 461Basic Communications

Systems

Session 6

15 Mai 2001

Agenda • T1 Framing

• Packet Switching

– Packet Headers

• The Internet– Structure

– Applications

– IP Addresses

– The World Wide Web• Universal Resource

Locators (URLs)• HTML

– The Domain Name System (DNS)

• Intranets– Applications

– Security

T1 Details• Bipolar Representation

– T1 uses Bipolar Coding to represent 1 and 0 bits

– ‘1’ bit represented by alternating +3 volt, -3 volt pulses

– ‘0’ bits represented by no voltage

• Framed Format

Bipolar Representation

0 0 1 1 0 1 0 0 0 1Data

+3V

-3V

Voltage

T1 Frame Format

1 b it 8 b its 8 b its8 b its8 b its

F DS0 #1 DS0 #24DS0 #3DS0 #2 . . .

Each DS0 called a time slot 8000 frames/sec * 8 bits/slot = 64 Kbps 24 * 8 + 1 = 193 bits/frame 8000 frames/sec * 193 bits/frame = 1.544 Mbps 8000 Framing bits sent per second

T1 Framing Bits• Framing Bits used for

– Allow receiver to find the start-of-frame (frame synchronization).

– Group sets of 12 frames into superframes– Indicate which frames contain signaling bits– Provide error checking (CRC) (ESF T1)– Provide Facilities Data Link channel to transmit

network management messages (ESF T1)

T1 Framing Bits (D4 Frame)• D4 Framing - Superframe T1 (1970)

– F-bit pattern marks 12-frame superframes– F-bit pattern: 100011011100

• Odd frames: 101010 (Basic framing pattern)

• Even frames: 001110 (marks 6th, 12th frames)

– One Signaling Bit “robbed” in 6th & 12th frames for each Time Slot

• Each signaling bit indicates whether that Time Slot is currently IDLE (1) or IN-USE (0)

D4 Frame Format

1 b it 8 b its 8 b its8 b its8 b its

F DS0 #1 DS0 #24DS0 #3DS0 #2 . . .

Frames 6, 12:

Frames 1-5, 7-11:

1 b it 7 b its 7 b its7 b its7 b its

F DS0 #1 DS0 #24DS0 #3DS0 #2 . . .1 111SI

G

SI

G

SI

G

SI

G

T1 Framing Bits (ESF Frame)• D5 Framing - Extended Superframe T1 (1983)

– F-bit pattern marks 24-frame extended superframes

– F-bit pattern:

• Odd frames: Facilities Data Link

• Every 4th frame: 001011 (Framing pattern)

• Every 4th frame: CRC for previous ESF

ESF Frame Advantages• Facilities Data Link

–

• Cyclic Redundancy Check (CRC)

–

–

Packet Data Services•

•

•

• Customer pays–

Packet Data Services

R outer

R outer

R outer

Carrier Network

R outer

ChicagoRouter

LAN Hub

Client

New YorkRouter

LAN Hub

Server

CustomerNetwork

CustomerNetwork

AccessLine

AccessLine

Packet Switching• Concepts:

– – – –

Packetizing• Example:

– I want to send a 50 Mbyte file– Max. packet length is 5000 bytes

Packet Switch Operations• Packet switch (router) is a store-

and-forward device • • • •

Packet Switching = Statistical Time Division Multiplexing

• A Packet Switch acts as a multiplexer, allowing multiple devices to share a single physical line.

Packet Switching = Statistical Time Division Multiplexing

CustomerPacketSwitch

CSU

CarrierPacketSw itch

CarrierPacketSw itch

Access L ine:Shared by

M ultip le devicesat Custom er site

Carrier Data T runk:Shared by

M ultip le custom ersof this carrier netw ork

Packet Switching = Statistical Time Division Multiplexing

• •

Packet Switching and OSI• Packet switching functions

performed by Network Layer (Layer 3) software

• Layer 3 protocol determines:– max. packet length– packet header format– address format

Example: X.25• X.25 protocol:

– Max packet length = 1024 bytes– Header format:

• 4-bit General Format Identifier• 4-bit Logical Group Number• 8-bit Logical Channel Number• 8-bit Packet Type Identifier

– Address: Logical Group/Channel

Example: Internet Protocol • IP protocol:

– Max packet length = 65,536 bytes– Header format:

• 4-bit Version Number• 4-bit Header Type• 8-bit Service Type• 16-bit Length• 16-bit Identifier

Example: Internet Protocol • IP protocol header format(cnt’d):

• 16-bit Offset• 8-bit Time-to-Live• 8-bit Protocol• 16-bit Checksum• 32-bit Source Address• 32-bit Destination Address

– Address: 4-byte IP address

Packets inside Frames• Layer 3 packets can be carried

inside any type of Layer 2 frame– Layer 3 protocol determines end-to-

end delivery– Layer 2 protocol just used on one

physical transmission link

Packets inside Frames• Example: IP packet carried inside

Ethernet frame

EthernetHeader

IPHeader Data

EthernetTrailer

Internet Protocol (IP) packet

Ethernet Fram e

Packets and Frames• Routers modify frame headers & trailers so

packet can travel end-to-end over many links

Router RouterCSU CSUC.O .

