Facts on Data Communication & Networks:
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Data Communication & Networks is one of the important subjects in UGC NET exam on which lots of questions are asked. So, prepare this subject well. The syllabus of the subject is vast, but is one of the easiest subjects to prepare.
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ETHERNET: Ethernet frames must carry a minimum payload of 46 bytes, which ensures that a valid Ethernet frame is 512-bits long (considering bits of header section also). The shortest Ethernet frame is 64 bytes in length, which carry Control messages.
Ethernet frames don't have a way to indicate end-of-frame, but an inter-frame
gap (of time required to send 96 bit of data, i.e. 9.6 micro sec.) is
used.
Horn
antennas
are very popular at UHF (300 MHz-3 GHz) and higher frequencies. They have a
wide impedance bandwidth, implying that the input impedance is slowly varying
over a wide frequency range.
S-parameters
describe the input-output relationship between ports (or terminals) in an
electrical system. A port can be defined as any place where we can deliver
voltage and current.
Waveforms
are not made up of a discrete number of frequencies, but rather a continuous
range of frequencies (sum of frequencies).
Through
modulation the waveforms can be relocated to separate frequency bands.
Television is broadcast primarily at 54-216 MHz. FM radio operates between
87.5-108 MHz.
Frequency Band Name
|
Frequency Range
|
Wavelength (Meters)
|
Application
|
Extremely Low Frequency (ELF)
|
3-30 Hz
|
10,000-100,000 km
|
Underwater Communication
|
Super Low Frequency (SLF)
|
30-300 Hz
|
1,000-10,000 km
|
AC Power (though not a transmitted wave)
|
Ultra Low Frequency (ULF)
|
300-3000 Hz
|
100-1,000 km
|
Earth Mode Communications*
|
Very Low Frequency (VLF)
|
3-30 kHz
|
10-100 km
|
Navigational Beacons
|
Low Frequency (LF)
|
30-300 kHz
|
1-10 km
|
AM Radio
|
Medium Frequency (MF)
|
300-3000 kHz
|
100-1,000 m
|
Aviation and AM Radio
|
High Frequency (HF)
|
3-30 MHz
|
10-100 m
|
Shortwave Radio
|
Very High Frequency (VHF)
|
30-300 MHz
|
1-10 m
|
FM Radio
|
Ultra High Frequency (UHF)
|
300-3000 MHz
|
10-100 cm
|
Television, Mobile Phones, GPS
|
Super High Frequency (SHF)
|
3-30 GHz
|
1-10 cm
|
Satellite Links, Wireless Communication
|
Extremely High Frequency (EHF)
|
30-300 GHz
|
1-10 mm
|
Astronomy, Remote Sensing
|
Visible Spectrum
|
400-790 THz (4*10^14-7.9*10^14)
|
380-750 nm (nanometers)
|
Human Eye
|
*
Communications through the ground using conduction fields e.g. military
communications.
Slot Time: It is twice the
time it takes for an electronic pulse to travel the maximum distance between
two nodes. Thus Propagation delay takes half of the time of slot time since it
is only the measure of the time required for signal to reach from node A to B.
Slot time is used for half-duplex Ethernet network operation. It is 512 bit
times for Ethernet networks operating below 1 Gbit/s, and 4096 bit times for
Gigabit Ethernet. To reliably detect collisions, the minimum transmission time
for a complete frame must be at least one slot time, whereas the round-trip
propagation delay must be less than a slot time (half of slot time).
Back-off Algorithm: Once a collision is detected by simultaneous transmitters, they will follow a backoff algorithm, which requires each transmitter to wait an integral number of slot times (51.2 µs) before attempting a new transmission sequence. The integer is determined by the equation:
0<=r<2 power k where k = min (n, 10)
The variable k is actually the number of collisions capped at a maximum of 10. Therefore, r can range from 0 to 1023. The actual value for r is determined by a random process within each Ethernet node. As the number of consecutive collisions increases, the range of possible backoff times increases exponentially. The number of possible retries is max. 16.
There are no collisions with a full-duplex link, where each node is paired with a port on the hub.
Collision Domain - a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, in particular.
The 5-4-3 rule: A system can have up to five segments in series, with up to four repeaters and no more than three mixing segments (a segment that may be connected to more than two transceivers).
The FCS field in Ethernet frame provides only bit-level error detection, no error recovery.
Back-off Algorithm: Once a collision is detected by simultaneous transmitters, they will follow a backoff algorithm, which requires each transmitter to wait an integral number of slot times (51.2 µs) before attempting a new transmission sequence. The integer is determined by the equation:
0<=r<2 power k where k = min (n, 10)
The variable k is actually the number of collisions capped at a maximum of 10. Therefore, r can range from 0 to 1023. The actual value for r is determined by a random process within each Ethernet node. As the number of consecutive collisions increases, the range of possible backoff times increases exponentially. The number of possible retries is max. 16.
