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Lecture 2.11 Module 2. AVIATION TELECOMMUNICATION SYSTEMS Topic 2.11. MULTISTATION ACCESS IN SATELLITE COMUNICATION. Capacity Allocation. FDMA FAMA-FDMA DAMA-FDMA TDMA Advantages over FDMA. FDMA.
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Lecture 2.11Module 2. AVIATION TELECOMMUNICATION SYSTEMSTopic 2.11. MULTISTATION ACCESS IN SATELLITE COMUNICATION
Capacity Allocation • FDMA • FAMA-FDMA • DAMA-FDMA • TDMA • Advantages over FDMA
FDMA • Satellite frequency is already broken into bands, and is broken in to smaller channels in Frequency Division Multiple Access (FDMA). • Overall bandwidth within a frequency band is increased due to frequency reuse (a frequency is used by two carriers with orthogonal polarization).
FDMA (cont.) • The number of sub-channels is limited by three factors: • Thermal noise (too weak a signal will be effected by background noise). • Intermodulation noise (too strong a signal will cause noise). • Crosstalk (cause by excessive frequency reusing).
FDMA (cont.) • FDMA can be performed in two ways: • Fixed-assignment multiple access (FAMA): The sub-channel assignments are of a fixed allotment. Ideal for broadcast satellite communication. • Demand-assignment multiple access (DAMA): The sub-channel allotment changes based on demand. Ideal for point to point communication.
TDMA • TDMA (Time Division Multiple Access) breaks a transmission into multiple time slots, each one dedicated to a different transmitter. • TDMA is increasingly becoming more widespread in satellite communication. • TDMA uses the same techniques (FAMA and DAMA) as FDMA does.
TDMA (cont.) • Advantages of TDMA over FDMA. • Digital equipment used in time division multiplexing is increasingly becoming cheaper. • There are advantages in digital transmission techniques. Ex: error correction. • Lack of intermodulation noise means increased efficiency.
FIXED-ASSIGNED MULTIPLE ACCESSFixed-assigned multiple access (FAMA) is one of the two main techniques for allocating channels to users. In FAMA, each user is allocated a channel permanently, whether they use it or not.
Demand Assigned Multiple AccessDemand Assigned Multiple Access (DAMA) is a technology used to assign a bandwidth to clients that don't need to use it constantly. DAMA systems assign communication channels or circuits based on requests issued from user terminals to a network control system. When the circuit is no longer in use, the channels are then returned to the central pool for reuse by others.Channels are typically a pair of carrier frequencies (one for transmit and one for receive), but can be other fixed bandwidth resources such as timeslots in a TDMA burst plan. Once allocated to a pair of nodes this bandwidth is not available to other users in the network until their session is finished.
It allows utilizing of one channel (frequency band, timeslot, etc.) by many users at different times. This technology is mainly used by small clients, as opposed to PAMA (Permanently Assigned Multiple Access). By using DAMA technology the amount of users that can use a limited pool of circuits can be greatly increased.DAMA and PAMA are related only to bandwidth assignment and are not to be mixed with the Multiple access methods intended to divide a bandwidth between several users at one time, which include FDMA, TDMA, CDMA and others. These systems typically allow a more determenistic near real time allocation of bandwidth based on demands and data priority.
DAMA is widely used in satellite communications, especially in VSAT systems. It is very effective in environments comprising multiple users each having a low to moderate usage profile.DAMA is often used in military environments due to the relative simplicity of implementation, ease of modeling, and the fact that military usage profiles are a very good fit.
ALOHAnetALOHAnet, also known as ALOHA, was a pioneering computer networking system developed at the University of Hawaii. It was first deployed in 1970, and while the network itself is no longer used, one of the core concepts in the network is the basis for the widely used Ethernet.
The idea was to use low-cost amateur radio-like systems to create a computer network linking the far-flung campuses of the University. The original version of ALOHA used two distinct frequencies in a hub/star configuration, with the hub machine broadcasting packets to everyone on the "outbound" channel, and the various client machines sending data to the hub on the "inbound" channel. Data received was immediately re-sent, allowing clients to determine whether or not their data had been received properly. Any machine noticing corrupted data would wait a short time and then re-send the packet. This mechanism was also used to detect and correct for "collisions" created when two client machines both attempted to send a packet at the same time.