T1T1TokenRing

TokenRing

IP packets / TR frames

IP packets / PPP frames

Router modifies frame

IP packets / TR frames

Router modifies frame

X.25 Details

• • •

X.25 Details• Virtual Channel Communications

– To send data, you must•

•

•

X.25 Details

• Cost of X.25 service– Carrier monthly charges based on:

•

•

•

X.25 Details• Network Error Control

– Each Packet Switch in the network buffers customer data and handles ACKs and retransmission• •

X.25 Details• Packet Assembler / Disassemblers

(PADs)– –

M odem

X .25S w itch

X .25S w itch

X .25S w itch

X .25S w itch

X .25S w itch

LE C X .25N etw ork

National X .25Backbone

X .25S w itch

LE C X .25N etw ork

X .25S w itch

CSU

M ainfram e

LeasedA ccess L ine

PC withX.25 Software

Terminal

PAD

CSU

TerminalTerminal

M odemC ustom er

P A D

PAD

M odem

M odem

C arrie rP A D

PC withoutX.25 Software

LeasedA ccess L ine

D ia l-U p (P O TS )A ccess L ine

IP Comparison to X.25

• •

•

M odem

IPR outer

IPR outer

IPR outer

IPR outer

IPR outer

R eg iona l IS PN etw ork

National ISPBackbone

IPR outer

R eg iona l IS PN etw ork

IPR outer

PC withTCP/IP Software

and SLIP/PPP Software

M odem

M odem

M odem

PC withoutTCP/IP Software

LeasedA ccess L ine

D ia l-U p (P O TS )A ccess L ine :

S LIP /P P P A ccess

D ia l-U p (P O TS )A ccess L ine :H ost A ccess

UNIX Host

IP Router

CSU

C ustom er S iteIP R oute r

LANPCs with LAN cardand TCP/IP Software

What is the Internet? • A “Network of Networks”

• Multiple networks of many types are all interconnected

• The single common element: Internet Protocol (IP)–

–

The Key Players • End Users are businesses and individuals who

wish to use the Internet

• Internet Service Providers (ISPs) are carriers that maintain networks of IP routers to provide IP packet service –

–

• ISPs interconnect with each other at Metropolitan Area Exchanges (MAEs)

M odem

IPR outer

IPR outer

IPR outer

IPR outer

IPR outer

R eg iona l IS PN etw ork

National ISPBackbone

IPR outer

R eg iona l IS PN etw ork

IPR outer

PC withTCP/IP Software

and SLIP/PPP Software

M odem

M odem

M odem

PC withoutTCP/IP Software

LeasedA ccess L ine

D ia l-U p (P O TS )A ccess L ine :

S LIP /P P P A ccess

D ia l-U p (P O TS )A ccess L ine :H ost A ccess

UNIX Host

IP Router

CSU

C ustom er S iteIP R oute r

LANPCs with LAN cardand TCP/IP Software

Access to Internet • Access lines to ISP routers can be:

–

–

–

–

• See http://www.thelist.com for list of ISPs and pricing information

Who Makes Money? • End Users and Businesses

• Regional ISPs pay National ISPs

• ISPs pay telecommunications carriers

Application Layer Protocols • Telnet

– Allows remote login to another computer

• File Transfer Protocol (FTP)– Transfers files between computers

• Simple Mail Transport Protocol (SMTP)– Sends e-mail to another computer

• Post Office Protocol (POP)– Downloads e-mail from e-mail server

• Hypertext Transfer Protocol (HTTP)– Transfers files between Web servers and Web clients

running browser software

IP Addresses • Each IP address is 4 bytes long

• Format: each byte written in decimal separated by dots - “dotted decimal” –

–

IP Addresses • How does every device on the Internet get a

different IP address?

• IP Address Prefixes assigned to organizations by the Internet Assigned Numbers Authority (IANA)--now Network Solutions–

–

• These organizations then control all IP addresses starting with that prefix

IP Address Classes • Class A Address:

– First byte value between 1 and 127

– IANA specifies value of 1st byte

– Organization chooses IP address for each device by assigning value in last 3 bytes.

– Organization has (256 * 256 * 256) = 16 million different IP addresses!!

– Example: IP addresses 36.x.x.x are all controlled by Stanford University

IP Address Classes • Class B Address:

– First byte has value between 128 and 191

– IANA specifies value of 1st and 2nd bytes

– Organization chooses IP address for each device by

assigning value in last 2 bytes.

