MMDS and LMDS

Multichannel Multipoint Distribution Service (MMDS):

Multichannel Multipoint Distribution Service (MMDS), also known wireless cable, is another wireless broadband technology for Internet access. MMDS has been around since the 1970s and is a well−tested wireless technology, which has been used for TV signal transmission for more than 30 years.

MMDS channels come in 6 MHz chunks and run on licensed and unlicensed channels. Each channel can reach transfer rates as high as 27 Mbps (over unlicensed channels: 99 MHz, 2.4 GHz, and 5.7 to 5.8 GHz) or 1 Gbps (over licensed channels). Typically, a block of 200 MHz is allocated to a licensed carrier in an area.

MMDS is a broadcasting and communications service that operates in the ultra−high−frequency

(UHF) portion of the radio spectrum between 2.1 and 2.7 GHz. MMDS is also known as wireless cable. It was conceived as a substitute for conventional cable television (TV). However, it also has applications in telephone/fax and data communications. MMDS frequencies provide precise, clear, and wide−ranging signal coverage.

The MMDS wireless spectrum originally consisted of 33 analog video channels, which were 6 MHz wide. The evolution of video technology into digital capacities enables the carriers to convert these 33 analog MMDS channels into 99 digital, 10 Mbps data streams, enabling full Ethernet connectivity. Therefore, a carrier with a normal operation can have as much as 1 Gbps of capacity at a single transmitter providing adequate capacities for most applications. This capacity is also readily expandable by using a sector cell concept (see the analog cellular chapter to get a handle on sectors), which reuses the same frequency many times. The combination of super cells and sectors enable the carrier to reuse the same frequency many times by building multiple cell sites. When enough customers sign on and as their bandwidth demands grow, the growth in traffic can be handled expeditiously through a new cell or a new sector.

Limited Frequency Spectrum:

The limited number of channels available in the lower radio frequency (RF) bands characterizes MMDS networks. Only 200 MHz of spectrum (between 2.5 GHz and 2.7 GHz) is allocated for MMDS use. This constraint reduces the effective number of channels in a single MMDS system. For TV signals using 6 MHz of bandwidth, 33 channels can be fit into the spectrum.

System Configuration:

The typical configuration of an MMDS system

Signals for MMDS broadcast at the transmitter site originate from a variety of sources, just like at cable head−ends. Satellite, terrestrial, and cable delivered programs, in addition to local baseband services, comprise the material to be delivered over MMDS. All satellite−delivered baseband formats are remodulated and subsequently up−converted to microwave frequencies. Terrestrially delivered signals are usually passed through a heterodyne processor prior to up−conversion to the desired MMDS frequencies. Repeater stations can be used to direct MMDS signals to blocked areas. The typical range of a transmitting antenna can reach up to 35 miles, depending on the broadcast power. Transmitters usually operate in the 1 to 100 watt range. MMDS is a line−of−sight service, so it does not work well around mountains, but it will work in rural areas, where copper lines are not available.

How a wireless cable system works:

• The cable studio, along with the head−end, receives programming from a variety of sources (see the following sect ion) .  Each source is assigned a channel number, processed to improve quality, encoded, and then sent to a transmitter. The signal is broadcast in the super−high−frequency (SHF) range. Using an omni−directional transmit pattern, the signal reaches subscribers located up to 50 KM from the antenna, depending on the terrain and transmit power.
• Wireless cable signals are received by the subscriber's small rooftop antenna, decoded (pay TV), and down−converted to standard TV channels on the subscriber's TV set.
• One of the two systems are normally used for multiple−dwellings (condo, apartment, and so on) to receive and distribute wireless TV.

1. The building management pays for all units to receive the programming from a single communal antenna. This agreed fee is usually based on the number of potential viewers.
2. I n other buildings, a single community antenna is installed with each tenant subscribing separately and billed separately by the cable company

•    In all cases, deposits are paid by subscribers that cover receiver system costs, much like cable subscriber

Advantages of Using MMDS:

• It has chunks of under−utilized spectrum that will become increasingly valuable and flexible.
• System implementation, which is little more than putting an installed transmitter on a high tower and a small receiving antenna on the customer's balcony or roof, is quick and inexpensive.
• Because MMDS services have been around for 30 years, there is a wealth of experience regarding the use and distribution of the services.

