Low-Earth-Orbit Satellite (LEO)

                      Low Earth Orbit Satellite (LEO) is satellite communication technologies. One technology is called the Iridium. It is a mobile satellite system. Iridium was a concept of launching 66 satellites but it was originally 77 satellites when it was proposed on FCC. Iridium is owned by the Motorola. The application was submitted to FCC in the year 1990. Its original concept was to use 7 polar orbits with 11 satellites in each. The coverage of this is around the world. This concept uses line of sight coverage from point to point virtually any other point on the globe. This uses a spot beam from the radio communications services on board each of the satellites. This technology of the spot beam concept introduced the high frequency reuse capacities that had not been achieved before. The satellite that was used was small compared to the others that had been used. Its electronic circuits inside were very sophisticated.

                        The initial features available on LEO networks are Radio Determination Service (RDSS), Voice communications (VC), Paging (P), Facsimile (Fax) and Data Communications (DC). The RDSS is used at the location of vehicle fleets, aircraft, marine vehicles, etc.  Voice communication is used on dial up digital voice communications anywhere in the world. Paging is used as a one way paging service. It includes alphanumeric display for up to two lines, but will expand to short messaging services (SMS) at 160 characters. Fax is used as a two way facsimile service. Data communication is used in two way transmission of data.

Here is a figure of an arrangement concept of LEO:

                        In this particular arrangement, the satellites are located at the height of 400+ nautical miles above the earth, in a polar orbit. In the polar orbit, the satellite moves around the earth’s poles and passes over any specific point along its path very quickly. The satellites have a speed of 7,400 meters per second in different orbits. Gateway feeder links uses the spectrum in the Ka band. Iridium used approximately 16.5 MHz of bandwidth in the L band. 

Advantages to LEO satellite telemetry:

·         Global applications in developing a remote environmental monitoring communication system.

·         Two-way communications.

·         Easy to setup and low maintenance cost.

·         Low profile, non-directional whip helix antenna.

·         Easy access to data.

·         No FCC or other governmental agency requirements for data transmission.

·         Coverage in very remote areas.

·         Data is proprietary.

·         Stevens is able to quickly detect any problems with transmission of data.

·         Systems verifies that data has been transmitted which minimizes risk of missing data.

·         Event notification on line, by pager, etc.

·         Lower power transceivers compared to GEO transmitter system.

·         LEO transceivers hardware system is lower in cost than a GEO transmitter system.

Disadvantage to LEO satellite telemetry:  

·         Monthly service fee which could be expensive with frequent transmission of data.

·         Power outage at GCC would shut down the communication server, which could delay transmission of data to end user until power is restores. However, no data is lost.

·         LEO satellites have much shorter life span(five to eight years) than GEO satellites.

Sources:

Broadband Telecommunications Handbook by Regis Bates

http://www.stevenswater.com/telemetry_com/leo_info.aspx 

Third Generation (3G) Wireless System

               Today, internet connection is available in mobile phones. This is made possible by using GPRS technology, but now there are modern methods in accessing internet through mobile phones, these are 3G and 4G technologies.

                GPRS uses packet data technology. With packet networks, you can immediately access the mobile internet. This enables you to access the existing applications of the internet such as email and web browsing without dialing into an ISP. Using the medium or the radio link, several users can share the same radio channel very efficiently. With packet data, users will only pay for the amount of data they actually communicate and not the idle time. Packet based communication is compatible with all other types of communication. GPRS can support the Internet Protocol (IP) as well as X.25 protocol.

                   There is a new technology that takes cellular community closer to UMTS or the 3G. It has a new modulation scheme called 8-phase shift keying (PSK). It also provides higher data rates than GPRS. This technology is called Enhanced Data for Global Environment (EDGE). The data rates allocated for EDGE are started at 384 Kbps and above as a second stage to GPRS. EDGE uses a combination of FDMA and TDMA as the multiple access control methods. If we view the OSI stack model, EDGE uses FMDA and TDMA at the MAC layer.