– Organization has (256 * 256) = 65,536 different IP

addresses!!

– Example: IP addresses 140.192.x.x are all controlled

by DePaul University

IP Address Classes • Class C Address:

– First byte has value between 192 and 223

– IANA specifies value of 1st, 2nd and 3rd bytes

– Organization chooses particular IP address for each device by assigning value in last byte.

– Organization gets 256 different IP addresses

– Example: IP addresses 207.36.140.x are all controlled by Microsoft Corporation

IP Address Classes

32 Bit sRange o f hos taddr es s es

1.0 .0 .0 t o127.255.255.255

128.0 .0 .0 t o19 1.255.255.255

19 2.0 .0 .0 t o223.255.255.255

224 .0 .0 .0 t o239 .255.255.255

24 0 .0 .0 .0 t o24 7.255.255.255

Clas s

0 Net w or k Hos t

10 Net w or k Hos t

110 Net w or k Hos t

1110 M ult ic as t addr es s

11110 Res er v ed f o r f ut ur e us e

A

B

C

D

E

Other Ways to get IP Addresses

• Small businesses and individuals usually

don’t go through the IANA

–

–

The Web • The World Wide Web is a collection of

computers on the Internet that use– HTTP protocol to transfer files (web pages)– Hypertext Markup Language (HTML) to represent

• Formatting information for display• Clickable Hyperlinks to go to other pages

• Web browsers access HTML and other files from servers using HTTP

• Web Servers store files and make them available to browsers

Web Sites• To publish a Web Site, a user must

– Find a Web server, that is• Connected to the Internet at all times (not dial-up access)• Running web server software• Has disk space that can be used for storing files.

– Preferably, get good software to help in creating graphics and HTML pages.

• Cost:– Many ISPs provide disk space and web page

building tools to their clients for a small fee.

URLs • Universal Resource Locator (URL) is the format

used to specify a particular resource on the Web.<protocol>://<host>/<directory>/<file>

• Protocol – HTTP:, TELNET:, FTP:, etc.

• Host – Either DNS name or IP address of device on the

Internet

HTML • Hypertext Markup Language allows a text

file to be augmented with tags that specify– Formatting information (font size, bold, italics,

table formats, etc)– Locations to include image files (using GIF, JPEG

or other graphics formats)– Hyperlinks (highlighted text and an associated

URL to go to if clicked)– Associated executable applets (written in Java,

Javascript, ??)

Browser Operations <protocol>://<host>/<directory>/<file>

• Given this URL, a browser will–

–

–

–

Domain Name System • The Internet utilizes a system of Domain

Name System (DNS) servers.–

brew ster.cs.depaul.edu

H o s t s ubdo m a in to p -le ve ldo m a in

do m a in

Domain Name System • Top-Level Domains:

– 2-letter top-level domains for each country - .fr for France, .de for Germany, etc.

– .edu - Educational Institutions

– .com - Commercial

– .gov - Government

– .mil - Military

– .org - Organizations

– .net - Network providers

– .int - Internet organizations

Domain Name System • Top-Level DNS Servers

• Domain DNS Servers

Domain Name System • Example: You type “http://www.ibm.com” in

browser:– Your PC sends DNS request packet to DePaul DNS

server

– DePaul DNS server forwards packet to “.com” top-level DNS server

– Top-level DNS server forwards packet to “ibm.com” Domain DNS Server at IBM Corp.

– IBM DNS server looks up IP address and sends result back to your PC at DePaul.

Intranets

• Intranets are internal business networks that

use Internet Web technologies to improve

business processes.

• Employees use standard Web Browsers to

accomplish many business applications

Intranet Applications • Collaborative Software

– Share and modify documents simultaneously– In-house video and audio conferencing

• Information Sharing– Company Newsletters– Employee Handbooks– Help Desk

• “Paperless Office”– All paper-based processes can be moved onto web-

based forms on-line.

Intranet Advantages • Increased Access to Information

– Easy to look up answers on-line

• Reduced Costs

– Reduced management cost

– Reduced cost of disseminating information

– Increased productivity

• Increased Flexbility

• New Applications

Intranet Disadvantages • Increased Network Usage and Costs

– May be difficult to predict data traffic loads

– New applications may increase overall

management costs

• Increased Security Risks

– Risks from within organization

– Risks from outside organization (if connected to

the public Internet).

Intranet Security • Firewalls

– Filtering application gateways placed between internal networks and public Internet

– Watches all data packets going in both directions

• IP Address Translation– Can be used to hide internal IP addresses from

outside world

Router

The Internet(Untrusted)

External (Dirty) Network

PublicW eb

Server

D ia l-U pC ommunications

Server

FIREW ALL

CorporateNetw ork

PacketF ilteringR outer