GB2361145A

GB2361145A - Data communication system

Info

Publication number: GB2361145A
Application number: GB0001901A
Authority: GB
Inventors: Dale Harold Barnes; Stephen Temple
Original assignee: NTL Group Ltd
Current assignee: NTL Group Ltd
Priority date: 2000-01-27
Filing date: 2000-01-27
Publication date: 2001-10-10
Anticipated expiration: 2020-01-27
Also published as: GB0001901D0; GB2361145B

Abstract

The system comprises a DOCSIS-compatible cable modem network system 58 having a data input couplable to the Internet; a radio frequency transmitter 55 coupled to the cable modem network system and to a transmit antenna; at least one radio frequency receiver 50 coupled to a receive antenna; and at least one DOCSIS-compatible cable modem 16 coupled to the microwave receiver and having a data output. Internet Protocol data may thus be communicated over a wireless link.

Description

2361145 1 DATA COMMUNICATION SYSTEMS This application relates to apparatus

and methods for data communications, and in particular to systems and protocols for wireless data communication.

With increasingly heavy use of the Internet, both domestic and commercial users are demonstrating ever increasing. demands for greater bandwidth to allow faster access to data and to provide high quality sound and video services at acceptable quality levels. At present the majority of home Internet connections are made by dialling up a local service provider by telephone, using either a land line connection or a mobile telephone. It is generally understood that in such communication systems the bulk of the cost is in the last few kilometres of "local loop" cable connection to a given subscribers home. By contrast, radio and television sounds and pictures are transmitted over the air to many subscribers simultaneously.

Increasingly, the potentially high bandwidth capability of fibre optic and copper cable connections is being exploited to provide fast Internet access for business and domestic subscribers. This is seen as a more appropriate use of data communication pathways since cable connections are thought to be intrinsically capable of providing higher bandwidth services than radio links which suffer from spectrum congestion and data rate and hence bandwidth limiting factors such as signal fading and multi- path fading.

Multi-path fading is caused when a receiver picks up signals from a transmitter which have reached the receiver by travelling over paths of different effective lengths. This causes data bits originally emitted at different times to arrive at the receiver at the same time causing inter symbol interference and notches in the receiver frequency response. Typically these problems begin to appear at data rates of a few 1 OOK bits per second and above. Typical error correction schemes employ complicated adaptive equalisation, the 2 use of significant redundancy and data interleaving. Alternatively spread spectrum techniques can be used such as are employed in some wireless LAN systems. These problems are particularly apparent at the microwave frequencies (1 GHz and above) which are necessary to provide the required bandwidth and to avoid conflict with existing spectrum usage allocations. Thus the provision of wireless local loop data services has heretofore focused on providing specific tailor-made solutions involving sophisticated error correction and techniques for improving spectrum efficiency (traffic carrying capacity for a given bandwidth). Whilst high speed wireless data links are available the equipment required for robust operation is complex and expensive, typically costing ú250K for head end equipment and around ú2,000 for subscriber equipment.

The cost of this equipment and the risk in installing it provides a significant barrier to mass adoption of wireless local loop Internet access schemes. Nonetheless, wireless Internet access at an affordable price is highly desirable as it would allow new service providers to offer services without having to rely on using the existing copper or fibre optic cable infrastructure. It would also offer improved subscriber mobility and facilitate fast Internet access whilst on the move By contrast, known cable data transmission systems are relatively simple and cheap typically subscriber and apparatus costs only a couple of hundred pounds. The majority of cable modems operate using a communications protocol known as DOCSIS. This standard is well-known to those skilled in the art and provides a clear definition of the features of the protocol. The characteristics of optical fibre data connections are very different to those of radio data transmission links - optical fibres provide large bandwidths and low transmission losses and are often used, for example, for long, high speed telecommunications. They are also immune to electromagnetic interference and the multi-path effects encountered in rf transmission links.

