Sampling and Quantization

Digital Telephony:
- Sampling and Quantization
- synchronous networks
- telephone standards
SONET
ISDN, DSL, cable

telephone carries voice faithfully up to 3000 Hz, then drops quickly
assuming we don't need above 4,000 Hz, how do we encode that data digitally?
Nyquist theorem: if you signal is band-limited to x Hz, sampling 2x times per second will allow you to exactly reconstruct the signal
the samples are still analog: we divide the possible voltages linearly (logarithmically?) into 256 intervals, and encode to the nearest interval
for the telephone, every second, 8,000 samples of 8 bits, 64,000 b/s

Synchronous Networks

a synchronous network such as the telephone network has some fundamental differences from an asynchronous data network:
- any substantial delay or queueing of data is not an option
- TDM still works: each call has a slot and all calls are served in round-robin order
- a multiplexer may have to delay each payload to be synchronized with the envelopes it sends
everything in the digital telephone network is designed to carry data 8,000 times per second, or one message every 125 us
faster lines can carry more than one byte in that time (there are no lines slower than 64Kb/s)

a Data Service Unit connects to the computer side, a Channel Service Unit connects to the telephone side: together, a DSU/CSU (WAN modem)
T1/DS1: 24 (and 1/8) voice circuits, 1.544 Mb/s. E1: 32 voice circuits, 2.048 Mb/s
T3/DS3: 28 T1 circuits (and 27 voice circuits), 44.763 Mb/s
STS-1/OC-1: 51.840 Mb/s ("Synchronous Transfer Signal"/"Optical Carrier")
STS-3/OC-3: 155.520 Mb/s
OC-12: 622.080 Mb/s
OC-48: 2.488320 Gb/s

Framing technology for carrying data (not voice) over digital public lines
very simple header identifies virtual circuit (usually permanently set up -- "provisioned" -- at contract time)
payload delivered to the destination with high likelihood
used for bridging LANs (e.g. between branches of the same company), sometimes for Internet traffic

Synchronous Optical NETwork (Synchronous Digital Hierarchy/SDH)
SONET dictates:
- framing (multiples of 810 bytes, probabilistically identified)
- multiplexing
- payload encoding (payload "floats" in the frame, with part in one frame and part in the successive frame)
frame "header" is 2-dimensional, 3 out of every 90 bytes
very complex
a frame is sent every 125 us, meaning the minimum speed is? (in-class exercise)

1st digital technology to the home ("local loop")
two channels (4 wires), each capable of 64Kb/s, for a total of 128Kb/s
not popular:
- too expensive for most homes (much more expensive than a modem)
- too early for the web (you don't need that much bandwidth for plain-text email)

Asymmetric Digital Subscriber Line
most of the traffic is in the direction from the central office to the home (downstream)
so reserve most of the bandwidth for downstream: up to 6.144 Mb/s (only up to 640Kb/s upstream)
bandwidth is available on the twisted pair of the local loop: don't use baseband (like RS-232), but broadband (like radio channels)
adaptive technology: send more bits through the channels that have high S/N ratio
distance limitations (CO to CPE)

Symmetric DSL/SDSL -- different encoding, symmetric bandwidth
High-Speed DSL/HDSL -- symmetric, 1.544Mb/s in both directions
Very high-Speed DSL/VDSL -- symmetric, up to 52Mb/s in both directions, requires new wiring to bring switch closer to the CPE

the channel in a cable TV system not used for broadcasting TV signals can be used as an ether to broadcast data packets
cable modems are needed at both ends
maximum rate up to 36Mb/s (but depending on traffic and number of channels)
cable not always designed for two-directional communication, e.g. repeaters might only repeat in one direction (can use telephone uplink)
in Hawaii, chiefly road-runner