                    EDGE is very special because its bandwidth is efficient than the GMSK modulation scheme used in the GSM standard, it creates a new modulation scheme. This technology provides a new physical layer: 8- PSK modulation, instead of GMSK. EDGE still retains existing GSM parameters. These include frame length, eight time slots per frame, and a 270.833 KHz symbol rate. These techniques help EDGE to promote worldwide or globally into the market.

                         UMTS is part of a global family of 3G mobile communications systems. It takes full advantage of the trend of converging existing and future information networks, devices and services, and the potential synergies that can be derived from such convergence. 

                          Mobile internet has many applications. There are applications for moving to a wireless and a 3G environment. These applications are customized infotainment, multimedia messaging service, mobile intranet/extranet access, mobile internet access, location based services rich voice, wireless advertising, mobile information, business solutions, mobile transactions, mobile entertainment, person-person communications and bearer entrance and periodic.

                            There is an issue that needs to consider whether to simplify provide a wireless IP pipe to a service offering hosted elsewhere on the internet, or to go for an interoperable end-to-end solution. In wireless IP pipe business using tunneling protocols will become a commodity operation, where cost, coverage, and data rate are the only competitive dimensions.

Here are some advantages and disadvantages of 3G:

Advantages:

·         Overcrowding is relieved in existing systems with radio spectrum.

·         3G has more bandwidth, security and reliability.

·         Provides interoperability among service providers.

·         Availability of fixed and variable rates.

·         Support to devices with backward compatibility with existing networks.

·         Always online devices – 3G uses IP connectivity with existing networks.

·         Rich multimedia services are available

Disadvantages:

·         The cost of cellular infrastructure, upgrading base stations is very high.

·         Needs different handsets.

·         Roaming and data/voice work together has not yet been implemented.

·         Power consumption is high.

·         Requires closer base stations and are expensive.

·         Spectrum-license costs, network deployment cost and handset subsidies subscribers are tremendous. 

Sources:

Broadband Telecommunications Handbook by Regis J. Bates

http://www.umtsworld.com/applications/applications.htm

http://www.careerride.com/3g-advantages-and-disadvantages.aspx

General Packet Radio Service

            General Packet Radio Service (GPRS) is a data network for GSM. It offers new data services for users but it has an equivalent price.  GPRS began appearing in 1999 through 2000. It uses the GSM infrastructure that was already in place. The main Focus of the GPRS was the business users. The residential users are the second choice of the operators.  With GPRS, a mass of users will pay high rates. This will make their investment cost lower and they can offer much more great services.

             GPRS is an actual packet radio service for GSM. It uses Time Division Multiple Access (TDMA). It extends the packet data capabilities of the GSM networks from Packet Data on Signaling- channel Service (PDSS) to higher data rates and longer messages. GPRS is designed to coexist with the current GSM Public Land Mobile Network (PLMN).

Here is a network view of GPRA:

              GPRS PLNM has access to wired packet data networks.  GPRS provides standards to handle higher data speeds and the transition to 3G. With GPRS, users have access to faster data speeds and it suits the internet traffic. GPRS can handle voice and data calls at the same time. It can also provide connectivity to mobile internet. Users have access to emails with large file attachments, web surfing and access to corporate Local Area Networks (LAN).

                GPRS was developed because of the demand of wireless packet data service. Many users demand for wireless internet connection. With GPRS you can send data packets through the infrastructure of a GSM. GPRS also aims for a cost efficient market. They provided data service without replacing the entire infrastructure. The initial GPRS standards make use of standard GSM radio systems, GSM standard modulation schemes and TDMA framing structures. With this the cost implications are minimized in the cell equipment.

                  GPRS radio resources are only used when sending and receiving data, this is called the packet switching. The available radio resources are shared by several users instead of one mobile user using a single channel dedicated with a fixed amount of time. With this switching technique data traffic can be lessen. The number of users in a single channel depends on the application of the user while connected to the network.  With this the operator can maximize system usage and efficiency in a dynamic and flexible way. One problem is that when several users’ increases, collision can be present between the randomly arriving data packets. This can cause users mobile surfing to queuing delays on the downlink.

