by Dinesh Thakur

MIMO technology (Multiple Input Multiple Output) is not new, but it hits the market that at the end of the first decade of the 2000s due to a very complex implementation. MIMO aims to carry multiple streams in parallel on different antennas but using the same frequency.

 
by Dinesh Thakur

Random Access, which is to issue a completely random time, relies on the Aloha method. The latter takes its name from an experiment performed on a network connecting the various islands of the Hawaiian Archipelago early 1970. In this method, when a coupler has information to transmit, it sends it without worry about other users. If there is a collision, that is to say superposition of two signals or more users, the signals become indecipherable and are lost. They are subsequently transmitted, as shown in Figure, in which the couplers 1, 2 and 3 collide. The coupler 1 transmits its field first because he shot the smallest timer. Then, the module 2 emits, and its signals collide with the coupler 1. Both derive a random time of retransmission. The coupler 3 is listening while the couplers 1 and 2 are silent, so that the frame of the coupler 3 passes successfully. Technical aloha is the origin of all the random access methods.



 
by Dinesh Thakur

SONET/SDH standard was carry telephone speech, and it took many adaptations for the transport of frames and type of IP packets, ATM, or others. The successor of SONET/SDH has been launched in early 2002 and standardized by ITU-T as the OTN (Optical Transport Network). Its role is to channel packages on routes to 2,5, 10 and 40 Gbit/s. The corresponding recommendation G.709 carries the number.

 
by Dinesh Thakur

SDH recommendation was standardized by ITU-T (G.707 and G.708):

• G.707: Synchronous Digital Bit Rate;

• G.708: Network Node Interface for the Synchronous Digital Hierarchy.

Found in SDH flows at 155, 622 and 2488 Mbit/s SONET.

 
by Dinesh Thakur

To achieve simultaneous multiplexing multiple telephone lyrics on the same circuit, Americans have adopted a standard for multiplexing 24 channels of 64 kbit/s on a support at 1544 Kbit / s. This channel is called DS-1. Europeans have responded to this technique by the E-1 channel multiplexing 30 voice channels on a support 2048 Mbit/s. From this basic multiplexing, a hierarchy has been defined, it is a multiple of the base channel, as in the case of Europe, or a little more complex, as in the US case, because an area of supervision rate-dependent.

 
by Dinesh Thakur

In the 1970s, the Department of Defense, or DOD (Department Of Defense), decided before the proliferation of machines using different, incompatible communication protocols, to define its own architecture. This architecture, called TCP / IP, is the source of the Internet. It is also adopted by many private networks, called intranets.

 
by Dinesh Thakur

CRC (cyclical redundancy checking) is a rather sophisticated error checking routine used to assure that two communicating computers are really getting the correct data. There are a whole bunch of these error correction and checking methods out there, but they all have pretty much the same objective: making sure that no data is lost or misinterpreted when two computers are exchanging information over the phone lines (through a modem, of course).



 
by Dinesh Thakur

Data encapsulation simply means adding the IP header to the data. The IP header consists of five or six 32-bit words; the sixth word is attributed to the IP options field.

 
by Dinesh Thakur

TCP, an acronym for Transmission Control Protocol, corresponds to the fourth layer of OSI reference model. IP corresponds to the third layer of the same model. Each of these protocols has the following features:

 
by Dinesh Thakur

CDM is widely used in so-called second-generation (2G) and third-generation 3G wireless communications. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands. This is a combination of analog-to-digital conversion and spread spectrum technology.

 
by Dinesh Thakur

In case of TDM, time slots are allocated to channels, even if they have no information to transmit. This is just wastage of the bandwidth and to overcome this inefficiency of standard TDM, a technique known as STDM has been developed where time is allocated to lines only when it is required. This is achieved with the use of intelligent devices that are capable of identifying when a terminal is idle.



