MMDS and LMDS

The demand for affordable, fast data connections is increasing both in the United States and around the globe. There are several reasons why faster connections are not readily available and affordable. They are a complex mix of entrenched interests of the incumbent connection providers, the high costs of wireline upgrades and the associated slow pace, cumbersome regulations, and tariffs; and the difficulty of forcing more data through already crowded data pipes.

A new wireless broadband point-to-multipoint microwave technology called local multipoint distribution service (LMDS) stands ready to bypass those barriers to readily available broadband connections. In the United States, incumbent connection providers were prevented from owning or controlling the large block of LMDS microwave spectrum in their territory for a period of 36 months (from the auction); consequently, the chances of entrenched interests limiting bandwidth availability are small. In Canada, local multipoint communication service (LMCS) applications from entrenched landline providers were not accepted (see sidebar, "More Communication Choices for Canadians"). The 1 GHz of LMCS spectrum was awarded to newly established companies and consortiums.

LMDS is a wireless broadband service and consequently does not require landline wire upgrades, which makes it affordable when compared with landline technologies. And LMDS is lightly regulated and can be used for two-way transmission of voice, video, and data. Finally, the LMDS spectrum is immense. This large amount of radiofrequency (rf) spectrum allows operators to realize data rates above 1 billion bits per second (bps).

LMDS also provides high-capacity point-to-multipoint data access that is less investment-intensive. LMDS, with its wireless broadband delivery, combined with the significant amount of spectrum allocated, allows for a very high quality communications services. It transmits milliwave signals within small cells. Since it has been tested by the U.S. military and corporate pioneers like SpeedUs.com, Inc., it is undoubtedly a proven technology.

The key to business and consumer acceptance of LMDS as an attractive solution is the affordability and availability of the systems. Because of its point-to-multipoint nature, one LMDS cell with a single hub transceiver can serve hundreds or thousands of simultaneous customers. The affordability of the overall LMDS solution is therefore largely dependent on the cost of the customer premises equipment.

Internationally, governments are working quickly to enable the use of high-gigahertz microwave spectrum for wireless broadband data, voice, and video transport. In many ways, the opportunities internationally are greater than those domestically because of the poor state of the communications infrastructure internationally and the desire of the ministries of telecommunications to move rapidly to make their systems competitive with those in the United States.

LMDS Advantages

• Ease and speed of deployment

• Fast realization of revenue

• Easy network management

• Large bandwidth

• Small cell size

LMDS Disadvantages

• Requires Line of Sight

• Affected by rain, foliage and reflections

• Many cell sites are required

• Multiple cell sites cause interference

• Security concerns

MMDS stands for Mutlichannel Multipoint Distribution Service and this is most commonly termed as Wireless Cable. MMDS is really a wireless communications technology which can be used in high speed networking within the telecoms market. It is frequently utilized rather than a programming reception involving cable connection and is also most favored in India, Brazil, Australia, Pakistan, america, Barbados, Mexico, Russia, Belarus, Lebanon and various areas. Throughout these locations it is primarily used in non-urban parts which happen to have sparse population. This really is caused by a defieicency of regular use cables. Thats usually where laying wires is most popular subsequently MMDS assistance are usually accessible.

MMDS is specified by means of UHF or ultra-high-frequency communications. It operates inside the given FCC accredited frequency. In the usa the FCC is split up into Basic Trading Areas or BTA's which inturn sell the right to transfer MMDS in areas where service providers can be obtained.

MMDS uses BRS bands of 2.1 GHz and also from 2.5 GHz to 2.7 GHz microwave frequencies. Rooftop microwave antennas are widely used to pass on data indicators and also BRS-delivered television reception. Antennas are attached to a transceiver or down-converter which in turn obtains and transmits microwaves signals. They're afterward converted to the frequencies which can be compatible with TV tuners. This is similar to the way indicators are transformed into the frequencies for satellite dishes but instead these are suitable for TV coaxial cables.

The MMDS band is afterward segregated into channels of 33 6 MHz. Because of this these kinds of people can own a number of other channels, radio, multiplex televisions along with Internet data. Digital cable channels are then capable of regulate 64QAM as well as 30.34 Mbps together with 256QAM modulation having 42.88 Mbps.

