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TDC 461 Basic Communications Systems

Learn about T1 framing, packet switching, and the basics of communication systems. Understand how packets are switched and the role of T1 framing in transmitting data.

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TDC 461 Basic Communications Systems

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  1. TDC 461Basic Communications Systems Session 6 15 Mai 2001

  2. 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 Agenda

  3. 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

  4. Bipolar Representation

  5. T1 Frame Format • 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

  6. 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)

  7. 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)

  8. D4 Frame Format • Frames 1-5, 7-11: • Frames 6, 12:

  9. T1 Framing Bits (ESF Frame) • D5 Framing - Extended Superframe T1 (1983) • F-bit pattern marks 24-frame extendedsuperframes • F-bit pattern: • Odd frames: Facilities Data Link • Every 4th frame: 001011 (Framing pattern) • Every 4th frame: CRC for previous ESF

  10. ESF Frame Advantages • Facilities Data Link • Cyclic Redundancy Check (CRC)

  11. Packet Data Services • Customer pays

  12. Packet Data Services

  13. Packet Switching • Concepts:

  14. Packetizing • Example: • I want to send a 50 Mbyte file • Max. packet length is 5000 bytes

  15. Packet Switch Operations • Packet switch (router) is a store-and-forward device

  16. Packet Switching = Statistical Time Division Multiplexing • A Packet Switch acts as a multiplexer, allowing multiple devices to share a single physical line.

  17. Packet Switching = Statistical Time Division Multiplexing

  18. Packet Switching = Statistical Time Division Multiplexing

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. Packets inside Frames • Example: IP packet carried inside Ethernet frame

  25. Packets and Frames • Routers modify frame headers & trailers so packet can travel end-to-end over many links Router modifies frame IP packets / PPP frames IP packets / TR frames Router modifies frame IP packets / TR frames

  26. X.25 Details

  27. X.25 Details • Virtual Channel Communications • To send data, you must

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

  29. X.25 Details • Network Error Control • Each Packet Switch in the network buffers customer data and handles ACKs and retransmission

  30. X.25 Details • Packet Assembler / Disassemblers (PADs)

  31. IP Comparison to X.25

  32. What is the Internet? • A “Network of Networks” • Multiple networks of many types are all interconnected • The single common element: Internet Protocol (IP)

  33. 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)

  34. Access to Internet • Access lines to ISP routers can be: • See http://www.thelist.com for list of ISPs and pricing information

  35. Who Makes Money? • End Users and Businesses • Regional ISPs pay National ISPs • ISPs pay telecommunications carriers

  36. 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

  37. IP Addresses • Each IP address is 4 bytes long • Format: each byte written in decimal separated by dots - “dotted decimal”

  38. 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

  39. 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

  40. 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

  41. 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

  42. IP Address Classes

  43. Other Ways to get IP Addresses • Small businesses and individuals usually don’t go through the IANA

  44. 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

  45. 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.

  46. 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

  47. 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, ??)

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