 
by Dinesh Thakur

For digital sources, two alternative technologies have evolved for multiplexing. These are Time Division Multiplexing (TDM) and Code division Multiplexing (CDM). TDM provides a way to merge data from several sources into a single channel for communication over telephone lines, a microwave system or a satellite system. TDM can be implemented in two ways. These are synchronous TDM and asynchronous TDM. Asynchronous TDM is popularly known as Statistical TDM.

 
by Dinesh Thakur

In Frequency Division Multiplexing (FDM), multiple channels are combined onto a single aggregate signal for transmission. The channels are separated in the aggregate by their frequency. It is explained in the Figure where a frequency dimension is subdivided into several non-overlapping frequency bands. Each channel ci is allotted its own frequency band as depicted in Figure.

 
by Dinesh Thakur

CDMA (Code Division Multiple Access) also called spread-spectrum and code division multiplexing, one of the competing transmission technologies for digital MOBILE PHONES. The transmitter mixes the packets constituting a message into the digital signal stream in an order determined by a PSEUDO-RANDOM NUMBER sequence that is also known to the intended receiver, which uses. it to extract those parts of the signal intended for itself. Hence each different random sequence corresponds to a separate communication channel. CDMA is most used in the USA.

 
by Dinesh Thakur

• In TDMA, the bandwidth of channel is dividend amongst various stations on the basis of time.

• Each station is allocated a time slot during which it can sent its data i.e. each station can transmit its data in its allocated time slot only.

 
by Dinesh Thakur

• In FDMA, the available bandwidth is divided into various frequency bands.

• Each station is allocated a band to send its data. This band is reserved for that station for all the time.

 
by Dinesh Thakur

• CSMA/CA protocol is used in wireless networks because they cannot detect the collision so the only solution is collision avoidance.

• CSMA/CA avoids the collisions using three basic techniques.



 
by Dinesh Thakur

To reduce the impact of collisions on the network performance, Ethernet uses an algorithm called CSMA with Collision Detection (CSMA / CD): CSMA/CD is a protocol in which the station senses the carrier or channel before transmitting frame just as in persistent and non-persistent CSMA. If the channel is busy, the station waits. it listens at the same time on communication media to ensure that there is no collision with a packet sent by another station. In a collision, the issuer immediately cancel the sending of the package. This allows to limit the duration of collisions: we do not waste time to send a packet complete if it detects a collision. After a collision, the transmitter waits again silence and again, he continued his hold for a random number; but this time the random number is nearly double the previous one: it is this called back-off (that is to say, the "decline") exponential. In fact, the window collision is simply doubled (unless it has already reached a maximum). From a packet is transmitted successfully, the window will return to its original size.



 
by Dinesh Thakur

NRZ-L (No return to zero level): this kind of encoding uses negative voltage to represent a binary 1 and positive voltage to represent a binary 0. As shown under: non return to zero is related with the voltage i.e. voltage never returns to a value of zero and the value of the voltage during a bit time is known as level. bit time is related with the amount of time one bit of data occupies.

 
by Dinesh Thakur

Multiplexing - To combine multiple signals (analog or digital) for transmission over a single line or media. A common type of multiplexing combines several low-speed signals for transmission over a single high-speed connection.

 
by Dinesh Thakur

Telephone line or telephone circuit (or just line or circuit within the industry) is a single-user circuit on a telephone communication system. Typically this refers to the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, and usually also implies a single telephone number for billing purposes reserved for that user.

 
by Dinesh Thakur

Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite is the engine for the Internet and networks worldwide. Its simplicity and power has led to its becoming the single network protocol of choice in the world today. TCP/IP is a set of protocols developed to allow cooperating computers to share resources across the network. 



 
by Dinesh Thakur

Carrier Sensed Multiple Access (CSMA) : CSMA is a network access method used on shared network topologies such as Ethernet to control access to the network. Devices attached to the network cable listen (carrier sense) before transmitting. If the channel is in use, devices wait before transmitting. MA (Multiple Access) indicates that many devices can connect to and share the same network. All devices have equal access to use the network when it is clear.