MMDS Disadvantages

• Limited two way capabilities (upstream bandwidth is limited)

• Shadowing and interference prevents ubiquitous coverage

• Normal radio security concerns

MMDS vs. LMDS

• Supports high bandwidths but gets affected by
  rain, foliage and reflections
• Requires Line of Sight conditions to serve
• Operates in smaller radius and hence more
  number of cell sites required for a city coverage
• Supports relatively lower bandwidths but does
  not get affected by the rain or foliage conditions
• Also works in Near Line of Sight (NLoS)
  conditions
• Supports larger coverage area hence few cell
  sites required to cover a city

Originally designed for wireless digital television transmission, LMDS and multipoint microwave distribution system (MMDS) were predicted to serve wireless broadband subscription television needs. MMDS is also a broadband wireless communications service that operates at lower frequencies. Usually, LMDS operates at frequencies above the 10-GHz range and MMDS at frequencies below the 10-GHz range. Later on they were extended to offer other interactive services.

Sources:

http://www.networkcomputing.com/netdesign/bb3.html

http://www.eetimes.com/electronics-news/4039196/LMDS-MMDS-race-for-low-cost-implementation

http://www.networkcomputing.com/netdesign/1223wireless1.html

http://quantumwimax.com/index.php?page=History-of-Wimax

broadband telecommunications handbook

Microwave and Radio Based Systems, a simple discussion

Microwave system is always a Line-of-Sight system which is based on the visibility between the Transmit End & Receive End.

•It is mandatory to avoid any physical obstruction in the line of sight design.

•The obstructions would include High rise buildings, Hillocks & Vegetations like tall trees.

•To overcome these obstructions we may have to increase the number of towers or increase

the height of the towers.

•The microwave systems can operate in LOS mode for around a few hundred meters to over 60 Km.

•So it is imperative to determine the LOS over the complete hop length between

 he Transmit end & Receive end

When a microwave signal is sent it travels from the transmit end to the receive end the signals take the form of an ellipsoid. The size of ellipsoid depends on the frequency of operation. The higher the operating frequency the smaller is the size of the ellipsoid. The size of the ellipsoid is biggest at the center of the LOS. If any obstruction is allowed into the fresnel zone, the obstruction will reflect the signal. This reflected signal will cancel out and distort the main signal thus reducing the strength of the main signal.

The height of the LOS should be high enough to not permit any obstructions to enter the fresnel zone.

The microwave signals do not travel in a straight line but tend to travel in a curved path following the curvature of the earth. The reason for the effect is the refractive index of the atmosphere reduces with the increase in altitude. This is known as the earth bulge factor ( k).

The antenna is a device that converts the electrical signals into the electromagnetic waves that propagate through free space.

 In Microwave systems the Standard Antenna used are Parabolic Antennas.

Antenna gain is a measure of the antenna’s ability to transmit the waves in a specific direction instead of in all direction.

Whats with Microwave Radio Based System?

Series 875 LAN+T1 Microwave Radio System is a low cost, high bandwidth, radio system capable of transporting full bandwidth LAN traffic and T1 telephony up to 3 miles. The radio system offers the following features:

The Ethernet interface of the radio system supports all 802.3 protocols and includes an AUI connection to network devices and BNC connections to the microwave unit. The T1 data interface of the radio system supports Bell Standard 100 ohm, two-wire twisted pair designed to connect to telephony and multimedia interfaces. No on-site programming is required and no additional test equipment is needed.

The Series 875 LAN+T1 system provides point-to-point connections for hubs, bridges, routers and repeaters allowing full 10 Mbps Ethernet connectivity. The 875 LAN+T1 interface has been designed to connect directly to standard T1 PBX's, Channel Banks or Telecom Multiplexers without the need for additional equipment. 

The Series 875 LAN+T1 is designed for rapid installation and alignment without special tools or test equipment. Rugged, modular radio design for ease of service and field support for years of trouble free operation in harsh weather conditions.

Why Microwave Radio Based Sytem?

