Lecture 5 - Notes

January 18, 2016

Physical Layer Internet Backbone

Internet access

• Phone Line
  • Dialup
  • Digital Subscriber Line (DSL)
  • Asynchronous Digital Subscriber Line (ADSL)
• Cable Line
  • Broadband Modem
• Ethernet
  • Local Area Network (LAN)
• Wireless
  • WiFi
  • Wireless Local Area Network (WLAN)
  • Wireless Metropolitan Area Network (WMAN)
  • Cellular

Switching Technologies

• Circuit Switched: Each message requests a transmission line, when granted it transmits on an assigned line.
• Message Switched: Messages are sent sequentially on separate lines.
• Packet Switching: Packets are sent on separate lines and decoded at the end.

Multiplexing

• A way of sending multiple signals or streams of information over communications link at the same time.

Frequency-Division Multiplexing (FDM)

• The total available bandwith in a channel is divided into no overlapping sub channels each of which is used to carry a separate signal
• Typically used for radio

• Example: Suppose a long-distance cable is available with a bandwidth allotment of three megahertz (3 MHz). This is 3,000 kHz, so in theory, it is possible to place 1,000 signals, each 3 kHz wide, into the long-distance channel. The circuit that does this is known as a multiplexer. It accepts the input from each individual end user, and generates a signal on a different frequency for each of the inputs. source

Wavelength Division Multiplexing (WDM)

• Multiplexing is done by using different wavelengths (colors) of light as the sub channels.

• WDM is multiplexing for optical channels

Time-Division Multiplexing (TDM)

• Synchronized switches at each end of the transmission line multiplex and demultiplex the streams of data so that each signal appears on the line only a fraction of time in an alternating pattern.

• In the image, when 4 streams are multiplexed using TDM the bits from any particular stream are spaced apart at interval of 4. So bits at 4*i (i is an integer), Come from stream 1 and 4*i + 1 come from stream 2 and so on.

Internet Backbone

• TO DO