                    There are several applications in GPRS mode. Its features and application can be met with other technologies.  These applications are chat, textual and visual information, still images, moving images, web browsing, document sharing/collaborative working, audio, job dispatch, corporate e-mail, Internet E-mail, vehicle positioning, remote LAN access, file transfer and home automation.

                      Chat is a general information service. The user can send information with a chat protocol. The information intensity tends to be lower with chat.

                      Textual and visual information is an application that can deliver pictures, sports scores, weather, flight information, news headline, prayer reminders, lottery results, jokes, horoscopes, traffics and others. This information may be maps or other types of visual information.

                      Still images are photographs, pictures, postcards, greeting cards, presentations and static web pages. These information can be sent or received over the mobile network.

                      Moving Images are video messages, movie previews being downloaded and data streaming via mobile device. With this application you can download movies or streaming it online. 

                     Web browsing are viewing webpages. But users don’t like web browsing on mobile phones because of delay in download or viewing a page. It takes time to view a page that has lots of images.  

Sources: 

Broadband Telecommunications Handbook by Regis J. Bates

MMDS AND LMDS

           Multichannel Multipoint Distribution Service (MMDS) is another form of a wireless communication system. It is known as the wireless cable. It is used in television broadcasting and voice communications. It operates on the Ultra High Frequency band (UHF) from 2.1 GHz to 2.7 GHz.

           MMDS uses Omni-directional antenna for broadcasting. The MMDS belongs to the medium power level. The radius of the transmitter can reach up to 70 miles in flat terrain, without mountains. MMDS has a service monthly fee. MMDS are immune to snow and fog. The MMDS originally started broadcasting at 33 analog video channels. When the analog channels where converted to digital, the 33 analog channels became 99 digital channels.

Here is a figure of a typical MMDS arrangement source: AMD

            MMDS system consists of head-end equipment and reception equipment. Head-end equipment’s are the radio transmitters and other broadcast equipment and also transmission antenna. The reception equipment is the antenna, frequency conversion device, and set-top device.  The typical range of a transmitting antenna can reach up to 35 miles, depending on the broadcast power. Usually transmitters broadcast in 1 to 100 watts range. One of the disadvantages of MMDS is that it transmits on line of sight so it is not applicable in areas that have mountains. MMDS antennas are designed to receive signals in horizontal or vertical polarized signals or even both at the subscribers’ location.  

Advantages of using MMDS:

1.    It has chunks of under-utilized spectrum that will become increasingly valuable and flexible.

2.    System implementation, which is little more than putting an installed transmitter on a high tower and a small receiving antenna on the costumers balcony or roof, is quick and inexpensive.

3.    MMDS services have been around for 30 years. There is a wealth of experience regarding the use and distribution of the services.

Internet Access is one of the applications of MMDS. This application differs from the one way communication or half duplex used in television broadcasting.

           MMDS consist of key elements. These are head-end, transmit antenna, transmission line and channel combiners. The head end functions as the distributor of the signals. It distributes the signal to as many as subscribers possible. With a good elevation and clear line of sight to the service area, provides real dividends. Community antennas are used by the CATV companies which then delivers the signal over coax cables.

            The Transmit antenna can handle a bandwidth of 200 to over 300 MHz and depending on the number of channels and their spacing. Transmit antenna requires wide bandwidth together with downward tilt and horizontal radiation patterns to focus the signal in a service are or in the subscribers area.

             In MMDS the transmission line is the free space path because it uses radio waves to transmit data. Major head-end site operates typically on 50 to 100 watts in their transmitters but often only 50 percent of this signal reach the antenna of the receiver after passing through channel combiners and transmission feeders.

             Channel combiners functions as a special filter that is used to combine the outputs of the transmitters to the transmission feeder and antenna. Combiners are designed critical to ensure that they are reliable, stable with temperature, have low return loss and provide low pass band loss.

            Local Distribution Service (LMDS) is a broadband wireless point to point specification utilizing microwave communications. It operates on FCC licensed frequencies.

Sources: 

• Broadband Telecommunications Handbook by Regis J. Bates.

