Internet Delivery on CATV (Overview)

THE BASICS
A basic question that is often not answered is " What really IS the Internet? " The Internet is simply a network of computers that are linked together. This is easily the world's largest network of any kind. In fact, Internet traffic or content is the most rapidly growing phenomena in human history. It is estimated that the requirement for Internet content is doubling every 100 days !Any computer can be linked to this network of computers using a simple telephone line and a telephone modem

INTERNET INTERACTIVITY
When any computer (user) gets onto the Internet, the user requests to go to a particular site by typing in the site address. This information is sent into the Internet and information from that particular site is then transferred down into the computer. Clearly the signal travels in 2 directions. A signal is sent from the user computer to the Internet (up loaded). This is only a very small amount of information - the site address. On receiving the site address, a much larger amount of information is then fed into the users computer (down loaded). This brings into focus a very important fact - on the Internet most of the time, users simply down load information, i.e. is pick up information from the Internet and load it into their computers. Only very small amount of information is usually up loaded or sent by the user into the Internet.

DELIVERY THROUGH CATV
From the basic description of the Internet given above it is clear that any network that needs to deliver Internet content must be bi-directional i.e. the network must have a Reverse Path. The other fact that emerges is that the cable network needs to deliver only a relatively small amount of data in the Reverse Path. The bulk of the data transfer takes place in the Forward Path i.e. from the Cable Headend to the customer. which indicates how a telephone network or a Cable Headend located in between the Internet and the user helps deliver Internet content to the
customer

DOCSIS

Data Over Cable Service Interface Specification (DOCSIS) is an international standard developed by CableLabs and contributing companies that include ARRIS, BigBand Networks, Broadcom, Cisco, Conexant, Correlant, Harmonic, Intel, Motorola, Netgear, Terayon, and Texas Instruments. DOCSIS defines the communications and operation support interface requirements for a data over cable system. It permits the addition of high-speed data transfer to an existing Cable TV (CATV) system. It is employed by many cable television operators to provide Internet access (see cable internet) over their existing hybrid fibre coaxial (HFC) infrastructure. The first DOCSIS specification was version 1.0, issued in March 1997, with revision 1.1 (adding Quality of Service (QoS) capabilities) following in April 1999. Because of increased demand for symmetric services such as IP telephony, DOCSIS was revised to enhance upstream transmission speeds; DOCSIS 2.0 was released in December 2001. Most recently, the specification was revised to significantly increase transmissions speeds (this time both upstream and downstream) and introduce support for Internet Protocol version 6 (IPv6). This version, DOCSIS 3.0, was released in August 2006. Cross-version compatibility has been maintained across all versions of DOCSIS, with the devices falling back to the highest supported version in common between both endpoints: cable modem and cable modem termination system (CMTS).As frequency allocation band plans differ between U.S. and European CATV systems, DOCSIS standards have been modified for use in Europe. These changes were published under the name of "EuroDOCSIS". The main differences account for differing TV channel bandwidths; European cable channels conform to PAL TV standards and are 8 MHz wide, whereas in North-America cable channels conform to NTSC standards which specify 6 MHz. The wider bandwidth in EuroDOCSIS architectures permits more bandwidth to be allocated to the downstream data path (toward the user). EuroDOCSIS certification testing is executed by Excentis (formerly known as ComLabs), while DOCSIS certification testing is executed by CableLabs. Typically, CPE gear receives "Certification", while CMTS equipment receives "Qualification".

Frame relay

Frame Relay consists of an efficient data transmission technique used to send digital information. It is a message forwarding "relay race" like system in which data packets, called frames, are passed from one or many start-points to one or many destinations via a series of intermediate node points.

Network providers commonly implement frame relay for voice and data as an encapsulation technique, used between local area networks (LANs) over a wide area network (WAN). Each end-user gets a private line (or leased line) to a frame-relay node. The frame-relay network handles the transmission over a frequently-changing path transparent to all end-users.

With the advent of MPLS, VPN and dedicated broadband services such as cable modem and DSL, the end may loom for the frame relay protocol and encapsulation. However many rural areas remain lacking DSL and cable modem services. In such cases the least expensive type of "always-on" connection remains a 64-kbit/s frame-relay line. Thus a retail chain, for instance, may use frame relay for connecting rural stores into their corporate WAN.

Asynchronous Transfer Mode

In electronic digital data transmission systems, the network protocol Asynchronous Transfer Mode (ATM) encodes data traffic into small fixed-sized cells. The standards for ATM were first developed in the mid 1980s. The goal was to design a single networking strategy that could transport real-time video and audio as well as image files, text and email. Two groups, the International Telecommunications Union and the ATM Forum were involved in the creation of the standards.