Key Elements:

The key elements of an MMDS system consist of the following pieces.

• The Head−End - The key elements in optimizing transmitted signal levels are the selection of the head−end site and the transmitting antenna,   transmission feeders, channel combiners, channel diplexers, and transmitters. The head−end's task is to distribute the signal to as many subscribers as possible.
• The Transmit Antenna - The bandwidth allocated to MMDS operators can vary from 200 to over 300 MHz, depending on the number of channels and their spacing.
• The Transmission Line - This is another critical component that can have a substantial effect on system losses. Major Head−end sites typically use 50 or 100 watt transmitters, yet often only 50 percent of this power reaches the antenna after passing through channel combiners and transmission feeders.
• Channel Combiners - MMDS sites normally transmit a number of channels. Special filters (channel combiners) are used to combine the outputs of the transmitters to the transmission feeder and antenna.

Local Multipoint Distribution Service (LMDS):

                LMDS, as its name implies, is a broadband wireless technology that is used to deliver the multiple service offerings in a localized area. The services possible with LMDS include the following:

• Voice dial−up services
• Data
• Internet access
• Video

LMDS operates in the higher frequencies, the radio signals are limited to approximately five miles of point−to−point service.

Architectural concept for the LMDS operation

Modulation and Access Techniques:

                The modulation and access method falls into two primary categories, FDMA and TDMA. Each of these techniques differs but also creates other sub modulation capabilities. For the broadband LMDS services, the system is usually separated into both phase and amplitude modulation of the RF. Phase−shift keying (PSK) and amplitude modulation combinations have been successfully used to achieve high rates of multiplexing and carrying capacities.

Summary of modulation techniques available for LMDS in FDMA

Two−Way Service:

The TDMA and FDMA modulation techniques on the LMDS network allow for the bidirectional flow between the carrier and the end user. In many cases, a different upstream is required than the downstream.

Microwave and Radio Based System

Microwave:

                Microwave are radio wave with wavelength ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz.

                Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do. Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it. A disadvantage is that microwaves are limited to line of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can.

Properties of a Microwave:

• Suitable over line-of-sight transmission links without obstacles
• Provides large useful bandwidth when compared to lower frequencies (HF, VHF, UHF)
• Affected by the refractive index (temperature, pressure and humidity) of the atmosphere, rain, snow and hail, sand storms, clouds, mist and fog, strongly depending on the frequency.

Comparison of frequency bands and distances:

                There are even differences between one type of microwave and another. The differences are due primarily to their respective operating frequencies. Some frequencies are good for distances of 30 or 40 miles and others can barely get you across an office park. Some can only support a couple of T1s or a single video channel and others go to 10 to 45 Mb.

Table that show’s comparison of frequency bands and distances

Other Application of Microwave:

               

                A laptop computer with a credit card−sized PRISM radio chip set can now convert incoming microwave messages into binary code for computer processing and then convert them back into microwaves for transmission.

Similarly, microwave transmission is used in LANs, on corporate or college campuses, in airports, and elsewhere. Whether it is collecting data, relaying conversations, or beaming messages from space, microwave makes the wireless revolution possible.

                Laptop computers can now send and receive microwave radio transmissions.

Microwave Radio Relay:

                Microwave radio relay is a technology for transmitting digital and analog signals, such as long-distance telephone calls, television programs, and computer data, between two locations on a line-of-sight radio path. In microwave radio relay, radio waves are transmitted between the two locations with directional antenna, forming a fixed radio connection between the two points. Long daisy-chained series of such links form transcontinental telephone and/or television communication systems.

xDigital Subscriber line (xDSL)

xDSL belongs to DSL family, the lower case x in xDSL stands for the many variation including the fallowing.

• Asymmetrical digital subscriber line (ADSL)

ADSL is the new modem technology to converge the existing twisted pair telephone lines into the high−speed communications access capability for various services.

ADSL is a modem technology used to transmit speeds of between 1.5 Mbps and 6 Mbps under current technology.

Data rates for ADSL, based on installed wiring at varying gauges.

NOTE: The speeds and distances shown here are the theoretical limits based on good copper.

• Digital subscriber line (IDSL)

                DSL refers to a pair of modems that are installed on the local loop to facilitate higher speeds for data transmission.

