Instrumentation to measure the quality of networks CATV/HFC

Highlights :

Test & Measurement for O.F.

In this article some measuring equipment for the supervision and maintenance of networks CATV/HFC appear. The last new features incorporate capacities for the monitoring of digital channels, signals VoIP and DOCSIS.

The modern networks of telecommunications HFC must be prepared to be able to offer an ample fan of applications and services to their subscribers. The majority of these services requires of the network the capacity to contact bidirectional between the head and the terminal teams of subscriber, and therefore they need the existence of an ascending return or communication channel. Like example, in figure 1 is the infrastructure of a bidirectional network HFC. The optical nodes are in charge to vice versa realise the conversion between the optical and electrical signal for the down link and for the connection of return, reason why they need an optical transmitter. The channel of return in networks HFC occupies the low part of the spectrum. This bandwidth shares all the homes served by a same optical node. The returns of different nodes arrive at the head by different multiplexados routes or from different wavelengths. In this way, a signal generated by the terminal team of a subscriber crosses the network of distribution of coaxial in ascending sense, crossing two-way amplifiers until arriving at the optical node. Here the signals of return of all the subscribers converge, who become optical signals by means of the return laser and they are transmitted towards the head.

In order to be able to use the same receivers that are used with the earth TV, the mesh wirings use for the down link the same frequency band, although systems HFC expand this band until the 862 MHz or 1 GHz, even leaving the low part of the spectrum (5-42 MHz in the United States and 5-65 MHz in Europe) like channel of return for orders, because this region is interferences more subject. In the useful band they are possible to be included a great number of analogical TV channels if one considers that, with American standard NTSC a TV channel occupies a bandwidth of 6 MHz, whereas with the European PAL or SECAM this one is increased only until the 7/8 MHz. In the case of DS, with a channel RF of 6 MHz can be provided a 27 data flow of Mbit/s from the network head to the end user making use of techniques of transmission 64QAM, or even greater (up to 38 Mbit/s) if 256QAM were used. In inverse sense, of the user to the network head simpler modulations are used user to the network head are used simpler modulations like QPSK, which provides a speed between 256 kbit/s and 5 Mbit/s, and does not require of sophisticated equipment in the home of the user. Like alternative, DPSK can also be used, that is something slower but it offers major immunity against interferences. Cable modems: standards DOCSIS The access to Internet at speeds every time majors is one of the great businesses of the new networks of broadband access. Networks HFC, by means of the use of modems especially designed for the digital communications in mesh wirings, have capacity to offer services of access to data networks like Internet at speeds superiors to the attainable ones by means of ADSL. In fact, the cable modems have turned the networks CATV/HFC into true suppliers of services of video telecommunication, voice and data.

A typical cable modem is characterized for being asymmetric: Mbit/s receives data at speeds of up to 30 and can transmit up to 10 Mbit/s. F type is connected to network HFC by means of a coaxial cable connector, and to the PC of the subscriber by means of an Ethernet card. The reception of data is realised through a channel among 6 and 8 MHz of the band 606-862 MHz, normally with digital modulation 64QAM. The cable modem demodulates the received signal and encapsulates the bit stream in Ethernet packages. Of this form, the subscriber sees network HFC outside as if an enormous LAN. In ascending sense, the cable modem disturbs the Ethernet packages that receives from the PC it turns and them into cells ATM or plots with propietary format. Later, using a channel of about 2 MHz of the return spectrum, the data transmit towards head using modulation QPSK. In figure 2 is a cable-modem example. The standards applicable to this type of modems have been evolving with time, although to day of today the great majority is based on standard DOCSIS. Over data Service Cable Specification Interface (DOCSIS) is an international standard developed by CableLabs with the contribution of a whole series of companies, between which they are included: ARRIS, Broadcom, Cisco, Conexant, Correlant, Intel, Motorola, Netgear, Terayon and Texas Instruments. In particular, it defines the requirements of cable interface digital modem for his use in systems of distribution of data on cable, that is to say, services on networks CATV/HFC. Therefore, DOCSIS allows to the data transfer at high speed using the channels of RF of the system CATV, making possible that operators CATV offer access to Internet through their existing infrastructure HFC.