Microwave and Radio Based System

         Many people have just taken for granted the importance of microwave communication. Microwave communication is very important in our daily lives. It is used for communication in great distances about a several hundred miles away. It is used in voice communication, television broadcasting, data communication and others. Microwave system suppliers have a great market on microwave communication systems. On point to point microwave radios services the suppliers have reached the sum of $4 billion of their products in 2006. 

          Microwaves have a wavelength of 1mm and 30cm long and has an operating frequency of 300 MHz to 300Ghz. Microwave was first used on the year 1930. A microwave was first discovered on World War II when two scientists invented the Magnetron which produces microwaves. It is used in Britain’s radar system to detect incoming war planes.  Microwave radio was used on 1950 for long distance voice communication. Bulky vacuum tubes were used and 2,100 watts is needed for the system to operate. But now with the help of the transistors and IC’s all has been in compact into a smaller device and much more reliable device. Microwave systems are more reliable than telephone landlines because it cannot be flooded away or no wires can be damage by animals or other means of nature. 

Here is an image of a cellular microwave radio:

           When a microwave system is transmitting on more than 40 miles a repeater is needed for the signal to reach the receiver. Repeaters are composed of a receiver and a transmitter just like a transceiver.  A repeater receives a signal then amplifies it and then reshapes the signal for transmitting into another repeater or to receiving station.

           Most television system and radio companies are using microwave systems. On television companies they use satellites for repeaters to broadcast into other countries or distant places. Another is the news station, when taking actual news on the streets the camera is connected to a van that has a portable microwave system and transmits the video via microwave to the base station to the TV news room.

          

Advantages of Microwave Radio:

1.    Radio systems do not require a right of way acquisition between stations.

2.    Each station requires the purchase or lease of only a small area of land.

3.    Because of their high operating frequencies, microwave radio systems can carry large quantities of information.

4.    High frequencies mean short wavelengths, which require relatively small antennas.

5.    Radio signals are more easily propagated around physical obstacles such as water and high mountains.

6.    Fewer repeaters are necessary for amplification.

7.    Underground facilities are minimized.

8.    Minimum delay times are introduced. Minimal crosstalk exists between voice channels.

9.    Increased reliability and less maintenance are important factors.

Disadvantages of Microwave Radio:

1.    It is more difficult to analyze and design circuits at microwave frequencies.

2.    Measuring techniques are more difficult to perfect and implement at microwave frequencies.

3.    It is difficult to implement conventional circuit components at microwave frequencies like resistors, capacitors, inductors, and so on).

4.    Transient time is more critical at microwave frequencies.

5.    It is often necessary to use specialized components for microwave frequencies.

6.    Microwave frequencies propagate in a straight line, which limits their use to line of sight applications.

Here are some pictures of an old microwave system that was used in World War II:

Sources: 

• Broadband Telecommunications Handbook, Second Edition by Regis J. Bates
• Electronic Communication Systems by Wayne Tomasi
• http://www.smecc.org/arnold_acker.htm
• http://bigdesignevents.com/2011/09/innovations-from-world-war-ii/

Asynchronous Transfer Mode

        ATM belongs to the fast packet switching family. It also uses Time Division Multiplexing (TDM) technique in transmission of data and so called traffics which also carry data or information. It sends asynchronously to the network. Traffics are sent in a form of cells or slots. This carries data to one network to the other. When there is no traffic to send the cell contains idle or empty cells because the network is synchronous.  ATM is the combination of the TDM technique and the transportation of data or traffic in a form of cells. This is an advantage for both end users of the network. ATM uses Permanent Virtual Circuits (PVC) and Switched Virtual Circuits (SVC).

        There are two choices of connections types, the PVC and SVC. The protocol is connection oriented. The circuits here are not the same. It depends on where the traffic is present on the network. When a user want’s to use it, a service provider will provide a committed bandwidth for the user to access the network. This is the basis of an ATM network.  ATM networks are on demand and has high speed communication network.

Here is a figure for an end to end connection through the network:

               In the figure the network uses VPI switching. It is handled by the network switched. It means that the switches use the virtual path for mapping through the network and will remap from one virtual path to the other while the virtual channel number is held consistent.

