Point to Point (SCPC) VSAT links
Single Channel per Carrier (SCPC) technology provides dedicated, point to point connectivity using Very Small Aperture Terminals (VSAT), making it the wireless equivalent of conventional leased lines. Using SCPC, communication links can be provisioned anywhere in the world with typical bandwidth rates ranging from 64 kbps to 2048 kbps.
The typical components of a SCPC link are the Ground Segment and the Space Segment.
The Ground Segment or Earth Station includes
- Antenna assembly: converts the Radio Frequency (RF) signal to electrical signal and vice versa
- Radio Frequency Terminal (RFT): performs Frequency Up Conversion, Frequency Down Conversion and RF Signal Power Amplification
- Satellite Modem: performs various functions including modulation/demodulation and Error Correction
- Terminal Equipment: includes the routers/switches, servers and hosts on the customer’s network
Each satellite in orbit carries a number of transponders, with each transponder supporting typically 40 MHz of bandwidth. This bandwidth is divided into separate channels using Frequency Division Multiple Access (FDMA). Depending on the requirements, different number of channels with various bandwidths can be allocated to the respective customers. The channel(s) and associated bandwidth leased to the customer(s) comprises the Space Segment.
Additionally, SCPC VSAT communication utilizes Geostationary satellites. Orbiting at the height of 35,786 km above the equator, the Geostationary satellite travels in the same direction and at the same speed as the Earth's rotation on its axis, taking 24 hours to complete a full trip around the globe. Thus, as long as a satellite is positioned over the equator in an assigned orbital location, it will appear to be "stationary" with respect to a specific location on the Earth. A single SCPC VSAT link can provide connectivity for any two sites located within the footprint (coverage area) of a Geostationary satellite.
- Distance Independent: links can be provisioned between any two points within the footprint of a satellite. With typical satellite footprints covering thousands of square kilometers and spanning multiple countries, the technology is essentially distance independent
- Quick deployment and commissioning: typical deployment time for a link (possibly spanning thousands of kilometers) is only two to three days. This enables rapid response to market needs.
- Rapid and direct access to telecommunications
- High Reliability: minimal points of failure make SCPC links inherently reliable
- Transparent: the technology offers a transparent transmission medium
- No “last mile” problem
Following are some of the prominent technical characteristics of SCPC VSAT links
- Frequency Bands: SCPC links typically operate using the C-Band and Ku-Band frequency ranges.
- C-band uses 4 GHz for the downlink and 6 GHz for the uplink. It is the most commonly used frequency band for SCPC operation and offers the following benefits
o Well Established Technology
o Lower Rain Attenuation
o Lower Space Segment Cost
o Available Everywhere
o Wide Coverage
- Following are some of the considerations when using C-Band for SCPC links
o Interference from terrestrial links operating in the 3 GHz band
o Larger Antennas
Ku-band uses 12 GHz for the downlink and 14 GHz for the uplink.
o Lower or no Terrestrial Interference
o More Satellite Power
o Smaller Antennas
- Following are some of the considerations when using Ku-Band for SCPC links
o Higher Rain Attenuation
o Higher Space Segment Cost
o Limited Coverage Area
- Antenna types: Various antenna types are used for SCPC link operation. The three most commonly used types are described below
- Parabolic/Center feed: The parabolic shape of the dish reflects the signal to the dish’s focal point. Mounted on brackets at the dish's focal point is a device called a feedhorn. This feedhorn is the front-end of a waveguide that gathers the signals at or near the focal point
- Cassegrain: In a Cassegrain antenna, the parabolic shape of the dish reflects the signal to the dish’s focal point. Mounted on brackets at the dish's focal point is a secondary reflector (or sub-reflector). This secondary reflector directs the signal towards the feedhorn
- Offset feed: In an offset antenna, the reflector focuses the signals on to a point that is offset from the center of the antenna.
- Antenna Sizes: The typical antenna sizes (diameter) are as follows
- C-band: 1.8 - 2.4 m
- Ku-band: 1.2 - 1.8 m
- lnterfaces: a variety of interfaces are available on SCPC links including Ethernet, G.703 (T1, E1, T3 and E3) and Serial interfaces (EIA-422 and EIA-232, HSSI, V.35)
- Round trip delay: the typical Round Trip Delay (RTD) for a SCPC VSAT link is 540 ms.
Applications and Services
SCPC VSAT can be used for a very wide range of applications and services, including (but not limited to) the following:
- High Speed Point to Point services: for reasons of availability, security and economics, customers may choose VSAT over other means of communication
- Corporate Data service: some examples include
- File transfer
- LAN extension
- Video Applications: current compression technologies enable data rates as low as 64 kbps. However, the best tradeoff between quality and cost is achieved at 384 kbps. SCPC VSAT users generally implement outbound video at 384 kbps and inbound video at 64 kbps.
- Interactive Voice: this category consists of the following voice services
- Voice services for private networks and corporations
- Voice services to extend the PSTN facilities to rural or remote areas
- High-speed internet access
- Supervisory Control and Data Acquisition (SCADA): data retrieval from remote sensing on oil drillings, pipe lines, gas, electric and transport industries.
- Interactive distance learning
- Cellular network backhaul: backhaul from Base Stations sites in remote areas not served by telecommunication infrastructure
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