First specification DOCSIS (version 1.0) was published in March of 1997, along with a later revision 1,1 of April of 1999. But due to the increasing demand of symmetrical services of real time, like for example telephony IP (VoIP), DOCSIS was reviewed again with the purpose of to improve the rates of transmission of the ascent connection and to incorporate capacities of QoS. This revision (DOCSIS 2,0) was sent in January of 2002. The Union the International of Telecommunications has adopted both variants like international standards: DOCSIS 1,1 in its recommendation ITU-T J.112 and DOCSIS 2,0 in its J.122 recommendation. Finally, recently the specification DOCSIS 3,0 has been published already that involves a series of improvements such as: greater capacity by channel, improvement of the security of the network, extension of the address space of network elements (IPv6) and unfolds of new supplies of services. Because the plans of frequency of systems CATV of Europe and the United States differ to each other, standards DOCSIS were modified for their use in Europe. These changes have been published with the name of EuroDOCSIS. The main differences make refereferencia to the bandwidths of the TV channels: the European cable channels adjust to the standard PAL TV with a bandwidth of 8 MHz, whereas in North America the channels of standard NTSC present/display a bandwidth of 6 MHz. Indeed this greater bandwidth of the EuroDOCSIS architectures allows to accommodate a greater capacity of data in the connections of the down link. Of similar form, in Japan also other variants of DOCSIS are used. In following articles we will present/display with greater level of detail the specifications and requirements of standard DOCSIS, as well as the tests of test that are realised on systems and networks HFC based on this standard.

Intrumentación of test Network-viewpoints-103-5Entre the most typical measures to realise in these networks are included: losses of signal, sweeping of the system, monitoring of channels of analogical and digital video, test on watch DOCSIS, VoIP, maintenance of the passages direct and return, etc. As a example, next some of the equipment are described that can be found available commercially. JDSU has a specific line of instruments to test of networks HFC. Like example, in figure 3 is the DSAM-6000. One is multímetro of sweeping to measure the quality of channels of analogical and digital video, as well as carrying DOCSIS and VoIP (PacketCable). It covers a margin with frequencies from 4 to 1000 MHz, reason why the slope connection analyzes so much as the return channels. Between his capacities it includes: measures of signal levels (CNR, “tilt”), complete monitoring of the band (up to 999 channels), diagrams of constellation for the digital modulations, To see, MER (error rate of the modulation), IS (seconds with errorres), SES (seconds with many errors), etc. On the other hand, company PROMAX also emphasizes by its line of products of test of telecommunication networks. With regard to TV by cable, it offers all a digital series of equipment of monitoring of cable channels and modulations. Among them we can emphasize equipment PROMAX-26 (figure 4). He is a system analyzer of transmission DOCSIS and EuroDOCSIS designed for the installation and maintenance of interactive services for video, voice and data at high speed on networks CATV. It includes the Adquisición function (LOGGER), that allows to realise and to store in memory a series of measures later power to be reviewed, to be transferred to a PC or printed. Equally, it allows to monitor parameters of quality previously very similar to the commented ones. Finally, it also agrees to enunciate to the company Sunrise Telecom like supplier of solutions of test of broadband networks CATV. It has an ample supply of solutions of test including capacities VoIP and DOCSIS. As it shows, in figure 4 an example of these equipment can be seen. The distribution of services “triple-play” (voice, video and data) through networks HFC requires the selection and use of applications and suitable equipment of test. In the market multiple solutions of test that cover from the network head to the remote nodes, and even the teams of the client exist. With regard to the section of optical fiber, equipment such as OTDRs or analyzers of optical spectrums already was presented/displayed in previous articles. However, networks CATV/HFC need additional equipment for the detection and location problems of activation of network, certification, test on watch, maintenance and monitoring.