ATM, as a connection-oriented technology, establishes a virtual circuit between the two endpoints before the actual data exchange begins. ATM is a cell relay, packet switching protocol which provides data link layer services that run over Layer 1 links. This differs from other technologies based on packet-switched networks (such as the Internet Protocol or Ethernet), in which variable sized packets (known as frames when referencing Layer 2) are used. ATM exposes properties from both circuit- and packet switched networking, making it suitable for wide area data networking as well as real-time media transport. It is a core protocol used in the SONET/SDH backbone of the public switched telephone network.

ATM ADDRESSING
A Virtual Channel (VC) provides the transport of ATM cells which have the same unique identifier, called the Virtual Channel Identifier (VCI). This identifier is encoded in the cell header. A virtual channel represents the basic means of communication between two end-points, and is analogous to an X.25 virtual circuit.

A Virtual Path (VP) transports ATM cells belonging to virtual channels which share a common identifier, called the Virtual Path Identifier (VPI), which is also encoded in the cell header. A virtual path, in other words, is a grouping of virtual channels which connect the same end-points, and which share a traffic allocation. This two layer approach can be used to separate the management of routers and bandwidth from the setup of individual connections.

Synchronous Digital Hierarchy

Synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH), are two closely related multiplexing protocols for transferring multiple digital bit streams using lasers or light-emitting diodes (LEDs) over the same optical fiber. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting larger amounts of telephone calls and data traffic over the same fiber wire without synchronization problems.

SONET and SDH were originally designed to transport circuit mode communications (eg, T1, T3) from a variety of different sources. The primary difficulty in doing this prior to SONET was that the synchronization source of these different circuits were different, meaning each circuit was actually operating at a slightly different rate and with different phase. SONET allowed for the simultaneous transport of many different circuits of differing origin within one single framing protocol. In a sense, then, SONET is not itself a communications protocol per se, but a transport protocol.

Due to SONET's essential protocol neturality and transport-oriented features, SONET was the obvious choice for transporting ATM (Asynchronous Transfer Mode) frames, and so quickly evolved mapping structures and concatenated payload containers so as to transport ATM connections. In other words, for ATM (and eventually other protocols such as TCP/IP and ethernet), the internal complex structure previously used to transport circuit-oriented connections is removed, and replaced with a large and concatenated frame (such as STS-3c) into which ATM frames, IP packets, or ethernet is placed.

Both SDH and SONET are widely used today: SONET in the U.S. and Canada and SDH in the rest of the world. Although the SONET standards were developed before SDH, their relative penetrations in the worldwide market dictate that SONET now is considered the variation.

Plesiochronous Digital Hierarchy

The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. The term plesiochronous is derived from Greek plesio, meaning near, and chronos, time, and refers to the fact that PDH networks run in a state where different parts of the network are nearly, but not quite perfectly, synchronised.

PDH is now being replaced by Synchronous Digital Hierarchy (SDH) equipment in most telecommunications networks.

PDH allows transmission of data streams that are nominally running at the same rate, but allowing some variation on the speed around a nominal rate. By analogy, any two watches are nominally running at the same rate, clocking up 60 seconds every minute. However, there is no link between watches to guarantee they run at exactly the same rate, and it is highly likely that one is running slightly faster than the other.

Implementation
The European and American versions of the PDH system differ slightly in the detail of their working, but the principles are the same. The European E-carrier system is described below.

The basic data transfer rate is a data stream of 2048 kilobits/s (kilobits/second). For speech transmission, this is broken down into thirty 64 kbit/s (kilobits/second) channels plus two 64 kbit/s channels used for signalling and synchronisation. Alternatively, the whole 2 Mbit/s (megabits/second) may be used for non speech purposes, for example, data transmission.

The exact data rate of the 2 Mbit/s data stream is controlled by a clock in the equipment generating the data. The exact rate is allowed to vary some percentage (+/- 50 ppm) on either side of an exact 2.048 Mbit/s. This means that different 2 Mbit/s data streams can be (probably are) running at slightly different rates to one another.

In order to move multiple 2 Mbit/s data streams from one place to another, they are combined together, or "multiplexed" in groups of four. This is done by taking 1 bit from stream #1, followed by 1 bit from stream #2, then #3, then #4. The transmitting multiplexer also adds additional bits in order to allow the far end receiving multiplexer to decode which bits belong to which 2-Meg data stream, and so correctly reconstitute the original data streams. These additional bits are called "justification" or "stuffing" bits.

