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Unformatted text preview: Digital Subscriber Line, ADSL, A for
Downstream data rate depends on local loop quality.
Maximum of 8 megabits/second is allowed for.
Trunks and Multiplexing - sharing a common
cable, wire or optical fiber.
Categories of multiplexing
1. Time division multiplexing (TDM), Synchronous or
2. Frequency division multiplexing (FDM).
3. Combined TDM and FDM.
4. Wavelength division multiplexing in optical fiber. Frequency multiplexing with carriers. T elephone network digital voice. Used by telephone network after A/D conversion of
voice signals - 8000 samples/second. 1/8000 = 125
microsec. frames time multiplex 24 channels.
Raw bit rate is (24 x 8 + 1) x 8000 = 1.544
Megabits/second. Called a T1 carrier
The channel data rate is 7 x 8000 = 56 kb/s.
Hierarchies of successively higher rates
AT&T had T2 = 4 T1 Streams; T3 = 7 T2 streams; T4 = 6
T3 streams. On voice A/D conversion - redundancy allows data
Can be as low as 8 kb/s, even 4 kb/s, with some quality
The Mobile Telephone System
First generation: Analog voice AMPS (Advanced Mobile
Second Generation Mobile Phones: Digital Voice
1. D- AMPS (Digital Advanced Mobile Phone Service) 3 users can share a 30 khz channel, using time multiplexing.
(or six with 4 kb/s coding).
2. GSM (Global System for Mobile Communications)
Frequency channels are 200 khz instead of 30 khz.
Time Division Multiplex 8 users instead of 3 users - more
data rate per user (200/8 > 30/3).
3. CDMA (Code Division Multiple Access) The Data Link Layer
Grouping bits into frames, handling errors,
regulating flow, proper sequencing of
frames. Link layer takes into its payload field
whatever higher layers give it.
Framing - breaking a bit stream into frames.
first task -define the start and end of a frame.
Asynchronous case - determine the bit sampling
time and start of packet. Physical...
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- Fall '08