Microwave Radio System offers the following features Standard IEEE 802.3 LAN Interface Standard Bell T1 1.544 Mbps Interface Standard 10 Mbps or Full Duplex Ethernet Lightweight package 7 Ibs (3.2kg) Interference free operation -30C to +55C temperature range EMI/RFI protection Compact Size 9" antenna Easy to Install Easy to Maintain

Sources:

http://www.arcelect.com/Wireless_875_LAN+T1_23GHz.htm

http://www.ehow.com/list_6137210_microwave-radio-communications-advantages-disadvantages.html

nystec.com/files/Microwave.pdf

http://www.dpstele.com/dpsnews/techinfo/microwave_knowledge_base/microwave_system.php

Broadband Telecommunications Handbook

Digital Subscriber Line (xDSL)

 DSL or xDSL is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, but as of 2009 the term digital subscriber line has been widely adopted as a more marketing-friendly term for ADSL, the most popular version of consumer-ready DSL. DSL can be used at the same time and on the same telephone line with regular telephone, as it uses high frequency bands, while regular telephone uses low frequency.

The DSL family includes several variations of what is known as digital subscriber line. The lower case x in front of the xDSL stands for the many variations.

ADSL is the new modem technology to converge the existing twisted pair telephone lines into the high−speed communications access capability for various services. Most people consider ADSL as a transmission system instead of a modification to the existing transmission facilities. the IDSL technique is 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. ISDN was very slow to catch on, but the movement to the Internet created a whole new set of demands for the carriers to deal with. HDSL was developed as a more efficient way of transmitting over the existing copper wires. HDSL does not require the repeaters on a local loop of up to 12K. Bridge taps will not bother the service, and the splices are left in place. This means that the provider can offer HDSL as a more efficient delivery of 1.544 Mbps. The modulation rate on the

HDSL service is more advanced. The goal of the DSL family was to continue to support and use the local copper cable plant. It was developed to provide high−speed communications on that single cable pair but at distances no greater than 10K. SDSL uses only one pair of wires, but is limited in its distance to provide duplex, high−speed communications. Not all users require symmetrical speeds at the same time. ADSL was, therefore, designed to support differing speeds in both directions over a single cable pair at distances of up to 18K. Because the speeds requested are typically for access to the Internet, most users look for higher speeds in a download direction and the lower speed for an upward direction. If the line conditions vary, the speed will be dependent on the sensitivity of the equipment. In order to achieve variations in the throughput and be sensitive to the line conditions, RADSL was developed. This gives the flexibility to adapt to the changing conditions and adjust the speeds in each direction to potentially maximize the throughput on each line.

CDSL does not use, nor need, a splitter on the line. Moreover, speeds of up to 1 Mbps in the download direction and 160 Kbps in the upward direction are provided. One of the most significant improvements SHDSL brings to the business market is increased reach — at least 30 percent greater than any earlier symmetric DSL technology. Furthermore, SHDSL supports repeaters, which further increase the reach capability of this technology. Clearly, changes will always occur as we demand faster and more reliable communications capabilities. It was only a matter of time until some users demanded higher−speed communications than was offered by the current DSL technologies. As a result, VDSL was introduced to achieve the higher speeds.

The download speed of consumer DSL services typically ranges from 256 kilobits per second (kbit/s) t o24,000 kbit/s, depending on DSL technology. Line conditions and service-level implementation, typically, upload speed is lower than download speed for Asymmetric Digital Subscriber Line and equal to download speed for the rarer Symmetric Digital Subscriber Line

Voice and data

Comparing DSL & Dial-Up

DSL (VDSL) typically works by dividing the frequencies used in a single phone-line into two primary "bands". The ISP data uses the high-frequency band (25 kHz and above) whereas the voice utilizes the lower-frequency band (4 kHz and below). (See the ADSL article for information on the subdivision of the high-frequency band.) The user typically installs a DSL filter on each phone outlet. This filters out the high frequencies from the phone line, so that the phone only sends or receives the lower frequencies and the user hears only the human voice. The DSL modem and the normal telephone equipment can be used simultaneously on the line without interference from each other provided filters are used for all voice devices.

History and science

DSL, like many other forms of communication, stems directly from Claude Shannon's seminal 1948 scientific paper: A Mathematical Theory of Communication. Employees at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wideband digital signals above the existing baseband analog voice signal carried between telephone-company central offices and customers on conventional twisted pair cabling.

U.S. telephone companies promote DSL to compete with cable Internet. the first DSL service ran over a dedicated "dry loop", but when the FCC required the incumbent local exchange carriers (ILECs) to lease their lines to competing providers such as Earthlink, shared-line DSL became common. Also known as DSL over Unbundled Network Element, this allows a single pair to carry data (via a digital subscriber line access multiplexer [DSLAM]) and analog voice (via a circuit switched telephone switch) at the same time.