There are no collisions with a full-duplex link, where each node is paired with a port on the hub.
Collision Domain - a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, in particular.
The 5-4-3 rule: A system can have up to five segments in series, with up to four repeaters and no more than three mixing segments (a segment that may be connected to more than two transceivers).
The FCS field in Ethernet frame provides only bit-level error detection, no error recovery.
UDP is also known as laissez-faire protocol
TCP is used for unicast addresses only, so multicast applications must use the
UDP transport protocol.
In asynchronous transmission, the Start bit always has a value of 0 (a Space). The Stop Bit always has a value of 1 (a Mark). This means that there will always be a Mark (1) to Space (0) transition on the line at the start of every word.
Application layer is free to send any size of data, there is no upper limit
defined by standards. The lower layers divides the data if needed.
A channel with x bps may not necessarily transmit data at x rate, since protocols, encryption, and other factors can add may overheads.
A channel with x bps may not necessarily transmit data at x rate, since protocols, encryption, and other factors can add may overheads.
The asymptotic bandwidth (formally asymptotic throughput) for a network is the
measure of maximum throughput for a greedy source (a traffic generator that
generates data at the maximum rate possible and at the earliest opportunity
possible).
CIDR: Classless Inter-Domain Routing, known as supernetting, is a solution to limited address space problem in a network. It allocates address space to ISPs and end users on any address bit boundary, instead of on 8-bit segments (which is class based addressing). It appends to the IP address a slash character and the decimal number as routing prefix, e.g., "192.168.2.0/24" for IPv4, and 2001:db8::/32 for IPv6. The value after / indicates how many bits are used for the network prefix, leaving the remaining bits to identify the specific host.
CIDR: Classless Inter-Domain Routing, known as supernetting, is a solution to limited address space problem in a network. It allocates address space to ISPs and end users on any address bit boundary, instead of on 8-bit segments (which is class based addressing). It appends to the IP address a slash character and the decimal number as routing prefix, e.g., "192.168.2.0/24" for IPv4, and 2001:db8::/32 for IPv6. The value after / indicates how many bits are used for the network prefix, leaving the remaining bits to identify the specific host.
CIDR currently uses prefixes anywhere from 13 to 27 bits. This solution fits an
organization's specific needs. It helps in reducing number of entries in global
routing tables. It is the concept of subnetting within the internet itself.
The industrial, scientific and medical (ISM) radio bands are radio bands (portions of the radio spectrum) reserved internationally for industrial, scientific and medical purposes other than communications. These are for unlicensed operations. Cordless phones, Bluetooth devices, near field communication (NFC) devices, and wireless computer networks all use frequencies allocated to low power communications as well as ISM.
The industrial, scientific and medical (ISM) radio bands are radio bands (portions of the radio spectrum) reserved internationally for industrial, scientific and medical purposes other than communications. These are for unlicensed operations. Cordless phones, Bluetooth devices, near field communication (NFC) devices, and wireless computer networks all use frequencies allocated to low power communications as well as ISM.
Hartley's law- "The maximum
data rate of a physical communication link is proportional to its bandwidth in hertz, which is sometimes called frequency
bandwidth, spectral bandwidth, RF bandwidth, signal
bandwidth or analog bandwidth."
A
Baud Rate represents the number of
bits that are actually being sent over the media, not the amount of data that
is actually moved from one DTE device to the other. That means, baud rate
decides the actual bit rate. For example, the bit rate is 9600
The Intelligent Network (IN) is the
standard network architecture which allows
telecom operators to differentiate themselves by providing value-added services in addition
to the standard telecom services. The intelligence is provided by network nodes
on the service layer (a conceptual
layer within a network service provider architecture. It aims at providing middleware that serves
third-party value-added services and
applications at a higher application layer.)
The Internet
protocol suite ( TCP/IP Model), occasionally known as the DoD model due to the
foundational influence of the ARPANET. The TCP/IP model and related protocols
are maintained by the Internet Engineering Task Force (IETF).
PORT: Each process that wants to communicate with
another process identifies itself to the TCP/IP protocol suite by one or more
ports. Application Layer talks with Transport layer through ports. A port
number helps the transport layer protocols (like TCP) to know the type of
content residing inside the packet.
A port is a
16-bit number, used by the host-to-host protocol to identify to which higher
level protocol or application program (process) it must deliver incoming
messages. There are two types of ports. Well-known port numbers(0-1023) are
typically odd, because early systems using the port concept required an
odd/even pair of ports for duplex operations.
The well-known
ports are controlled and assigned by the Internet Assigned Number Authority
(IANA) and on most systems can only be used by system processes or by programs
executed by privileged users. Ephemeral ports are opposite to well-known ports.
