Architecture of IP transmission
Architecture of IP transmission

Triple Play (4)


It is essential to have a modem cable to be able to benefit from the Internet and cable telephony.
Indeed digital data signals are transmitted through signals on the cable radio frequency spectrum. Cable modems allow you to convert digital data into radio frequency modulated signal and vice versa.

Cable modems are bidirectional and allow sends it and receive IP data at the same time. These modems can reach speeds of 43 Mbps in amount and 10 Mbps down.
They use techniques PSK or QAM (Quadrature Amplitude Modulation) (Phase Shift Keying). The cable modem is modulation between cable and network Ethernet Converter.
The data are transmitted on different cable network frequencies than those used by the TV.

For transmitting voice operators have chosen to rely on existing technologies. It is through the voice over IP telephony with cable operators settled.
IP phones, faxes, computers and televisions are connected to the modem via an Ethernet bus cable.
An application DOCSIS layer allows to connect the HFC network to an IP via the Headend or CMTS (Cable Modem Termnation System) network.
It is the head of network equipment. It is located in the local station and connects all the subscribers of the area. It will convert the data from the IP network to signal radio frequency to transmit on the HFC network. He will also perform the reverse operation. It allows you to communicate with other networks such as the Internet or the network analog CATV HFC network.

The CMTS is the equivalent of the DSLAM for xDSL technologies. Typically for domestic consumers the speed of the stream swallow is limited between 512 Kbps to 10 Mbps and upstream flow from 256 Kbps to 1 Mbps.
There are three types of communication that spread on a wired network :
• Downstream Channel Broadcast : allowing the diffusion of television.
• Interactive Downstream Channel : communication from the operator to the Subscriber.
• Upstream Channel Return : communication from the Subscriber back to the operator.

Upstream Return and Downstream Interactive bandwidths are distributed by the CMTS for different Subscribers.








DVB/DAVIC


The technology of \Digital Video Broadcast\ (DVB) and \Digital Audio Video Council\ (DAVIC) is the European standard for digital television. They have to be in line with the guidelines and European standards.
Specification DVD 2.0 also called DVD - RCC (Return Channel for Cable) is standardized under the name ETSS 200800.
The standard is based on a layer ATM using LLC/SNAP (RFC 1483) and an AAL5 adaptation layer to encapsulate the IP packet (IP switching to an IP routing on an ATM switch).
For the communication between the subscriber terminal (STM) and the facilities of the local station (TDR), he uses the ATM signaling protocol or a proxy with DSM - CC.










DOCSIS and PacketCable


The consortium CableLabs has been created in May 1988 and brings together operators, actors or the cable TV industry.
He has established several specifications for the transport of IP packet : DOCSIS and PacketCable.

The American standard DOCSIS (Data Over Cable Service Interface Specification) defines the requirements for the interfaces of the cable modems used for the dissemination of data at great speed by cable TV.
In March 98 ITU (International Telecommunications Union) accepts DOCSIS as standard for modems cables under the name ITU or DOCSIS 1.0.
To deliver data services DOCSIS Wired on a network, a channel of radio frequency of 6 MHz in the 50-750 MHz frequency band is allocated to the descending traffic and another channel of 6 Mhz in the band 5-42 MHz for the traffic amount.

Headend equipment CMTS communicating through these channels with modems cables located at the Subscriber.
These are external equipment that connect to the computer through an Ethernet card or a USB interface. The method used by DOCSIS TDMA(Time Division Multiple Access)/SCDMA(Synchronous Code Division Multiple Access).
This access method differs from the Ethernet system, as the DOCSIS system experiences no collision.
The software in initialization stage DOCSIS is usually (highly simplified) :

• 1 the modem sends a request DHCP way to know the network configuration to use.
• 2 the CMTS returns its local IP address (not to be confused with the IP address of the computer on the Internet), its gateway, and more specifically the IP address of the server TFTP and the name of the configuration file has to go look for.
• 3 the modem connects to the server TFTP and request the configuration file named above, this file contains, among other relative information the connection speed of the modem, its priority on the network, and the number of authorized computers access the modem at the same time.
4. the modem • informs him CMTS that it has received the file, and that it is ready to operate (synchronization phase).

After this, the computers connected to the modem may themselves request their login information via the DHCP, and act like on a quite conventional LAN.
Finally in February 2002 arrives DOCSIS 2.0. This new standard brings a symmetry of the bandwidth between the rising and falling tracks.
This is due to the evolution of Internet usage by users with the advent of the peer-to-peer, voice over IP, video conferencing...

IPsec must ensure the security of data outside the cable network. H.263 for video compression and G.711, G.726 and G.729 for audio compression.
Finally for managing centralized telephone internetwork signalling CableLabs uses MGCP. MGCP (Medio Gateway Control Protocol) to ensure signaling cable telephony services.






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