Operation

Regular DSL

Telephone engineers initially developed the local loop of the public switched telephone network (PSTN) to carry POTS voice communication and signaling: no requirement for data communication as we know it today existed. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as voice band or commercial bandwidth.

The local telephone exchange or central office generally digitizes speech signals into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore, according to the Nyquist theorem, any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).

Because DSL operates above the 3.4 kHz voice limit, it cannot pass through a load coil. Load coils are, in essence, filters that block out any non-voice frequency. They are commonly set at regular intervals in lines placed only for POTS service. A DSL signal cannot pass through a properly installed and working load coil, while voice service cannot be maintained past a certain distance without such coils. Therefore, some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this, phone companies are endeavoring to remove load coils on copper loops that can operate without them, and conditioning lines to avoid them through the use of fiber to the neighborhood or node FTTN.

The commercial success of DSL and similar technologies largely reflects the advances made in electronics that, over the past few decades, have been getting faster and cheaper even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive.

Naked DSL

Dry-loop DSL or "naked DSL," which does not require the subscriber to have traditional land-line telephone service, started making a comeback in the US in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of AT&T's merger with SBC, and Verizon's merger with MCI, those telephone companies have an obligation to offer naked DSL to consumers.

Even without the regulatory mandate, however, many ILECs offer naked DSL to consumers. The number of telephone landlines in the US dropped from 188 million in 2000 to 172 million in 2005, while the number of cellular subscribers has grown to 195 million.. This lack of demand for landline service has resulted in the expansion of naked DSL availability.

Typical setup and connection procedures

Physical connection must come first. On the customer side, the DSL Transceiver, or ATU-R, or more commonly known as a DSL modem, is hooked up to a phone line. The telephone company connects the other end of the line to a DSLAM, which concentrates a large number of individual DSL connections into a single box. The location of the DSLAM depends on the telephone company but it cannot be located too far from the user because of attenuation, the loss of data due to the large amount of electrical resistance encountered as the data moves between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM.

When the DSL modem powers up it goes through a sync procedure. The actual process varies from modem to modem but generally involves the following steps:

1.     The DSL Transceiver does a self-test.
2.   The DSL Transceiver checks the connection between the DSL Transceiver and the computer.
3.    The DSL Transceiver then attempts to synchronize with the DSLAM. Data can only come into the computer when the DSLAM and the modem are synchronized.

Modern DSL gateways have more functionality and usually go through an initialization procedure very similar to a PC boot up.

Equipment

The customer end of the connection consists of a Terminal Adaptor or in layman's terms “DSL Modem” This converts data from the digital signals used by computers into a voltage signal of a suitable frequency range which is then applied to the phone line.

In some DSL variations (for example, HDSL), the terminal adapter connects directly to the computer via a serial interface, using protocols such as RS-232 or V.35. In other cases (particularly ADSL), it is common for the customer equipment to be integrated with higher level functionality, such as routing, firewalling, or other application-specific hardware and software. In this case, the entire equipment is usually referred to as a DSL router or DSL gateway.

Some kinds of DSL technology require installation of appropriate filters to separate, or "split", the DSL signal from the low frequency voice signal. The separation can take place either at the demarcation point, or with filters installed at the telephone outlets inside the customer premises.

At the exchange, a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other networking transports. In the case of ADSL, the voice component is also separated at this step, either by a filter integrated in the DSLAM or by a specialized filtering equipment installed before it. The DSLAM terminates all connections and recovers the original digital information.

Protocols and configurations

Many DSL technologies implement an ATM layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.

DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnet. The earliest implementations used DHCP to provide network details such as the IP address to the subscriber equipment, with authentication via MAC address or an assigned host name. Later implementations often use PPP over Ethernet or ATM (PPPoE or PPPoA), while authenticating with a userid and password and using PPP mechanisms to provide network details.

DSL technologies

The line-length limitations from telephone exchange to subscriber impose more restrictions on higher data-transmission rates. Technologies such as VDSL provide very high speed, short-range links as a method of delivering "triple play" services (typically implemented in fiber to the curb network architectures). Technologies likes GDSL can further increase the data rate of DSL. Fiber Optic technologies exist today that allow the conversion of copper based IDSN, ADSL and DSL over fiber optics.

