TDC 461 Basic Communications Systems Session 6 15 Mai 2001
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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
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