TDC 461
Basic Communications
Systems
Session 6
15 Mai 2001
• T1 Framing
• Packet Switching
– Packet Headers
Agenda
• 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
Data
+3V
Voltage
-3V
0 0 1 1 0 1 0 0 0 1
T1 Frame Format
1 bit
8 bits
8 bits
8 bits
F
DS0 #1
DS0 #2
DS0 #3
8 bits
. . .
DS0 #24
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


Frames 1-5, 7-11:
1 bit
8 bits
8 bits
8 bits
F
DS0 #1
DS0 #2
DS0 #3
8 bits
. . .
DS0 #24
Frames 6, 12:
1 bit
F
7 bits
1
S
DS0 #1 I
G
7 bits
1
S
DS0 #2 I
G
7 bits
1
DS0 #3
S
I
G
. . .
7 bits
1
DS0 #24
S
I
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
Access
Line
Access
Line
Router
New York
Chicago
Router
Router
Router
Router
Customer
Network
Router
Carrier Network
LAN Hub
Client
Customer
Network
LAN Hub
Server
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 storeand-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
Access Line:
Shared by
Multiple devices
at Customer site
Customer
Packet
Switch
CSU
Carrier
Packet
Switch
Carrier
Packet
Switch
Carrier Data Trunk:
Shared by
Multiple customers
of this carrier network
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-toend delivery
– Layer 2 protocol just used on one
physical transmission link
Packets inside Frames
• Example: IP packet carried inside
Ethernet frame
Ethernet
Header
IP
Header
Data
Internet Protocol (IP) packet
Ethernet Frame
Ethernet
Trailer
Packets and Frames
• Routers modify frame headers &
trailers so packet can travel end-toend over many links
T1
Token
Ring
IP packets /
TR frames
C.O.
Router
Router
modifies
frame
CSU
T1
CSU
IP packets /
PPP frames
Router
Router
modifies
frame
Token
Ring
IP packets /
TR frames
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)
–
–
Terminal
Terminal
PC without
X.25 Software
Terminal
Modem
Modem
Customer
PAD
PC with
X.25 Software
PAD
CSU
Dial-Up (POTS)
Access Line
Modem
Carrier
PAD
Leased
Access Line
PAD
Modem
X.25
Switch
LEC X.25
Network
X.25
Switch
LEC X.25
Network
Mainframe
CSU
X.25
Switch
Leased
Access Line
X.25
Switch
X.25
Switch
X.25
Switch
National X.25
Backbone
X.25
Switch
IP Comparison to X.25
•
•
•
PCs with LAN card
and TCP/IP Software
LAN
PC without
TCP/IP Software
Customer Site
IP Router
IP Router
Dial-Up (POTS)
Access Line:
Host Access
Modem
PC with
TCP/IP Software
and SLIP/PPP Software
Dial-Up (POTS)
Access Line:
SLIP/PPP Access
CSU
Modem
Modem
Leased
Access Line
UNIX Host
IP
Router
Regional ISP
Network
IP
Router
Modem
Regional ISP
Network
IP
Router
IP
Router
IP
Router
IP
Router
National ISP
Backbone
IP
Router
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)
PCs with LAN card
and TCP/IP Software
LAN
PC without
TCP/IP Software
Customer Site
IP Router
IP Router
Dial-Up (POTS)
Access Line:
Host Access
Modem
PC with
TCP/IP Software
and SLIP/PPP Software
Dial-Up (POTS)
Access Line:
SLIP/PPP Access
CSU
Modem
Modem
Leased
Access Line
UNIX Host
IP
Router
Regional ISP
Network
IP
Router
Modem
Regional ISP
Network
IP
Router
IP
Router
IP
Router
IP
Router
National ISP
Backbone
IP
Router
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
A
B
C
D
E
Range of host
addresses
32 Bits
Class
0
Network
10
Network
110
1110
11110
1.0.0.0 to
127.255.255.255
Host
128.0.0.0 to
191.255.255.255
Host
Network
Host
192.0.0.0 to
223.255.255.255
Multicast address
224.0.0.0 to
239.255.255.255
Reserved for future use
240.0.0.0 to
247.255.255.255
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.
–
brewster.cs.depaul.edu
Host
subdomain
domain
top-level
domain
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” toplevel 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
Corporate
Network
Dial-Up
Communications
Server
FIREWALL
External (Dirty) Network
Router
Packet
Filtering
Router
The Internet
(Untrusted)
Public
Web
Server