Transcript DCN-4-Communications_Equipment

Communications Equipment
•Communication Equipments:
–Equipments that are,
•Not a direct part of the network (like node,
client, terminal, server etc.) but,
•Indirectly they are a part of network and,
–Are required for communication between the source
and the destination.
Communications Equipment
Multiplexer
MUX
Demultiplexer
DEMUX
Communications Equipment
•Multiplexer (Mux) / De-multiplexer (Demux):
–Multiplexer:
•Device that,
–Receives the input signals from several devices,
–Combines them into a single stream of data, and then
–Transmits the data over a single communications line.
•Process of combining various signals into one is called:
–Multiplexing
–De-multiplexer:
•Device that,
–Recovers the input signal and divides them as,
–Separate signals again.
•Process of separating the signals from a single signal is called:
–De-multiplexing
–Most multiplexers today include,
•Both multiplexing and de-multiplexing functionality.
Multiplexer / Demultiplexer
•Advantages:
–Increases the efficiency of communications by reducing the,
•Cost of communications by,
–Combining the data of various communication lines onto a single
communication line.
•If each client had its own dedicated line,
–The line would be idle most of the time.
–The process of combining the links onto 1 transmission line
is,
•Transparent and hence,
•The network behaves as if each connection was occurring over
a separate physical line.
–Organizations need to lease (rent) only,
•A single long-distance line to,
•Transmit many voice and data signals between two offices in
different cities.
Multiplexer / Demultiplexer
•Multipoint line v/s Multiplexed line:
–Multipoint line:
•Physically a single line and logically,
–Also a single communication channel.
•Allows only,
–1 client to transmit over the line at a time.
•Requires only,
–1 port / interface for connection.
•Requires algorithm to avoid congestion.
–Multiplexed line:
•Physically a single line is logically,
–Divided into multiple communication channels.
•Allows,
–Many clients to transmit at the same time.
•Requires,
–Many ports / intefaces for connection because,
»The multiplexed line needs to be demultiplexed at some point
of time into separate lines.
•Algorithm to avoid congestion is not required.
Communications Equipment
•Multiplexing:
–Depending on how the signals are multiplexed
(joined/combined) together, classification is:
•Guided Media/Signal:
–Frequency Division Multiplexing (FDM)
–Time Division Multiplexing (TDM)
–Statistical Time Division Multiplexing (STDM)
–Wave Division Multiplexing (WDM)
•Unguided Media/Signal:
–Frequency Division Multiple Access (FDMA)
–Time Division Multiple Access (TDMA)
»Used in GSM cellular phones.
–Code Division Multiple Access (CDMA)
»Used in CDMA cellular phones.
Multiplexing
Frequency Division Multiplexing (FDM)
Interference? No. Because of Guardbands.
12000 to 16000 Hz / 12 to 16 KHz (4000 Hz)
8000 to 12000 Hz / 8 to 12 KHz (4000 Hz)
4000 to 8000 Hz / 4 to 8 KHz (4000 Hz)
0 to 4000 Hz / 0 to 4 KHz (4000 Hz)
Bandwidth
16000 Hz / 16 KHz
4,000 Hz
Guardband (4,000 – 3,300) = 700 Hz
3,300 Hz
Data (3,300 – 300) = 3,000 Hz
300 Hz
Guardband (300 - 0) = 300 Hz
0 Hz
Frequency Division Multiplexing
(FDM)
Multiplexing
•Frequency Division Multiplexing (FDM):
–The bandwidth of the entire communications line is,
•Logically divided into,
–Narrower (thin) bandwidths / channels so that,
–Each client on the line can use one frequency range to transmit
data.
–The gaps that occur between the frequency ranges are
available for,
•Guardbands which are,
–Frequency ranges used to separate the individual signals so that,
–They do not interfere with each other.
–Example:
•FDM might assign following frequency channels to each 4 KHz
telephone line that is connected:
–1st channel: 0 KHz to 4 KHz
–2nd channel: 4 KHz to 8 KHz
–3rd channel: 8 KHz to 12 KHz and so on.
•Guardbands in each 4 KHz channel prevent,
–The voice or data signals from interfering with one another.
Multiplexing
•Frequency Division Multiplexing (FDM):
–Types:
•Group:
–If 12 of these 4 KHz lines are combined onto a,
»12x4 = 48 KHz line.
•SuperGroup:
–If 60 such lines are combined onto a,
»60x4 = 240 KHz line.
•MasterGroup:
–If 600 such lines are combined onto a,
»600x4 = 2400 KHz (2.4 MHz) line.
