Transcript CS335 Networking & Network Administration

CS335
Networking &
Network Administration
Wednesday, April 14, 2010
Long Distance WANS




Digital Telephony
Prior to networks being important
Early telephone used analog signals which
degrade over long distance needing
amplifiers and introduce noise
Digital audio avoids noise by encoding
original audio into digital form
Audio digitization

Closest integer to signal = sampling =
continuous stream of numbers
Audio to digital converter






Sample the signal
Compute a number closest to level of signal
Continuous stream of numbers
Human voice has frequencies up to 4000 Hz
If sampled at twice the highest frequencies
the original signal can be reconstructed from
the samples
Digitized voice sampled 8000 times/sec
(once every 125 microseconds)
Pulse Code Modulation






World-wide standard for digital telephony
Need to choose a range of values
More values = more accurate but also more
data
Uses integers between 0 and 255 for voice
PCM is the sampling scheme
Digitized data sent over long distance and
converted back to audio at destination
Synchronous communication





Telephone industry has complex digital
communications systems – long distance
Digitized voice is synchronous or clocked
Data is moved at precise rates
Voice apps can’t tolerate delay or noise in the
same way as data
Continuous transmission synchronized to
have constant data rate
Digital phone circuits


Point to point digital circuits - data uses
different standards than voice
DSU/CSU’s (Data Service Unit /Channel
Service Unit) at each end of phone circuit
The demarc



Network Interface Unit (NIU) is the last piece
of telephone company equipment
CSU portion handles line termination and
diagnostics to test if line is connected
DSU translates data between the digital
format of the phone circuit and the format
needed by computer network
Data rates


Fractional t1’s can also be leased
Phone company uses Time Division Multiplexing to
subdivide T1 into 24/64 Kbps fractions =1536
Intermediate capacity


Need more than a T1 but less than a T3
Inverse multiplexor allows multiple T1’s to be
combined
Highest Capacity Circuits


Synchronous Transport Signal (STS)
Optical Carrier (OC)
SONET (Synchronous Optical
Network)




In addition to STS and OC standards phone
companies have defined SONET
Specifies details such as how data is framed
How lower capacity circuits are multiplexed
into a high capacity circuit
How synchronous clock info is sent along
with the data
SONET Frame



SONET Encoding
Size of a SONET frame depends on bit rate
STS-3 circuit each frame holds 2430 octets
IDSN





Digitized voice and data over conventional local loop
wiring (twisted pair copper same as the analog
phone system – POTS: plain old telephone service)
Two B channels operating at 64Kbps
B channels can be combined into a single channel
to get 128 KbPS
One D channel operating at 16 Kbps as a control
channel
When it was begun 64 Kbps was much faster than
dialup modem, now it is an expensive alternative
with little throughput
ADSL








Multiple flavors so we refer to it as xDSL
ADSL, SDSL, HDSL
ADSL –Asymmetric digital subscriber line
Higher bit rate downstream than upstream
Most users request downstream data – browsing the
internet
ADSL can operate at 6.144 Mbps downstream and
576 upstream
Doesn’t require changes in local loop wiring
Can run simultaneously over the same wires as
phone service
ADSL
ADSL



Adaptive to conditions on the local line in terms of
choosing frequencies based on the interference that
the modems find on the line when they power up
Probe many frequencies on the line between
modems and select frequencies and modulation
techniques that give optimal results
286 separate frequencies or subchannels


255 downstream and 31 upstream with 2 channels for
control information
Choose frequencies above 4 Khz to not interfere with voice
Cable Modems








Higher speed than telephone wiring
Less susceptible to EM interference
Already has infrastructure in place
Multiplexed onto the cable with other TV signal
Can deliver up to 36 Mbps, but is a shared capacity can be as
little as 1/n where n=subscribers
CATV was never designed for upstream communication
Early attempts used a dual path approach – upstream data
handled by a dialup modem
New approach is HFC )hybrid fiber coax) but it requires major
upgrade of existing cable infrastructure
Satellite and Wireless



Wireless is usually line-of-site
Receiving antenna needs to be in line with
transmitter
Satellite also uses another path for upstream
traffic, making it more complex