DOCSIS stands for Data Over Cable Service Interface Specification, and is also known as the CableLabs (Registered Trade Mark) Certified (Trade Mark) Cable Modems

3 project. It defines interface requirements for cable modems involved in high-speed data distribution over cable television networks, and is widely used in the cable television industry, particularly in the USA, for the provision of high-speed Internet services over a cable television infrastructure. When used in such a way it adds a data path from a cable operator's head end(s) to local subscribers which is transparent to the Internet Protocol (IP). The specification also defines optional operations support system functions for, inter alia, security, network management, accounting, and configuration and fault management. The specification includes physical, link and network specifications and provides definitions for aspects of the system including features such as rf levels, multiplexing, contention control and the like.

In outline, a box is located at the head end of the cable network. One side produces data flows that pass to and from the Internet. The other side of the box is interfaced with the cable TV plant. The down-stream output is up converted to a suitable part of the cable network frequency spectrum and is broadcast to all connected customers. The upstream input takes signals that have been sent by a particular customer. In the customer premises a cable modem customer premises equipment is located. One side is connected to the cable TV network by a coaxial cable. The other side is a standard 10 Base T Ethemet connection to a Personal Computer.

Figure 1 shows a simplified block diagram of a data-over-cable system using DOCSIS equipment. Cable network 10 typically has a tree and branch structure with a single input 12, normally an optical fibre, fanning out to a plurality of outputs 14a-d, frequently to actual cable links, to local subscribers. Often, although not always, the "input" 12 and the "outputs" 14 are bi-directional, and in the DOCSIS specification one frequency band (for example 120 to 850 MHz) is allocated to downstream links from the head end to the subscriber ends whilst a second frequency band (for example below 80 MHz) is allocated to upstream links from subscribers to the head end or hub. It can be seen that data broadcast from the head end on line 12 is normally received by all the subscribers; similarly, the possibility exists of collisions between signals sent to the head end simultaneously from two different subscribers.

4 A subscriber link 14a is coupled to a cable modem 16 located at the subscriber's premises. Cable modem 16 provides an output 18 to a communications port on personal computer 20. The DOCSIS-compatible cable modem 16 forwards Internet Protocol traffic, for example using transparent bridging or network-layer forwarding, and may also support other network layer protocols. The link 18 to PC 20 is typically an Ethernet connection (1 0Base-T, MEE 802.3); the PC runs standard TCP/IP software. Fibre optical cable 12 is coupled to physical interface 22 which in turn is coupled to Cable Modem Termination System (CMTS) 24 via link 26. Again the CMTS 24 provides network layer forwarding or transparent bridging for Internet Protocol traffic via link 28 to network side interface (NSI) 30. The NSI 30 links to one or more of local server(s) 32, remote server(s) 34 and Internet 36. Both cable modem 16 and CMTS 24 function as IP hosts. The system can transparently transfer IP traffic of a variety of types including broadcast and multicast IP Group addressing&.

In the DOCSIS specification, broadcast data is encrypted and subscriberspecific data is extracted by a subscribers cable modem. Upstream bandwidth from a subscriber to the head end is divided into a stream of mini-slots and the CMTS controls access to the slots by the cable modems, determining which mini-slots are subject to collisions (subscribers initially transmit a short "request to send" message). Further information on the DOCSIS specification can be found at www.cablemodem. com and at cablelabs.com.

The above problems with the provision of wireless local loop data services are addressed by the present invention according to which there is provided a wireless data communication system comprising: a DOCSIS compatible cable modem network system having a data input couplable to the Internet; a radio frequency (rf) transmitter coupled to the cable modem network system and to a transmit antenna; at least one radio frequency (rf) receiver coupled to a receive antenna; and at least one DOCSIScompatible cable modem coupled to the microwave receiver, and having a data output; whereby data from the data input is transmissible to the data output.

In other aspects the invention provides a method of communicating Intemet Protocol data over a wireless rf link comprising: encoding the data using a DOC SI S -compatible cable modem network system to provide a radio frequency (rf) encoded data output; transmitting the encoded data over a wireless link to a receiver; receiving the encoded data using the receiver; and decoding the encoded data using a DOCSIS-compatible cable modem to provide a decoded data output and a use for DOCSIS-compatible cable modem devices in such a method and system.