                The IDSL technique is an all digital operating at two channels of 64 Kbps for voice or non voice operation and a 16 Kbps data channel for signaling, control, and data packets.

                A  DSL is used to deliver I SDN services. As the deployment of IDSL was speeding up on the local loop, the providers developed a new twist, called "always on, ISDN" mimicking a leased set of channels that are always connected.

The IDSL line connection enables 128 Kbps in total simultaneously.

• High bit−rate Digital Subscriber Line (HDSL)

HDSL was the first DSL to use higher frequency spectrum of copper, twisted pair cable.  HDSL was developed in the US, as a better technology for high-speed, synchronous circuits typically used to interconnect local exchange carrier system, and also to carry high-speed corporate data links and voice channels, using T1 lines.

• Consumer Digital Subscriber Line (CDSL)

                CDSL is a model of DSL developed for the consumer that does not who is not looking for symmetrical high−speed communication. With other forms of DSL (such as ADSL and RADSL), splitters are used on the line to separate the voice and the data communications. CDSL does not use, nor need, a splitter on the line. Speeds of up to 1 Mbps in the download direction and 160 Kbps in the upward direction are provided. It is expected that the speeds and DSL will meet the needs of the average consumer for some time to come.

• Single High Speed DSL (SHDSL)

                SHDSL is a data communications technology that enables faster data transmission over copper telephone line than a conventional voice modem can provide. Compare to ADSL, SHDSL employs TC-PAM modulation and frequencies that include those used by analog plain old telephone service to provide equal transmit and receive data rates. SHDSL features symmetrical data rates in both the upstream and downstream directions, From 192 kbit/s to 2,312 kbit/s of payload in 8 kbit/s increments for one pair and 384 kbit/s to 4,624 kbit/s in 16 kbit/s increments for two pairs of wires.

• Rate−adaptive digital subscriber line (RADSL)

                 RADSL is a variation of asymmetric digital subscriber line technology. In RADSL the DSL modem adjust the upstream bandwidth to create a wider frequency band for the downstream traffic. Using this technique the line is more tolerant of errors caused by noise and signal loss.

• Very high−bit rate digital subscriber line (VDSL)

                VDSL was introduced to achieve the higher speeds. If, in fact, speeds of up to 50 Mbps are demanded, then the distance limitations of the local cable plant will be a factor. In order to achieve the speeds, you can expect that a fiber feed will be used to deliver VDSL.

DSL speeds and operations using current methods

xDSL Coding Techniques

• Discreet Multitone - DMT uses multiple narrowband carriers, all transmitting simultaneously in a parallel transmission mode. Each of these carriers carries a portion of the information being transmitted. These multiple discrete bands, or, in the world of frequency division multiplexing, sub-channels, are modulated independently of each other using a carrier frequency located in the center of the frequency being used. These carriers are then processed in parallel form. 
• Carrier-less Amplitude Phase Modulation CAP - CAP is closely aligned to QAM. QAM as a technique is widely understood in the industry and well deployed in our older modems. Both CAP and QAM are a single−carrier signal technique. The data rate is divided into two and modulated onto two different orthogonal carriers before being combined and transmitted. The main difference between CAP and QAM is in the way they are implemented. QAM generates two signals with a sine/cosine mixer and combines them onto the analog domain. CAP, on the other hand, generates its two orthogonal signals and executes them digitally. Using two digital transversal bandpass filters with equal amplitude characteristics and a p/2 difference in phase response, the signals are combined and fed into a digital−to−analog converter. Then the data is transmitted. The advantage of CAP over QAM is that CAP is done in silicon, which is more efficient and less expensive.

A design of an ADSL model and its model components

The intent of the model is to show the infrastructure of the network from the customer premises to the network provider.

Source:

http://en.wikipedia.org

Broadband Telecommunication Handbook 2nd edition by: Regis J. "BUD" Bates

Asynchronous transfer mode (ATM)

ATM is a member of the fast packet−switching family called cell relay. As part of its heritage, it is an evolution from many other sets of protocols. In fact, ATM is a statistical time−division multiplexed (TDMed) form of traffic that is designed to carry any form of traffic and enables the traffic to be delivered asynchronously to the network.


How an ATM sends data?

Speed comparison between ATM over other technology and protocols.

