Understanding Virtual Channels, Cell Size in ATM Switching, and Radiocommunication Principles
There is the concept of a virtual channel and a virtual path.
- To grant the sequence of packets. Although there are no specific control mechanisms, cells maintain their order within virtual channels, preserving their positional sense.
- Congestion control and flow over the network:
- Strategies are incorporated to manage cell selection and discarding in connections causing network congestion.
- Users can be notified of congestion by flags in the delivered cells.
- Control of delayed cells delivered: It is possible to prioritize cells of some virtual connections over others, depending on the needs.
3 Why Such a Small Cell Size?
ATM switching transfers data in discrete 53-byte cells (48 bytes of data + 5-byte header) transmitted at high speed.
Given ATM’s flexibility, there’s a trade-off regarding cell size. Switching data benefits from a large payload, while switching voice and other delay-sensitive data requires minimal delay.
Advantages:
- Reduced delays caused by data bundling, storage, and forwarding.
- The average delay at a station bundling a 64 Kbits/s flow into ATM cells is:
It takes 6.6 ms to assemble and send each cell. - Packetization and forwarding delay at an intermediate station managing a 155 Mbits/s data flow is:
Therefore, delay is directly proportional to packet size, making voice communication impossible if cell sizes cause delays exceeding 30 ms.
Disadvantages:
- The header occupies almost 10% of the total cell length (5 of 53 bytes), resulting in 10% bandwidth overhead for control information.
Radiocommunications – General
Electromagnetic waves can be propagated through free space by appropriately sized antennas and received over long distances. In a vacuum, all electromagnetic waves travel at the speed of light, c, approximately:
m/s. In copper or fiber, the speed is two-thirds of c and is highly frequency-dependent.
Frequency (f), wavelength (λ), and the speed of light (c) are related by: fλ = c
1 Division of the Spectrum
The electromagnetic spectrum can transmit information by modulating signal frequency, amplitude, or phase.
2 Signal Propagation
Due to Earth and the ionosphere, radio propagation between transmitting antenna A and receiving antenna B can occur in the following ways:
2.1 Surface Wave
A wavefront spreading along the ground. This occurs at low frequencies where the ground acts as a good conductor. Wetter and more saline soil enhances conductivity. Signal attenuation varies with distance, frequency, and soil characteristics.
Disadvantages of using these frequencies include significant electrical noise and limited bandwidth.
2.2 Ionospheric Wave
A powerful propagation mode involving reflection of frequencies (3 MHz – 30 MHz) in the ionosphere. This atmospheric layer, starting around 80 km above Earth, is ionized by solar radiation. Used for long-distance and maritime communication, though satellite communication is becoming more prevalent.
2.3 Spatial Wave
Waves traveling directly through the troposphere, often with ground reflections. Atmospheric refraction, dispersion, and absorption affect this mode. Line-of-sight between antennas is usually required, typical for frequencies above 30 MHz.
Mobile Automatic 02/10 (TMA)
TMA manages numerous mobile subscribers over a large area automatically, addressing aspects like:
- Automatic switching and communication continuity
- Paging a mobile prior to communication
- Maintaining quality switching with automatic station selection
TMA systems require broad coverage, high traffic capacity, and limited frequencies, achieved through frequency reuse in cellular structures.
1 TMA System Elements
The basic elements are the Central Mobile (CTM), Base Stations (BS), Coverage Area (ZC), and Mobile Stations (MS).
1.1 Base Stations
These establish contact with mobile phones, determining radio coverage. They consist of a computer, transceiver, and antenna, connected to the CTM via dedicated circuits and to mobile stations via assigned radio channels.