In a further aspect the invention provides a wireless local loop system designed to provide the same functionality as a cable modem system over a cable TV network in such a way that a cable modem on a customer premises designed to the DOCSIS technical specification for use over a cable TV network can be plugged into the wireless local loop system without modification and successfully provide the same ftmetions.

In a still further aspect the invention concerns the linking together of equipment intended to be placed at the head end of a cable TV network (by virtue of its compliance with the DOCSIS technical specification) with equipment on a customers premises that is intended to be connected to the termination of a cable TV network (by virtue of its compliance with the DOCSIS technical specification) by means a wireless transmission system instead of a cable TV transmission system.

This concept simplifies the design of the microwave transceiver and harnesses the complex electronics to something being produced in huge volumes and thus low prices. In this way a significant advance in the pricelperformance of wireless local loop systems can be achieved.

The applicant has recognised that the tree and branch structure of a cable network is an environment which has functional similarities to a point to multi-point microwave communication system, in that both systems share requirements for selective data reception and for collision handling for data sent by subscribers to the head end or, in the case of an RF system, to a local base station. The applicant has conducted a number 6 of experiments and has discovered the surprising and unexpected result that DOCSIScompatible cable modem equipment provides acceptable performance levels when used in a wireless local loop system, particularly when operating at relatively high frequencies, such as microwave frequencies where radio wave propagation is relatively directional. This discovery runs directly counter to the prevailing technical prejudice.

The wireless data communication system need only provide a one-way link from a base station to the subscribers as a return path can be provided by other means, for example the public switched telephone network (PSTN). However, preferably the system employs transceivers at both the base station and subscriber ends to provide a bidirectional radio frequency link as this simplifies both lower layer communications such as rerequesting packets with errors, and a higher level data communications such as web-telephony and video. Although the system is primarily envisaged for the transmission of Internet data (which includes text, audio and video data), it can also be used for transmitting other data, for example suitably packaged MPEG data and/or ATM data. Advantageously DOCSIS forward error correction is enabled for improved robustness.

In a preferred arrangement there is a plurality of local subscribers each with a transceiver and DOCSIS compatible-cable modem. To reduce the effects of multi-path it is preferable that a subscribers antenna is relatively directional, with a gain of 3,6,9,12 dBi or greater. Advantageously a patch antenna can be used to provide sufficient directionality. Radio frequency digital data transmission systems tend not to degrade linearly but, to a first approximation, either function acceptably or fail to function at all, so that it is possible to clearly determine whether a system is working or not.

The system can be used with RF signals of any frequency but preferably it is used at frequencies above 1 GHz, and more preferably above 1 OGHz or, for greater directionality and bandwidth, above 30GHz.

7 These and other aspects of the present invention will now be further described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows a simplified block diagram of a known data over-cable system using DOCSIS equipment; Figure 2 shows a wireless local loop system; and Figure 3 shows a wireless data communication system according to an embodiment of the present invention.

Referring to Figure 2, this shows the concept of a wireless local loop data communications system. A base station 40a is provided with an antenna 42a for transmission and reception of RF signals 48 between the base station and a plurality of data communications services subscribers 44a-c, each with respective antennas 46a-c. Typically further base stations such as 40b and 40c in Figure 2 are provided to service further subscribers and the base stations are networked.

Referring now to Figure 3, this shows the equipment located at a base station 40 and subscriber premises 44. New subscriber premises includes a microwave antenna 46 coupled to a microwave transceiver 50, although a preamplifier or low noise block down converter LNB (not shown) may be used for some applications. The microwave transceiver 50 interfaces to the DOCSIS cable modem 16 by means of a simple coaxial cable connection 52. The equipment from electrical interface AA onwards towards PC 20 is the same as that used in a cable network system, as shown in Figure 1. Typically connection 18 is a Cat 5 cable connection to an Ethernet NIC (network) interface card (installed in PC 20), although other types of customer premises equipment, for example, a television can also be used.

Antenna 46 can be a simple printed patch antenna or can be a patch array or yagi if greater directionality is desired. If necessary, to reduce the bit error rate additional 8 filtering or equalisation can be included in microwave transceiver 50. Antenna 46 can be chosen taking into account physical size, bandwidth and filter requirements. Microwave transceiver 50 may also include an up converter and/or power amplifier (not shown) for return transmissions to the base station at microwave frequencies.