                With this information we can derive is that the ATM technique is a combination of TDM, with cells using pre-assigned slots, and Statistical TDM, with cells using whatever slots are available or needed to handle a particular traffic flow. It is also a connection−oriented protocol much the same as dialup voice communications services, but it uses virtual circuits, such as permanent virtual circuits (PVCs) and switched virtual circuits (SVC), to handle the connection.

Advantage of ATM over other technology and protocols

                ATM was designed from the ground up to work across the various places where we communicate: the Local Area Network (LAN), the Campus Area Network (CAN), and the Metropolitan

Area Network (MAN), also the Wide Area Network (WAN).

Speed comparison between ATM over other technology and protocols.

ATM features and functions

ATM Protocols

It takes many protocols to support an ATM network, which is one of the issues that continually Comes up as a negative from the supporters of the gigabit Ethernet crowd. The actual protocols needed depend on where the traffic originates, what transport mechanisms must be traversed, and where the traffic will terminate.

To understand more about this protocol here is an image that will show you the graphic representation of the ATM protocol interfaces and a table that shows where protocols are used for ATM

The problem with ATM is that in order to support the older legacy systems, many protocol points and interfaces are necessary. To get around the problem of "forklift" changes, the necessary protocols have been developed.

Mapping Circuits through an ATM Network

ATM uses one of two connection types, namely PVC and SVC. There is actually no permanency to the circuits. They are logically mapped through the network and are used when Needed for PVC or dial−connected when using the SVC, the carriers promise only to make a best attempt to serve the needs of the end user when the time is appropriate.

The concept is that the network provider will provide a committed bandwidth available to the user on demand whenever the user wants to use it. This forms the basis of what ATM networks are all about: on−demand, high−speed communications networks.

The connection is built into a routing table in each of the switches involved with the connection from end to end. As such, the switches only need to look up a table for the incoming port and channel and then determine the mapping (in the same table) for the output port and channel. Using virtual path identifiers (VPI) and virtual channel identifiers (VCI), the carrier maps the table.

This figure shows a full virtual connection is mapped through the various switches across the network. Here the end−user device is connected across an ATM access link through a switch. The switches provide the cross connect ion and link to the next downstream node. Note that the connection from the end user to the network may be on a T1, T3, or OC−n. From the first switch out, the network will use Synchronous Optical Network (SONET) or synchronous Digital Hierarchy (SDH) capabilities possibly mapped onto a Dense Wave Division Multiplexer (DWDM). The network carrier will use whatever services and bandwidth is available at the connection points.

The network switches handle the mapping on the basis of VPI switching.  VPI switching means that the switches use the virtual path for mapping through the network and will remap from one virtual path to another, while the virtual channel number is held consistent through the entire network.

A second alternative is to use VPI/VCI Switching, In this case, the ATM switches along the route will switch and remap both on a virtual path and a virtual channel, the virtual path and virtual channel switches process and remap both elements.

The OSI and ATM Layered Architecture

This is the upper-layer services of ATM.

The types of ALL and services offered.

ATM Traffic Management

                Some requirements are needed for the ATM to manage the traffic.

• ATM must be flexible. It must meet the constantly changing demands of the user population. These goals mean that the demands for traffic will rise or fall as necessary, and therefore managing this traffic is of paramount importance.
• ATM must meet the diverse needs of the end−user population. Many users will have varying demands for both high− and low−speed traffic across the network. Using a QoS capability throughout the ATM network, a user can determine the performance and the capabilities of how the ATM network will meet their demands.
• Cost efficiency is a must, If ATM is truly to succeed, and traffic management must also include the effective usage of all of the circuitry available. ATM is designed to reduce the inefficient circuit usage by efficiently mapping cells into dead spaces, particularly when data is involved.
• Robustness in the event of failure or in the event of excess demand is a requirement of the traffic management goals. s. If the network is to be readily available for all users to be able to transmit information on demand, then the network must be very robust to accommodate failures,   link downtime, and so on.

Traffic in the form of asynchronous bursts of information (cells) enters the network at random times. This randomness is what causes the confusion and the unpredictability of the data. To manage the traffic flow, buffers are used to enable the flow and ebb of traffic volumes. Because data tends to be very bursty, it is extremely difficult to predict the demands of the network and the capacity needed at a given time.

The use of "leaky buckets" in the buffering of the traffic helps to manage and control the flow of           traffic onto and through the network. The leaky bucket, as the name implies, is a buffer that is constantly flowing.

