INTERNET CONNECTIVITY
Local Loop Connectivity - Internet Broadcast
Local Loop Connectivity - Internet VSATs |
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Over the last few years the use and acceptance of the Internet, particularly the World Wide Web (WWW), has increased dramatically. With this has come two significant traffic related problems for users and for the Internet Service Providers (ISPs) themselves.
The first problem results from the concentration of WWW sites and infrastructure in the US. Currently, some 80% of all WWW sites are located in the USA. It is common for a connection between, for example, two European countries to have to be routed via the US backbone network. Such a concentration of traffic around the US slows traffic flow for two reasons. As traffic hops from server to server on its journey to and from the USA delays are unavoidably introduced at each server. Some connections can require several dozen server to server hops. Secondly, links to and from the US and within the USA are subject to blocking at peak times resulting in very long delays at times.
Traffic dynamics are continuously evolving however, and as more content is developed on a regional basis, traffic flow is changing from being US-centric to being primarily intra-regional.
The second problem relates from the lack of access to local Internet Service Providers at a high enough data rate to be useful (particularly for the WWW) in some geographical areas. This is a result of archaic or inadequate local infrastructure in many countries.
This problem is being exacerbated, even in developed countries with a well developed access infrastructure, because of evolving user expectations. The introduction of broadband terrestrial services, even if only in major population centres, has created much media hype with the result that frustration with the "World Wide Wait" is growing and unsatisfied demand for broadband is being cultivated.
Satellite solutions have been implemented to ease both of these problems.
Firstly, high speed trunks are used to provide high data rate direct connections between gateways on the US Internet backbone and European ISPs. Intra-regional trunks between major European Internet gateways are also being established, though much of this traffic is being carried by fibre.
Secondly, satellite connections are used to provide the local loop between national ISPs and their commercial and private customers. Three technologies are available for this:
These are discussed in detail in the sections below.
IP over DVB Transmission
For IP applications it is becoming common practice to use an IP encapsulator to convert raw IP data into a DVB/MPEG-2 transport stream which is then transmitted by satellite.
This implementation is becoming a de facto industry standard for Internet/Intranet/Extranet and multimedia transmission.
IP Encapsulation
MPEG-2 is a video standard which defines the compression of the video data both spatially (within each image frame) and temporally (from frame to frame). It also provides for the compression of stereo audio and the multiplexing of this with the video data stream. Finally it provides for an integrated transport mechanism for the multiplexing of the video, audio and data. The DVB standard for satellite delivery is an extension of MPEG 2 and specifies additional implementation functions including modulation, forward error correction and service information specification.
Each MPEG 2 packet is 188 bytes long, consisting of 4 bytes of header and 184 bytes of payload. The payload can have several formats, including:
A number of satellite service providers and operators now provide point to point links which allow non-US ISPs to connect directly to the US Internet backbone network. This greatly reduces the number of server-server hops that each packet has to make and, along with the extra bandwidth available, can greatly reduce response times.
Internet Backbone Configuration
Signal Types and Characteristics
Internet trunk links such as these to the US backbone are a type of point to point SCPC connection. Transmissions are generally duplex with a carrier in each direction. Because of the asymmetry in data traffic between transmissions to and from the US backbone, the data rates carried on the two SCPC carriers are different. Often the data rate from the US is a factor of ten or more times higher than the rate to the US.
Both carriers are transmitted continuously (duty cycle 100%) and generally carry IP over DVB transmission.
Internet Backbone Connectivity Spectrum
Internet trunks carrying traffic between European ISPs and the US backbone generally use large or medium sized earth stations rather than small terminals. These earth stations generally consist of:
Local Loop Connectivity - Internet Broadcast
Several satellite service providers and equipment manufacturers now offer products which allow a remote Internet user to connect directly into an ISP by satellite at rates up to several hundred kb/s.
Taking advantage of the fact that local loop Internet data flow is highly asymmetric (the data received by an Internet subscriber is generally one hundred or more times higher than that sent), current systems use a conventional modem link via the PSTN to send data requests to the ISP. The requested pages are then transmitted by broadcasting them over the satellite connection.
Some service providers take the concept a step further, caching commonly requested pages at the hub. Some service providers also cache all pages which are linked to pages being downloaded to increase the access speed perceived by the subscriber.
An alternative approach adopted by some service suppliers is to broadcast popular pages to each subscriber, where they are cached locally on each subscriber's hard drive.
Internet Broadcast Configuration
Signal Types and Characteristics
Internet broadcast networks generally have a single IP over DVB carrier which can have a data rate anywhere from 500 kb/s to 55 Mb/s depending on the equipment manufacturer and the traffic requirements.
Internet Broadcast Spectrum
The hub station consists of:
The gateway equipment for an earth station typically costs of the order of 10 kEuro. Costs for the antenna, RFT and civil works are additional to this.
The remote terminal consists of an antenna with an LNB mounted on the feed and a card which is mounted inside a PC. The typical cost for the remote equipment is less than 500 Euro.
Local Loop Connectivity - Internet VSATs
A few manufacturers now also supply interactive hubbed VSATs configured for carrying Internet type traffic. These VSATs are the same as interactive hubbed VSATs with the following main differences:
Other than these differences, the networks are very similar and the technology will not be discussed further here.
Internet VSAT Configuration
Signal Types and Characteristics
Internet VSAT Spectrum
Local Loop Connectivity - Interactive TV
A future development, which has been planned for some time by broadcasters, will see the traffic from ISP to users carried by satellite on channels which are shared by television broadcasters. Many broadcasters see the WWW as a natural extension to their normal activities which has the advantage that there are very few regulatory constraints. Interactive TV is seen as a way to integrate the WWW with conventional TV broadcasting. To ease this transition a number of broadcasters have recently become ISPs in their own right.
Initially many broadcasters intend to provide a terrestrial return link by modem over the PSTN using a similar network configuration to Internet Broadcast as described above. The ultimate aim, for satellite broadcasters, is to have the return link also carried by satellite.
Eventually each subscriber will, in effect, have a personal interactive VSAT terminal capable of receiving downloaded data at several hundred kb/s, but which will only transmit at a few kb/s. Both reception and transmission will be at Ku or Ka band.
SES, the Luxembourg operator of the Astra satellites will begin offering interactive TV based services using this technology in 2000 following the launch of the Astra 1H satellite.