Friday 14 March 2014

What is Patch Cable ?

 patch cable or patch cord or patch lead is an electrical or optical cable used to connect ("patch-in") one electronic or optical device to another for signal routing. Devices of different types (e.g., a switch connected to a computer, or a switch to a router) are connected with patch cords. Patch cords are usually produced in many different colors so as to be easily distinguishable, and are relatively short, perhaps no longer than two metres. Types of patch cords include microphone cables, headphone extension cables, XLR connectorTiny Telephone (TT) connector,RCA connector and ¼" TRS phone connector cables (as well as modular Ethernet cables), and thicker, hose-like cords (snake cable) used to carry video or amplified signals. However, patch cords typically refer only to short cords used with patch panels.
Patch cords can be as short as 3 inches (ca. 8 cm), to connect stacked components or route signals through a patch bay, or as long as twenty feet (ca. 6 m) or more in length for snake cables. As length increases, the cables are usually thicker and/or made with more shielding, to prevent signal loss (attenuation) and the introduction of unwanted radio frequencies and hum (electromagnetic interference).
Patch cords are often made of coaxial cables, with the signal carried through a shielded core, and the electrical ground or earthed return connection carried through a wire mesh surrounding the core. Each end of the cable is attached to a connector so that the cord may be plugged in. Connector types may vary widely, particularly with adapting cables.

Patch cords may be:

Sunday 19 January 2014

Stratums in NTP (Network Time Protocol)


Network Time Protocol is a hierarchical protocol and is divided into stratum which define the distance from the reference clock. A reference clock source that relays UTC (Coordinated Universal Time) time and has little or no delay is known as a stratum-0 device. Stratum-0 servers cannot be used on the network, instead, they are directly connected to computers which then operate as primary time servers.
A primary server that receives a time signal from a stratum 0 device either through the GPS network or national time and frequency transmission is known as a stratum-1 device. On a network a stratum 1 time server supplies the time to other devices on the network which are known as stratum-2 devices. These also can be used as a time source and equipment that connects to a stratum-2 device to receive it become stratum-3 and so on.
NTP can handle up to 16 different stratum levels, although the lower down the hierarchy you go the less accurate the devices become. However, to make the system more reliable, each client can receive a time source from multiple servers. Stratum 2 devices and below can also synchronise with each other. The NTP software monitors continuously the figures of stability and accuracy of all the servers and always chooses a server with the best figures.
Multiple stratum are used an in larger networks because to bombard a single stratum-1 time server with NTP requests from thousands of machines could cause it to overload or block the network itself with workstations/routers etc repeatedly waiting for their time synchronisation requests to go through.

Thursday 2 January 2014

Star Delta Transformer ?

delta-wye transformer is a type of three-phase electric power transformer design that employs delta-connected windings on its primary and wye/star connected windings on its secondary. Aneutral wire can be provided on wye output side. It can be a single three-phase transformer, or built from three independent single-phase units. An equivalent term is delta-star transformer.


Delta-wye transformers are common in commercial, industrial, and high-density residential locations, to supply three-phase distribution systems.
An example would be a distribution transformer with a delta primary, running on three 11kV phases with no neutral or earth required, and a star (or wye) secondary providing a 3-phase supply at 415 V, with the domestic voltage of 240 available between each phase and the earthed (grounded) neutral point.
The delta winding allows third-harmonic currents to circulate within the transformer, and prevents third-harmonic currents from flowing in the supply line.[2] Delta connected windings are not common for higher transmission voltages (138 kV and above) owing to the higher cost of insulation compared with a wye connection