At base station 40 a microwave transceiver 54 and antenna 42 are also provided. As with transceiver 50 and antenna 46, up and down converters and pre- and power amplifiers and filters and equalisation (not shown) may also be included. A less directional antenna is preferred for base station 40 because of the need to transmit to subscribers located over a range of directions; antenna 42 can therefore be, for example, a dipole antenna. The transmission power of the base station will also generally be higher than that of a subscriber's transceiver, because of the greater coverage required and also because of the different frequency and data rates of the down and up links. The DOCSIS specification provides for 8MHz channels allocated to either analogue or digital TV signals or to cable modem data signals. In a preferred embodiment only 6MHz of an 8MHz channel slot is utilised for reasons of cost.

Microwave transceiver 54 interfaces to a DOCSIS cable modem network system 58 via link 56. Details of the cable modem network system are not shown but the system 58 will include a DOCSIS-compatible Cable Modem Termination System (CMTS), as shown in Figure 1.

The CMTS is provided with an appropriate physical interface to microwave transceiver 54, which will in general separate downstream and upstream data. The network side of the CMTS will generally be coupled to a switch and thence to the Internet. Typically the DOCSIS cable modem network system 58 will include a server complex including a DHCP server to verify each subscriber's MAC address, a caching server, and other local and remote servers, and will also be coupled to an ATM backbone. The interface to microwave transceiver 54 is, however, a single cable, such as a coax cable, carrying RF frequency multiplexed data. At the appropriate electrical interface, that is at the physical interface where such a single cable interface to the system exists,'microwave 9 transceiver 54 can be coupled to the cable modem network system 58 with substantially no modification to this network system.

No doubt many other effective alternatives will occur to those skilled in the art, and it should be understood that the invention is not limited to the described embodiments.

Claims

CLAIMS:

A wireless data communication system comprising: a DOCSIS-compatible cable modem network system having a data input couplable to the Intemet; a radio frequency (rf) transmitter coupled to the cable modem network system and to a transmit antenna; at least one radio frequency (rf) receiver coupled to a receive antenna; and at least one DOCSIS-compatible cable modem coupled to the microwave receiver, and having a data output; whereby data from the data input is transmissible to the data output.
2. A wireless data communication system as claimed in claim 1, comprising a plurality of rf receivers each coupled to a respective receive antenna and to a respective DOCSIS-compatible cable modem.
3. A wireless data communication system as claimed in claim 1 or 2, wherein the rf transmitter further comprises rf reception means; wherein a said rf receiver further comprises rf transmission means; and wherein a said DOCSIS-compatible cable modem includes an upstream data input, whereby upstream data is transmissible from a said cable modem to the cable modem network system.
4. A wireless data communication system as claimed in any preceding claim, wherein a said receive antenna is sufficiently directional to provide an overall bit error rate due to reflected signals of no more than 10-2.
5. A wireless data communication system as claimed in any preceding claim, wherein the DOCSIS-compatible cable modem network system comprises DOCSIScompatible cable modem termination system coupled to Intemet access means.
6. A wireless data communication system as claimed in any preceding claim, wherein the radio frequency is a microwave ftequency, in particular 10 GHz or greater.
7. A method of communicating Internet Protocol data over a wireless rf link comprising: encoding the data using a DOCSIS-compatible cable modem network system to provide a radio frequency (rf) encoded data output; transmitting the encoded data over a wireless link to a receiver; receiving the encoded data using the receiver; and decoding the encoded data using a DOC SI S -compatible cable modem to provide a decoded data output.
8. A method of communicating data as claimed in claim 8, further comprising up converting the encoded data to a microwave rf frequency and down converting the received encoded data from the microwave frequency before decoding.
9. A method of communicating data as claimed in claim 8 or 9, further comprising encrypting the encoded data; decrypting the data output from at least one of said corresponding cable modems; and wherein the step of transmitting broadcasts the encrypted data to a pluralityOf receivers and corresponding cable modems.
10. Use of a DOCSIS-compatible cable modem network system or DOCSIScompatible cable modem in the system or method of any preceding claim.