                Comparing the OSI model and the ATM. The ATM would probably belong to the two lower layers. The Data link layer and the Physical layer of the architecture. Such as SONET, ATM has been designed to run on a physical medium.  When comparing OSI and ATM, there is no exactly the same mapping on both architectures.

              In the ATM adaption layer. There are several sub layers present. The upper most portion of the layer is the Service Specific Convergence Sub layer (SSCS).  It is used on mapping Frame Relay and on Switched Multimegabit Data Service (SMDS). Under this layer is the Common Part Convergence Sub layer (CPCS).The combination of the SSCS and the CPCS make up the Convergence Sub layer (CS). This layer is the changing and melding of the data into a common interface for the ATM networks.  The next layer is the Segmentation And Reassembly (SAR). In this layer the data is prepared into a 48 byte payload prior to being submitted to the ATM layer for the header. 

Here are some benefits of ATM:

·   ATM uses dynamic bandwidth for burst of traffic meeting applications needs and delivering high utilization of networking resources.

·         High-speed communication

·         Fast, hardware-based switching

·         A single network connection that can reliably mix voice, video and data

·         Flexible and efficient allocation of network bandwidth

·         High speed communication

·         Opportunities for simplification via switched VC architecture; this is particularly for LAN based traffic that today is connectionless in nature.

·         High performance via hardware switching with terabit switches on the horizon 

Sources:

Broadband Telecommunications Handbook by Regis J. Bates

http://www.telecomspace.com/vop-atm.html

http://technet.microsoft.com/en-us/library/cc740081(WS.10).aspx

Frame Relay

       Frame relay was introduced in 1992.  It is a virtual circuit wide area network that was designed to respond to demands for a new type of WAN. Some wide area networks uses virtual circuit switching called X.25. This network carries packets from one place to another. Frame relay falls in to the category of a packet switching family.

         Packet switch is a switching technology that technology that store and forward messages. When messages are broken down into smaller parts it now called packets. Each packet contains a destination address and control information. It uses the time division multiplexing technique in transferring data. Packets are sent from the source to the destination through a shared network. Packet switching has several applications including electronic fund transfer, credit card approvals, point of sale equipment, short files and email.

      Combining packet switching and a fast network using high speed communications and delay networking are called Fast packet switching. It is designed to reduce the delay by using a hold and forward technology. With this technology it reduces overhead and processing, improve speed and reduce cost. It is also designed to run on high speed circuits with low error rates.

Frame Relay Layers

Physical Layer – Frame Relay uses whatever protocol is available. It supports any of the protocol supported by American National Standards Institute (ANSI).

Data Link Layer – In this layer the Frame Relay employs a simplified version of High Level Data Link Control (HDLC). HDLC provides extensive error and flow control fields that are not needed in Frame Relay. 

Here is a figure that show the format of a frame relay frame:

·         Address (DLCI) field – The 1^st part of the DLCI field is 6 bits with the first byte. The second part of the DCLI uses the first 4 bits of the second byte.

·         Command Response (C/R) – It allows the upper layer to identify a frame as either a command or a response.

·         Extended Address (EA) – It indicates whether the current byte is the final byte of the address.

·         Forward Explicit Congestion Notification (FECN) – It is set by any switch to indicate that traffic is congested in the direction in which the frame is traveling.

·         Backward Explicit Congestion Notification (BECN) – it is set to indicate a congestion problem in the direction opposite to the one in which the frame is traveling.

·         Discard Eligibility (DE) – It indicates the priority level of the frame.

·         Extended Address (EA) – The frame relay address has been extended from the original 2-byte address to 3 or 4 byte addresses.

Frame Relay was developed to carry data traffic across the WAN link and Local Area Networks (LANs) to other LANs. Here is a figure of the transmission of data across to the local loop to the local telephone company’s central office that is connected to the interexchange carriers:

Advantages of Frame Relay

•  The main advantage of Frame Relay over point-to-point leased lines is cost. Frame Relay can provide  performance similar to that of a leased line, but with significantly less cost over long distances.

Disadvantages of Frame Relay

• One main disadvantages of Frame Relay are slowdowns due to network congestions. Because all of the company’s costumers on Frame Relay uses common network and there are times that the network exceeds its capacity on data transmission.