•JumboGroup:
–If 3,600 such lines are combined onto a,
»3600x4 = 14400 KHz (14.4 MHz) line
Multiplexing
•Frequency Division Multiplexing (FDM):
–Disadvantages:
•If the client does not have any data to transmit,
–That logical part of transmission line remains idle.
•Some part of bandwidth is also wasted in,
–Trying to protect the signals from interference, i.e.
Guardbands.
–Use:
•For analog cable television transmission.
Multiplexing
Time Division Multiplexing (TDM)
C4
C3
C1
C2
C3
C4
C1
C2
C2
Bandwidth
4000 Hz / 4 KHz
or
3000 Hz / 3 KHz
C1
4,000 Hz
Guardband (4,000 – 3,300) = 700 Hz
3,300 Hz
Data (3,300 – 300) = 3,000 Hz
300 Hz
Guardband (300 - 0) = 300 Hz
0 Hz
Time Division Multiplexing
(TDM)
Multiplexing
•Time Division Multiplexing (TDM):
–TDM divides the transmission time slot into,
•Small Time Segments / Time Slices instead of,
–Using different frequency ranges to transmit data as in
FDM.
–Each client is assigned a,
•Fixed time slot in,
–Rotation, during which it can transmit data and,
•During that slot,
–The client is given the full transmission capacity/bandwidth
of the line.
–A major difference between TDM and FDM in how
the signals are separated is,
•FDM uses,
–Frequency guardbands to separate signals whereas,
•TDM uses,
–Time to separate the signals.
Multiplexing
•Time Division Multiplexing (TDM):
–Disadvantage:
•If the client does not have any data to transmit
during its fixed time slot,
–The transmission line remains idle during that time
and,
–No other client can use that time slot.
FDM v/s TDM
Multiplexing
Statistical Time Division Multiplexing (STDM)
C4
C3
C2
C1
C2
C3
C3
C2
Bandwidth
4000 Hz / 4 KHz
or
3000 Hz / 3 KHz
C1
4,000 Hz
Guardband (4,000 – 3,300) = 700 Hz
3,300 Hz
Data (3,300 – 300) = 3,000 Hz
300 Hz
Guardband (300 - 0) = 300 Hz
0 Hz
Multiplexing
•Statistical Time Division Multiplexing (STDM):
–Developed to overcome the problem of,
•Idle time on the transmission line and make better use of
bandwidth.
–Unlike TDM, in STDM,
•Time slots are assigned to the clients,
–Dynamically and not Statically (Fixed).
•So if a client does not have data to transmit,
–STDM will not leave an open time slot on the transmission
line.
–In most cases, this results in,
•Less idle time on the transmission line and improves
efficiency.
–Major difference between TDM and STDM is:
•TDM is driven by time whereas,
•STDM is driven by data.
Multiplexing
•Statistical Time Division Multiplexing
(STDM):
–Problem / Challenge / Disadvantage:
•If all the clients want to send data at the same
time then,
–STDM has/uses special feature called,
»Buffers to,
–Store the excess data in memory until,
–Bandwidth is available on the line for each
transmission.
•STDM is more costly than TDM because,
–It has more advanced functions than TDM.
Multiplexing
•Wavelength Division Multiplexing (WDM):
–Similar to FDM (Frequency Division Multiplexing) done on,
•Fiber-optic cables and so,
•WDM is the optical equivalent of FDM.
–Each signal is assigned to,
•A particular wavelength (i.e. particular frequency) on optical fiber
communications line which enables,
–Bi-directional communication over 1 optical fiber and,
–Increase in the capacity/bandwidth as well.
–Different frequencies (or wavelengths),
•Allow,
–Many different transmissions to exist on the same optical fiber at
the same time.
•Prevent,
–Individual signals from interfering with each other.
–Advanced form of WDM has also been invented called:
•DWDM: Dense Wavelength Division Multiplexing
Wavelength Division Multiplexing
(WDM)
Multiplexing
•Wavelength Division Multiplexing (WDM):
–BOTTLENECK Problem:
•Internet infrastructure uses WDM to give speeds of,
–MegaBytes per second (MBps) and,
–GigaBytes per per second (GBps) yet,
•Internet users get the speeds only in,
–KiloBits per second (Kbps).
–Reason:
•Ultimately optical fibers are always connected to,
–Networks that use copper wire.
•Copper networks can only carry,
–Electrical signals and not Light signals.
•So some kind of conversion needs to be done at both the ends
of transmission (Light to Electricity & vice-versa) and,
–These conversions take time and slow down end-to-end
transmission.
•Moreover, customer’s home and the local telephone company
end office are connected by,
–Slow-speed analog line.