Because each of the four 2 Mbit/s data streams is not necessarily running at the same rate, some compensation has to be made. The transmitting multiplexer combines the four data streams assuming that they are running at their maximum allowed rate. This means that occasionally, (unless the 2 Mbit/s really is running at the maximum rate) the multiplexer will look for the next bit but it will not have arrived. In this case, the multiplexer signals to the receiving multiplexer that a bit is "missing". This allows the receiving multiplexer to correctly reconstruct the original data for each of the four 2 Mbit/s data streams, and at the correct, different, plesiochronous rates.

The resulting data stream from the above process runs at 8,448 kbit/s (about 8 Mbit/s). Similar techniques are used to combine four x 8 Mbit/s together, giving 34 Mbit/s. Four x 34 Mbit/s, gives 140. Four x 140 gives 565.

565 Mbit/s is the rate typically used to transmit data over a fibre optic system for long distance transport. Recently, telecommunications companies have been replacing their PDH equipment with SDH equipment capable of much higher transmission rates.

LNB (Low-noise block converter)

A low-noise block converter (LNB, for low-noise block, or sometimes LNC, for low-noise converter) is the (receiving, or downlink) antenna of what is commonly called the parabolic (actually paraboloid) satellite dish commonly used for satellite TV reception. It is functionally equivalent to the dipole antenna used for most other TV reception purposes, although it is actually waveguide based. Whereas the dipole antenna is unable to adapt itself to various polarization planes without being rotated, the LNB can be switched electronically between horizontal and vertical polarization reception. The LNB is usually fixed on or in the satellite dish, for the reasons outlined below. The corresponding component in the uplink transmit link is called a Block upconverter (BUC).
Universal LNB

A universal LNB can receive both polarisations (Vertical and Horizontal) and the full range of frequencies in the satellite Ku band. Some models can receive both polarisations simultaneously (though rarely) through four different connectors Low/Hor, Low/Ver, High/Hor, High/Ver, and others are switchable (using 13 volt for Vertical and 17 or 18 volt for Horizontal) or fully adjustable in their polarisation (this is relatively rare as this requires a separate polarisor, and it's also not part of the Universal LNB specification).

Here is an example of Universal LNB specifications:
LO: 9.75 / 10.6 (or rarely 10.75) GHz (the 10.6 GHz Oscillator is selected by applying a 22 KHz tone to the cable)
Freq: 10.7 - 12.75 GHz (slightly wider for 10.75 GHz LOs)
NF: 0.7 dB (The Best LNBs have claimed values as low as 0.1 but it's unlikely that this is a true and reliable value)
Polarisation: Linear

Satellite dish

A satellite dish is a type of parabolic antenna that receives or transmits electromagnetic signals to and from another location typically a satellite. A satellite dish is a type of microwave antenna. Satellite dishes come in varying sizes and designs, and are commonly used to receive satellite television. Many of the offset type of satellite dishes are sections of a larger parabolic dish.
The parabolic shape of a 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 essentially the front-end of a waveguide that gathers the signals at or near the focal point and 'conducts' them to a low-noise block downconverter or LNB. The LNB converts the signals from electromagnetic or radio waves to electrical signals and shifts the signals from the downlinked C-band and/or Ku-band to the L-band range. Direct broadcast satellite dishes use an LNBF, which integrates the feedhorn with the LNB.  

The theoretical gain (directive gain) of a dish increases as the frequency increases. The actual gain depends on many factors including surface finish, accuracy of shape, feedhorn matching.

With lower frequencies, C-band for example, dish designers have a wider choice of materials. The large size of dish required for lower frequencies led to the dishes being constructed from metal mesh on a metal framework. At higher frequencies, mesh type designs are rarer though some designs have used a solid dish with perforations.

A common misconception is that the LNBF (low-noise block/feedhorn), the device at the front of the dish, receives the signal directly from the atmosphere. For instance, one BBC News countdown shows a "red data stream" being received by the LNBF directly instead of being beamed to the dish, which because of its parabolic shape will collect the signal into a smaller area and deliver it to the LNBF.

Modern dishes intended for home television use are generally 43 cm (18 in) to 80 cm (31 in) in diameter, and are fixed in one position, for Ku-band reception from one orbital position. Prior to the existence of direct broadcast satellite services, home users would generally have a motorised C-band satellite dish of up to 3 metres in diameter for reception of channels from different satellites. Overly small dishes can still cause problems, however, including rain fade and interference from adjacent satellites.