Such port number are used by clients and are contained in the UDP datagrams
sent to the server.
Normally, a
server will use either TCP or UDP, but there are exceptions. For example,
domain name servers use both UDP port 53 and TCP port 53.
SOCKET: A socket is a special type of file handle, which is used by a process to request network services from the
operating system. A socket
address is the triple:
<protocol, local-address, local-process>. For example, in the TCP/IP
suite:
<tcp,
193.44.234.3, 12345>
An association is the 5-tuple that completely specifies
the two processes that comprise a connection:
<protocol,
local-address, local-process, foreign-address, foreign-process>. In the
TCP/IP suite, the following could be a valid association:
<tcp,
193.44.234.3, 1500, 193.44.234.5, 21>
Two processes
communicate via TCP
sockets. The socket
model provides a process with a full-duplex byte stream connection to another
process.
UDP: UDP is basically an application interface to
IP. It adds no reliability, flow-control, or error recovery to IP. It simply
serves as a multiplexer/demultiplexer
for sending and
receiving datagrams, using ports to direct the datagrams. It requires the
application to take responsibility for error recovery and so on.
Slow-start
is one of the algorithms that TCP uses to control congestion inside the
network. It is also known as the exponential growth phase.
Broadband means "having
instantaneous bandwidths greater than 1 MHz and supporting data rates greater
than about 1.5 Mbit/s." In telecommunication, Broadband refers to a
communication bandwidth of at least 256 kbit/s. Each channel is 6 MHz wide.
The
additive-increase/multiplicative-decrease (AIMD)
algorithm is a feedback control algorithm used by TCP for Congestion Avoidance. The approach
taken is to increase the transmission rate (window size), probing for usable
bandwidth, until loss occurs.
IP
address classes:
Class
|
Leftmost bits
|
Start address
|
Finish address
|
A
|
0xxx
|
0.0.0.0
|
127.255.255.255
|
B
|
10xx
|
128.0.0.0
|
191.255.255.255
|
C
|
110x
|
192.0.0.0
|
223.255.255.255
|
D
|
1110
|
224.0.0.0
|
239.255.255.255
|
E
|
1111
|
240.0.0.0
|
255.255.255.255
|
IP address range for Intranets (Private
Networks):
Class
|
Private start
address
|
Private finish
address
|
A
|
10.0.0.0
|
10.255.255.255
|
B
|
172.16.0.0
|
172.31.255.255
|
C
|
192.168.0.0
|
192.168.255.255
|
IP packets addressed
by them cannot be transmitted onto the public Internet. If such a private
network needs to connect to the Internet, it must use either a network address
translator (NAT) gateway, or a proxy server.In IPV6, The address block fc00::/7 has been reserved for private networks.
IP
officially reserves the entire range
from 127.0.0.0 through 127.255.255.255 for loopback purposes.
IPv6
does not use classes.
IPv6 supports the following three IP address types: unicast, multicast and
anycast. IPv6 does not support broadcast. Multicast addresses in IPv6 start
with 'FF' (255) just like IPv4 addresses. Unicast addresses have 3 defined scopes, including link-local,
site-local and global; and multicast addresses have 14 scopes.
The
number of IPv6 addresses is 1028.
There is no ARP in V6. Currently, DHCP,
FTP, PPP, RIP, SNMP, VPN, L2TP and Telnet do not support IPv6.
IPv6 does not require NAT. NAT, too, doesn't
support V6. Currently, IPv6 packets are not forwarded.
IPv6
reserves just two special addresses: 0:0:0:0:0:0:0:0 and 0:0:0:0:0:0:0:1. IPv6
uses 0:0:0:0:0:0:0:0 internal to the protocol implementation, so nodes cannot
use it for their own communication purposes. IPv6 uses 0:0:0:0:0:0:0:1 as its
loopback address, equivalent to 127.0.0.1 in IPv4. The minimum size of an IP
datagram is 28 bytes, including 20 bytes of header.
BIND and NSD are the most widely used DNS
software on the Internet
Anycast
is a network addressing and routing methodology in which datagrams from a
single sender are routed to the topologically nearest node in a group of
potential receivers, though it may be sent to several nodes, all identified by
the same destination address. On the Internet, anycast is usually implemented
by using BGP.
In
denial-of-service attacks, a rogue
network host may advertise itself as an anycast server for a vital network
service, to provide false information or simply block service.
"6 to 4" is an Internet
transition mechanism for migrating from IPv4 to IPv6, a system that allows IPv6
packets to be transmitted over an IPv4 network. 6to4 does not facilitate
interoperation between IPv4-only hosts and IPv6-only hosts, but simply a
transparent mechanism used as a transport layer between IPv6 nodes.
The
network requests supporting DNS lookups run over TCP and UDP, port 53 by
default.
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