Example DSL technologies (sometimes called xDSL) include:

1.    ISDN Digital Subscriber Line (IDSL), uses ISDN based technology to provide data flow that is slightly higher than dual channel ISDN.

2.    High Data Rate Digital Subscriber Line (HDSL / HDSL2), was the first DSL technology that uses a higher frequency spectrum of copper, twisted pair cables.

3.    Symmetric Digital Subscriber Line (SDSL / SHDSL), the volume of data flow is equal in both directions..

4.    Asymmetric Digital Subscriber Line (ADSL), the volume of data flow is greater in one direction than the other.

5.    Rate-Adaptive Digital Subscriber Line (RADSL), designed to increase range and noise tolerance by sacrificing up stream speed

6.    Very High Speed Digital Subscriber Line (VDSL)

7.    Etherloop Ethernet Local Loop

8.    Gigabit Digital Subscriber Line (GDSL), based on binder MIMO technologies.

Transmission methods

Transmission methods vary by market, region, carrier, and equipment.

1.      2B1Q: Two-binary, one-quaternary, used for IDSL and HDSL
2.     CAP: Carrierless Amplitude Phase Modulation - deprecated in 1996 for ADSL, used for HDSL
3.    DMT: Discrete multitone modulation, the most numerous kind, also known as OFDM (Orthogonal frequency-division multiplexing)

 Sources:

Broadband Communications Handbook

www.aaxnet.com/topics/cblmdm.html

www.jawin.com/protocolxDSL.html

www.business.com

www.dsl-direct.com/

Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode is a high-speed network technology that supports the transportation of voice, data, and video signals over a single stream. ATM combines both circuit and packet switching methods into one flexible technology that makes for simple network processing functions. That digital data is encoded in the form of small fixed size cells instead of the variable sized packets used by Internet Protocol or Ethernet. This ensures that the packets can be sent quickly and easily.

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 form of traffic that is designed to carry any form of traffic and enables the traffic to be delivered asynchronously to the network. When traffic in the form of cells arrives, these cells are mapped onto the network and are transported to their next destination. When traffic is not available, the network will carry empty (idle) cells because the network is synchronous.

ATM is connection oriented, which means that data sent through the ATM network will always follow the same pre-defined path with the data arriving in the order it was sent.

ATM Cells

An ATM cell is 53 bytes long with a 5-byte header possessing information for control and signaling, and 48 bytes of data payload. Having fixed-size cells may reduce queuing delays for high priority cells. Because one knows the size of a cell beforehand, it becomes easier to implement the switching mechanism in hardware for efficient switching. The header information is generated in the ATM Layer, while the ATM Adaptation Layer (AAL) breaks the entire message into 48-byte data chunks. The cell header contains fields to help deal with congestion, maintenance, and error control problems. It is broken up into the following fields:

• Generic Flow Control (GFC), a mechanism used to alleviate shortterm overload conditions in the network. It is intended to provide efficient and equal utilization of the link between all the users.
• Virtual Path Identifier (VPI), which allows for more virtual paths to be supported within the network.
• Virtual Channel Identifier (VCI), which functions as a service access point as it is used for routing to and from the end user.
• Payload Type (PT), which is used to distinguish between user information and connection-associated layer management information.
• Cell Loss Priority (CLP), which is used to provide guidance to the network to discard the cell in case of congestion.
• Header Error Control (HEC), which contains the information that can be used by the physical layer for error detection or correction. It is calculated from the first 32 bits of the header.

VCI/VPI Connections

The entire ATM network is based on virtual connections set up by the switches upon initialization of a call. Virtual Channel Identifiers (VCI) and Virtual Path Identifiers (VPI) are used to identify these virtual connections. They are used to route information from one switch to another. VCI and VPI are not addresses; they are explicitly assigned to each segment within a network.

A Virtual Channel Connection (VCC) is set up between two end users through the network and used for full-duplex flow of cells. They are also used for user-network exchange (control signaling) and network-network exchange (network management and routing). The VCI label identifies a VCC between two ATM switches and may change at intermediate nodes within a route.

Virtual channels having the same endpoints are often grouped together to form a Virtual Path Connection (VPC). This grouping of channels makes the task of network management easier without losing flexibility.