Traffic enters into the buffers and is tagged, based on the amount of cells enabled by the carrier. If the user exceeds the amount of cell flow per increment (per second, and so on), then the buffer is filled and begins to empty out the bottom side. If more cells enter the buffer than are allowed, the cells are flagged for discard.

Shaping the Traffic

The ITU−TSS defines four possible situations when a cell enters an ATM network:

• Successfully delivered the cell arrives at the destination with less than time T−cell delay.
• An error cell occurs a cell arrives with at least one detected bit error in the information field in the cell. Another possibility is the severely error cell with information bits errors equal to n or n>1
• Lost cells a cell either never arrives or arrives after the time T−cell delay, in which case it is discarded at the destination.
•       An inserted cell a cell contains an undetected error or is misdirected by an ATM node and therefore shows up at the wrong destination.

This figure shows that if the user exceeds the network rate, the cells will be discarded, and the user will, therefore, have to retransmit at a later time.

And if the user does not exceeds the network rate the network should deliver all available cells delivered to the network. As shown in the figure bellow.

Source

Broadband telecommunication handbook 2nd edition by Regis J. "Bud" Bates

Frame Relay

Frame Relay:

It is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology. Originally designed for transport across integrated services digital network (ISDN) infrastructure, it may be used today in the context of many other network interfaces.

Network providers commonly implement Frame Relay for voice (VoFR) and data as an encapsulation technique, used between local area networks (LANs) over a wide area networks (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.

Frame Relay has become one of the most extensively-used WAN protocols. Its cheapness (compared to leased lines) provided one reason for its popularity. The extreme simplicity of configuring user equipment in a Frame Relay network offers another reason for Frame Relay's popularity.

Frame Relay began as a stripped-down version of the X.25 protocol, releasing itself from the error-correcting burden most commonly associated with X.25. When Frame Relay detects an error, it simply drops the offending packet. Frame Relay uses the concept of shared-access and relies on a technique referred to as "best-effort", whereby error-correction practically does not exist and practically no guarantee of reliable data delivery occurs. Frame Relay provides an industry-standard encapsulation utilizing the strengths of high-speed, packet-switched technology able to service multiple virtual circuits and protocols between connected devices, such as two routers.

Frame:

When Frame Relay was developed, the important part of the data−carrying capacity was the use of the frame to carry the traffic and not have the same overhead as an older technology (such as X.25). The frame was filled with data as necessary, but it handled the speed and throughput via the high−speed communications and lower overhead.

Frame relay vs OSI:

Frame Relay works at the data link layer to reduce the overhead associated

with the movement of data across the wide area. Because we refer to Frame Relay as a WAN

technology, it is natural that the protocols will work with the improvements made in the network over

the past decades.

In the older days, data was shipped across the layer three protocols (such as X.25) to assure the

reliability and integrity of the data.The X.25 protocol worked at layer three.The overhead associated with the transmission and reception of the data on the X.25 networks was inordinate. To facilitate better data throughout and eliminate some of the overhead, Frame Relay was developed. 

• The networks were now improved through the mass deployment of fiber−based networking technologies and the use of SONET protocols.
• The networking st rategies of  many end users were based on  router   technologies and LAN−to−WAN communications instead of the older terminal−to−host inter communications.

These two changes actually revamped the way we communicate. No longer did we have to use a

timing   relationship,  as in the older data networks. Any form of data transmission could be

accommodated across the newer improved techniques and protocols

With this comparison in mind, one will note that the Layer 2 protocol (in this case, Frame Relay)

eliminates some of the overhead associated with the transmission of data. The need for network

addressing using Layer 3 is reduced because many of the link architectures are based on

point−to−point circuits or private networking techniques.

Frame Relay Speed:

Carriers' Implementation of IP−Enabled Frame Relay:

Carriers are now offering IP−enabled services, enabling a customer to use an existing Frame Relay

access link to tap into a connection less Multiprotocol Label Switching (MPLS)−based IP backbone

or a private IP network. The primary benefit is that achieving mesh connectivity within a customer's

VPN requires just a single "access" PVC from each remote site.

Frame Relay vs IP:

Source:


www.wikipedia.com
Broadband Telecommunications Handbook second edition by Regis J. "BUD" BATES