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Multi-channel Multipoint Distribution Service ( MMDS)

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MMDS

MMDS also known wireless cable , isanother wireless broadband

technology .MMDS has been around since the1970s and is a well tested wirelesstechnology, which has been used for TV signal transmission for more than30 years.

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MMDS operates in the SHF between 2.1 and2.7 GHz.The MMDS originally consisted of 33 analogvideo channels, which were 6 MHz wide .The evolution of video technology into digitalcapacities enabled to convert 33 analog

channels into 99 digital, 10 Mbps datastreams , enabling full Ethernet connectivity.

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A service provider can have as muchas 1 Gbps of capacity at a single

transmitter, providing adequatecapacities for most applications.The capacity can be expanded bysector cell concept .

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The transmitter power allowed to aMMDS base station enables to service

an area of 50 km , but the subscriber antennas should be in LOS .MMDS can also be used to support twoway services . It is an alternative for broadband data service such asinternet access.

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MMDS has been used to compete with cableTV providers and to provide service in ruralareas not reached by broadcast TV or cableTV.The frequency band allocated for fixedwireless broad band service using MMDS isin the range of 2.15 to 2.68 GHz.There are five frequency bands in this rangeas shown in table.

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Key Elements

The key elements of an MMDS systemconsist of the following pieces.

The HeadEnd. The Transmit A ntenna. The Transmission Line. Channel Combiners.

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W orking of W ireless Cable

System The cable studio, along with the headend, receivesprogramming from a variety of sources .

Each source is assigned a channel number, processedto improve quality, encoded, and then sent to a

transmitter . The signal is broadcast in the superhighfrequency (SHF) range. Using an omnidirectional transmit pattern,the signal reaches subscribers located up to 50 K M fromthe antenna, depending on the terrain and transmitpower.

Wireless cable signals are received by the subscriber'ssmall rooftop antenna, decoded (pay TV ), anddownconverted to standard TV channels on thesubscriber's TV set.

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B roadband Service Delivery

The wireless cable TV did not pick up due toother competing technologies such asbroadcast TV, direct broadcast satellite

(DBS) and cable TV.in 1999, many service providers began touse the MMDS frequencies for a viablebroadband service delivery option.

The networks were upgraded with digitalcompression capabilities and a returnchannel to create interactive capability.

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Data Security

MMDS have adopted DOCSIS ( Data Over Cable S ervice Interface S pecification ) fromthe cable modem. The version for wirelessBB is DOCSIS+ .Data transport S ecurity is accomplishedunder MMDS by encrypting traffic flow

between the BB wireless modem and theWMTS (Wireless Modem TerminationS ystem )

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Data Security

DOCSIS+ reduces theft of service vulnerabilitiesunder MMDS by enforcing encryption on MMDS and employing authenticated client / server keymanagement protocol in which W MTS controlsthe distribution of keying material to BB wirelessmodems.MMDS wireless modems utilizes DOCSIS+ KeyManagement control protocols to obtainauthorization and traffic encryption.

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Data Over Cable System Interface Specification

(DOCSIS).

DOCSIS defines all the protocolsnecessary to transport data from a CableModem Transmission S ystem (CMTS) toa Cable Modem (CM).

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U pstream Communication

The CM checks the downstream channels for a specificpacket periodically sent by the CMTS . The packet asksany new CM to announce itself on a specific upstreamchannel.

The CMTS sends a packet to the CM, defining itsallocated downstream and upstream channels .

The CM then starts a process, called ranging , whichdetermines the distance between the CM and CMTS .This process is required for synchronization between allCMs and CMTS s for the mini slots used for timesharingof the upstream channels.

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U pstream Communication

The CM sends a packet to the ISP , asking for the Internet address .

The CM and CMTS then exchange somepackets to establish security parameters , whichare needed for a public network such as cableTV.

The CM sends its unique identifier to the CMTS . Upstream communication can start in theallocated upstream channel. The CM can

contend for the mini slots to send data.

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Downstream Communication

In the downstream direction, thecommunication is much simpler. There isno contention because there is only onesender.

The CMTS sends the packet with theaddress of the receiving CM, using theallocated downstream channel.

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advantages of using MMDS

It has chunks of under-utilized spectrum thatwill become increasingly valuable andflexible.

System implementation, which involves aninstalled transmitter on a high tower and asmall receiving antenna on the customer'sbalcony or roof, is quick and inexpensive .

MMDS services have been around for 30years, there is a wealth of experienceregarding the use and distribution of theservices.

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Advantages

MMDS uses lower frequency band andtherefore has larger cell size .

Equipment at lower frequencies is lessexpensive .MMDS signals dont get blocked easily byobjects and are less susceptible to rainabsorption.

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Disadvantage of MMDS

The main disadvantage of MMDS ascompared to L MDS is less bandwidth . A single channel can offer upstream transfer rates of 27 Mbps with individual subscriber rates of 300 Kbps to 3 Mbps.MMDS is likely to be used mainly byresidential users and small businesses .