Layers and Their Functions

ATM is a layered architecture allowing multiple services—voice, data, and video—to be carried over the network. It consists of three layers: the physical layer, the ATM layer, and the ATM adaptation layer.

The physical layer of ATM is similar to layer 1 of the Open Systems Interconnections (OSI) model and performs bit level functions. It defines electrical characteristics and network interfaces. It is further divided into two layers: Physical Medium (PM) and Transmission Convergence (TC) sub-layer.

The PM sublayer contains physical medium dependent functions and provides bit transmission capability including bit alignment.

The TC sublayer performs five primary functions. The lowest function is the generation and recovery of the transmission frame. Transmission frame adaptation adapts the cell flow according to the used payload structure of the transmission system in the sending direction, and extracts the cell flow from the transmission frame in the receiving direction.

ATM Layer

The ATM layer is next above the physical layer. The ATM layer takes the data to be sent and adds the 5-byte header information. It performs the following four actions:

• Cell header generation/extraction, which adds the appropriate ATM cell header to the received cell information field from the upper layer in the transmit direction. It does the opposite in the receive direction.
• Cell multiplex and demultiplex function, which multiplexes cells from individual virtual channels and virtual paths into one resulting cell stream in the transmit direction. It divides the arriving cell stream into individual cell flows to VCs or VPs in the receive direction.
• VPI and VCI translation, which is performed at the ATM switching and/or cross-connect nodes.
• Generic Flow Control (GFC), which supports control of the ATM traffic flow in a customer network.

ATM Adaptation Layer

The AAL performs the adaptation of OSI higher layer protocols, as most applications cannot deal directly with cells. The Adaptation Layer assures the appropriate service characteristics, and divides all types of data into the 48-byte payload that will make up the ATM cell. AAL is further divided into two sublayers: Segmentation and Reassembly (SAR) and Convergence Sublayer (CS).

The SAR sublayer performs segmentation of the higher layer information into a size suitable for the payload of the ATM cells of a virtual connection and, at the receiving side, it reassembles the contents of the cells of a virtual connection into data units to be delivered to the higher layers. The CS sublayer performs functions like message identification and time/clock recovery.

Mapping Circuits Through an ATM Network

ATM uses one of two connection types. The protocol is connection−oriented, so the two choices are a PVC or a 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 concept is that the network provider will provide a committed bandwidth available to the user on demand whenever the user wants to use it. The connection is built into a routing table in each of the switches involved with the connection from end to end.

ATM Traffic Management

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. These demands must be met in terms of the

delay or the actual delivery of the cells across the network.

Cost efficiency is a must. If ATM is truly to succeed, 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. 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. Through this process, the managing of traffic must

accommodate such diverse needs on a WAN.

Key Benefits of ATM

ATM offers significant benefits to users and those who design and maintain communications networks. Because network transport functions can be separated into those related to an individual logical connection and those related to a group of logical connections, ATM simplifies network management. ATM also allows for the integration of networks, improving efficiency and manageability and providing a single network for carrying voice, data, and video.

ATM increases network performance and reliability because the network is required to deal with fewer aggregated entities. There is also less processing needed and it takes less time to add new virtual channels because capacity is reserved beforehand on a virtual path connection. Finally, ATM offers a high degree of infrastructure compatibility. Because ATM is not based on a specific type of physical transport, it can be transported over twisted pair, coaxial, and fiber optic cables.

Two additional features of ATM that warrant discussion are its asynchronous operation and its connection-oriented operation. ATM cells are intermixed via multiplexing, and cells from individual connections are forwarded from switch to switch via a single-cell flow. However, the multiplexing of ATM cells occurs via asynchronous transfer, in which cells are transmitted only when data is present to send. In comparison, in conventional time division multiplexing, keep-alive or synchronization bytes are transmitted when there is no data to be sent. Concerning the connection-oriented technology used by ATM, this means that a connection between the ATM stations must be established before data transfer occurs. The connection process results in the specification of a transmission path between ATM switches and end stations, enabling the header in ATM cells to be used to route the cells on the required path through an ATM network.

Sources:

http://www.telecomdictionary.com/telecom_dictionary_ATM_definition.html

http://www.javvin.com/protocolATMLayer.html

Cisco - Creating ATM VLANs and Configuring LANE Services

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm