4. Technique for assuring that a transmitting entity does not overwhelm a receiving entity with data Necessary when data is being sent faster than it can be processed by receiver Prevents buffers from overflowing One of the primary functions performed at the data link layer of OSI

5. Two types of errors Lost frame A frame fails to arrive at the other side Damaged frame A recognizable frame does arrive, but some of the bits are in error Collectively referred to as automatic repeat request (ARQ), common techniques for error control are: Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgment and retransmission

6. The higher the data rate, the more cost- effective the transmission facility Cost per kbps declines with an increase in the data rate of the transmission facility Cost of transmission and receiving equipment, per kbps, declines with increasing data rate Most individual data communicating devices require relatively modest data rate support

11. Same general architecture as other FDM systems A number of sources generate a laser beam at different wavelengths These are sent to a multiplexer that consolidates the sources for transmission over a single fiber line Optical amplifiers amplify all of the wavelengths simultaneously The composite signal arrives at a demultiplexer where the component channels are separated and sent to receivers at the destination point

13. ADSL uses frequency-division modulation (FDM) to exploit the 1-MHz capacity of twisted pair Asymmetric because ADSL provides more capacity downstream (from the carrier’s central office to the customer’s site) than upstream (from customer to carrier) Provides a perfect fit for Internet access

15. Entire frequency band for the upstream channel overlaps the lower portion of the downstream channel Advantages The higher the frequency, the greater the attenuation More flexible for changing upstream capacity Disadvantages Need for echo cancellation logic on both ends of line

19. Long-distance carrier system designed to transmit voice signals over high-capacity transmission links such as optical fiber, coaxial cable, and microwave Evolution of these networks to digital involved adoption of synchronous TDM transmission structures

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22. Transmission facilities supporting DS-1 Often used for leased dedicated transmission between customer premises Private voice networks Private data network Video teleconferencing High-speed digital facsimile Internet access

23. SONET is an optical transmission interface that was originally designed for the public telephone network It has been standardized by ANSI for voice, long-haul data, and/or video traffic applications Synchronous Digital Hierarchy (SDH), a compatible version, has been published by ITU- T Specifications for taking advantage of the high-speed digital transmission capability of optical fiber

26. Frequency-division multiple access (FDMA) Radio spectrum used to connect mobile devices and cell towers is divided into separate frequency channels, each capable of carrying one call Frequency-division duplexing (FDD) Two distinct frequency bands are used – one band carries uplink channels (mobile device to cell tower) and the second carries downlink channels (cell tower to mobile device) These approaches are being replaced by time-division multiple access (TDMA) and code-division multiple access (CDMA) in cellular networks

27. Flow control Error control Motivation for multiplexing FDM WDM ADSL Chapter 6: Data Link Control and Multiplexing Synchronous TDM The TDM mechanism Digital carrier systems T-1 facilities SONET/SDH Cellular and cordless phone systems