Why star topology is fastest?

Star networks

In a star network, each instrument and actuator is connected directly to the supervisory computer by its own signal cable. One apparent advantage of a star network is that data can be transferred if necessary using a simple serial communication protocol such as RS232. This is an industry standard protocol and so compatibility problems do not arise, but it represents old technology in which data transfer is slow. Because of this speed problem, parallel communication is usually preferred even for star networks.

Although star networks are simple in structure, the central supervisory computer node is a critical point in the system and failure of this means total failure of the whole system. When any device in the network needs to communicate with another device, a request has to be made to the central supervisory computer and all data transferred are routed through this central node. If the central node is out of operation for any reason, data communication in the network is stopped.

16.2.2 Network Topologies

3.6.4 RANDOM ACCESS OF A SHARED FIBER SYSTEM VIA CODE-DIVISION MULTIPLEXING

When a transport medium is used as a shared broadcast medium, such as a star network, the benefit is that every user can hear the same information. The drawback is that every user signal acts as interference to other users signals. There are several standard techniques employed to work around the interference problem. Time-division and frequency-division techniques are commonly used. Code-division multiplexing, first used in defense communications and more recently in cellular communications, is a potential candidate for lightwave networks. In this scheme [3.20], each user encodes his or her messages using a unique signature code and broadcasts the resulting signal into the medium. The receiver uses a decoder to sort out the intended user signal, treating all other user signals as noise. This random access scheme is particularly attractive when time synchronization is difficult, as in the case of a sizable all-optical network. Generally, there is a significant bandwidth expansion of the message rate [as much as the number of users sharing the medium], to accommodate many users in the network; thus, this method is less attractive for high-rate lightwave systems except when it is being used in the low-rate signaling channel for network management and diagnostics. The ability to work without time synchronization is an attractive feature for network management because of its ease of operation, particularly during network cold starts.

20.7.1 Architecture

The coaxial amplifier cascade beyond the node is limited to 4, and the entire node distribution system contains 53 amplifiers. Three power supplies are required to power all the active devices, with a maximum power supply cascade of two. The total number of series-connected taps in any one distribution leg is about 20.

The initial analysis is based on the use of a generator, but no uninterruptible power supply [UPS], at the headend and field standby power supplies with 2-hour battery capacity. It is assumed that this results in a 30-second headend outage every time the commercial power fails [three times per year] until the generator kicks in, and that the field standby power supplies have the effect of reducing the field failure rates to 50% per year in each location, with 1 hour of unprotected outage when the batteries do run down [based on dispatching a crew with portable generators as a result of customer-reported outages]. It is assumed that there is no status monitoring of power supplies that would have allowed crews with portable generators to back-power supplies before the batteries expire.

System Description - Concurrent Communicating Process Specification

Triggered by an external interrupt, the read_decorr process reads periodically [at a rate of 3.4 kHz] the memory mapped decorrelator FPGA. This process sends data to the track_pilot&demod and the traffic_demod processes, which perform the tracking of the transmission carrier and the demodulation [i.e. gain, carrier phase and bit phase correction]. After a 1:3 rate conversion the demodulated traffic data is formatted by the traffic_manage_data process and via the send_vocoder process transmitted to a second, memory mapped processor. In contrast, the demodulated pilot data will be further processed on the same processor.

The track_pilot&demod process not only delivers its demodulated data to the pilot_manage_data process, it steers the frequency of the NCO [numerical controlled oscillator] in the preceeding analog demodulation part through use of the on-chip serial peripheral. Moreover, together with traffic_demod process it sends information concerning carrier synchronization to the display_LEDs process and write_antenna process. The channel decoding of demodulated pilot data is carried out by the pi1ot_DSP_functions process, which operates on a 1024 element frame basis, so a multi-rate transition is present between the pilot_manage_data and this latter process. The output data of the pilot channel decoding is sent to a PC computer using the on-chip DMA engine. The setup_DMA process is triggered when output data is available from the pilot_DSP_functions process and sets up and starts the DMA process.

Asynchronously with this chain of periodic processes, the read_sys_cmd and read_antenna process control the internal parameters of the demodulation processes. They respectively perform the man-machine interface connected to the system using a memory mapped flag, allowing the user to alter the system operating parameters, and the interface with the antenna controller which is connected via an external interrupt. The former is a sporadic process, since a user will adapt the parameters only once in a while, and is allowed to have a large response time. The latter is a time-critical process: when the antenna controller looses the beam, it will signal this immediately to the demodulator, which must take special re-tracking actions.

1.7 High-Level Architecture Changes

Through a continuing process of industry consolidation, individual cable systems have grown steadily larger. This, combined with decreasing node sizes, has made it simply impractical to serve every node directly from the headend in a simple star network. Not only is the cost of running multiple, dedicated fibers to each node very high, the huge optical cables that would be required in a major metropolitan system would create major single points of failure.

As a result, operators have developed distributed networks, in which at least some of the headend functionality is moved to multiple hubs. Typically, the transport between headend and each hub is entirely optical and redundant, with route-diverse transport for all critical signals. High levels of multiplexing, whether at the baseband digital level, through wavelength division multiplexing, or a combination of both, reduces the required fiber counts between major facilities. In the largest systems, the core headend signal-processing requirements are sometimes duplicated at two locations, so the entire headend is redundant.

Bluetooth [IEEE802.15.1 and .2]