• Another main disadvantage is that there are difficulties on ensuring the Quality of Service (QoS). It is due to the fact that Frame Relay uses variable length packets. It is easier to guarantee QoS when using a fixed-length packet.

Sources:

Broadband Telecommunications Handbook by Regis J. “BUD” Bates

Data Communications and Networking third edition by Behrouz A. Forouzan

Wan Design with Frame Delay by David Horton

Integrated Service Digital Network

 Integrated services Digital Network(ISDN) is a set of protocol or standards for digital transmission of voice, data, video, and other network services over the traditional circuits of the public switched telephone network.

        
         ISDN was a concept developed by the Consultative Committee on International Telegraph and Telephone (CCITT). The CCIT changed their name to  ITU-T. It is a UN treaty organization. Each country is entitled to send representatives that come from the government that run PTT monopoly to any committee meetings. The name was changed due to the privatization trend separating telephone business from the post office and the general elimination of telegraph service.

Here are some study groups of CCITT with their own area of expertise:

a.)  SG VII on public data networks (X.25) X−series standards

b.)  SG VIII terminal equipment for telematic services

c.)   SG XI ISDN and telephone network switching and signaling

d.)  SG XII transmission performance of telephone networks and terminals

e.)  SG XV transmission systems

f.)    SG XVII data transmission over public telephone networks

g.)  SG XVIII digital networks, including ISDN

     The Basic Rate Interface(BRI) was intended for the standard subscriber interface. It specifies the costumer interface I.45x. It has two bearer channels and a data channel. The costumers’ information is placed on the two bearers’ channels. The concept is that from analog calls to teleconferencing data and these would be switched channels. In the calling party, the analog telephone circuit is digitized to 64kbps at the local Telco office before being switched across the network. It is then turned back into analog at the far end before being delivered to the called party. But now we use two telephone circuits with BRI.

       Here’s a figure that shows the BRI graphically and indicates the bandwidth allocation on the ISDN interface. This is a time-division multiplexed interface where the B,D, and overhead bits are interleaved.

This figure represents the network terminal type of the BRI:

The NT1 creates a four−wire bus called the T interface onto which each ISDN device is connected. It was provided by the Telco as part of the ISDN service.

The NT2 terminal is the device that would do the switching. Allowing the standard eight devices to share the T bus by creating perhaps multiple S buses.

The terminal equipment type 1 (TE1) is a standard ISDN terminal that is

capable of dealing with the B and D channels.

The terminal equipment type 2 (TE2) is a standard device having an RS−232 or V.35 interface. But it doesn't have an ISDN interface capable of handling the D and B channels.

The terminal adapter (TA) is the semi−intelligent device that lets a TE2 connect to the S/T ISDN interface. Its primary function is to run the ISDN interface for our TE2.

Applications of the ISDN interface.

Multiple Channels - Its concept was to provide access to every possible home device and for up to 8 devices using two B channels and one D channels to share among eight devices.

Telephone - From the calling party which is digital instead of being analog from the handset to the central office where it becomes digitized. With ISDN the conversation can be digitized directly from the calling party and passed digitally to the central office and to the called party. It is converted directly into digital at the starting point.

Digital Fax - Fax machines now have to be digital. Therefore, the Group IV fax standard specifies 64 KBps fax operation.

Analog Fax - Analog fax are plug into telephones through a modem that would convert analog tones to digital at 64 KBps.

Computer/Video Conferencing - video conferencing equipment can use one of the 64 KBps or bond both bearer channels together for a 128 KBps digital channel across the network.

Signaling - The function of the data channels is to provide for signaling for setting up and tearing down of the switched bearer channels.

Sources:

1.)  www.wikipedia.com. Issue Date: May 1986 Volume: 4 Issue: 3 On page(s): 320 - 325 ISSN: 0733-                   8716

2.) Future trends R. Aaron, R. Wyndrum, AT&T Bell Laboratories, IEEE Communications Magazine, March 1986, volume 24 #3, pp 38-43. Retrieved 2007-09-02.

3.)   Broadband Telecommunications Handbook second edition by Regis J. Bates.