Like the ARPANET group, ALOHA was important because it used a shared medium for transmission. This revealed the need for more modern medium access control schemes such as CSMA/CD, used by Ethernet. Unlike the ARPANET where each node could only talk to a node on the other end of the wire, in ALOHA all nodes were communicating on the same frequency. This meant that some sort of system was needed to control who could talk at what time. ALOHA's situation was similar to issues faced by Ethernet (non-switched) and Wi-Fi networks.
This shared transmission medium system generated interest by others. ALOHA's scheme was very simple. Because data was sent via a teletype the data rate usually did not go beyond 80 characters per second. When two stations tried to talk at the same time, both transmissions were garbled. Then data had to be manually resent.
The ALOHA protocol is an OSI layer 2protocol for LAN networks with broadcasttopology.The first version of the protocol was basic:- If you have data to send, send the data- If the message collides with another transmission, try resending "later"Many people have made a study of the protocol. The critical aspect is the "later" concept. The quality of the backoff scheme chosen significantly influences the efficiency of the protocol, the ultimate channel capacity, and the predictability of its behavior.
Pure Aloha had a maximum throughput of about 18.4%. This means that about 81.6% of the total available bandwidth was essentially wasted due to losses from packet collisions. The basic throughput calculation involves the assumption that the aggregate arrival process follows a Poisson distribution with an average number of arrivals of 2G arrivals per 2X seconds. Therefore, the lambda parameter in the Poisson distribution becomes 2G. The mentioned peak is reached for G = 0.5 resulting in a maximum throughput of 0.184, i.e. 18.4%.
An improvement to the original Aloha protocol was Slotted Aloha, which introduced discrete timeslots and increased the maximum throughput to 36.8%. A station can send only at the beginning of a timeslot, and thus collisions are reduced. In this case, the average number of aggregate arrivals is G arrivals per 2X seconds. This leverages the lambda parameter to be G. The maximum throughput is reached for G = 1.
Because Listen before send (CSMA - Carrier Sense Multiple Access), as used in the Ethernet, works much better than Aloha for all cases where all the stations can hear each other, Slotted Aloha is used on low bandwidth tactical Satellite communications networks by the US Military, subscriber based Satellite communications networks, and contactless RFID technologies.
DescriptionPrior to ALOHAnet, most computer communications tended to share similar features. The data to be sent was turned into an analogsignal using a device similar to a modem, which would be sent over a known connection like a telephone line. The connection was point-to-point, and set up (typically) by manual control.
In contrast ALOHAnet was a true network. All of the computers "connected" to ALOHAnet could send data at any time without operator intervention, and any number of computers could be involved. Since the medium was a radio, there were no fixed costs so the channel was "left open" and could be used at any time.Using a shared signal in this way leads to an important problem: If two systems on the network – known as nodes – send at the same time, both signals will be ruined. Some sort of mechanism needs to be in place to avoid this problem. There are a number of ways to do this.
One would be to use a different radio frequency for every node, a system known as frequency multiplexing. However this would require each node added to able to be "tuned in" by all of the other machines. Soon there would be hundreds of such frequencies, and radios capable of listening to this number of frequencies at the same time are very expensive.Another solution is to have "time slots" into which each node is allowed to send, known as time division multiplexing. This is easier to implement because the nodes can continue to share a single radio frequency. On the downside if a particular node has nothing to send, their slot goes wasted. This leads to situations where the available time is largely empty and the one node with data to send has to do so very slowly just in case one of the other 100 nodes decides to send something.
ALOHAnet instead utilised a new solution to the problem, one that has since been developed to become the standard, carrier sense multiple access. In the Aloha system nodes which needed to send simply sent out their frames as soon as they were ready.Normally this would mean that the first node to start using the radio would have it for as long as it wanted, which means the other nodes couldn't "get a word in edgewise". In order to avoid this problem the ALOHAnet made the nodes break down their messages into small packets, and send them one at a time with gaps between them. This allowed other nodes to send out their packets in between, so everyone could share the medium at the same time.
There is one last problem to consider: if two nodes attempt to start their broadcast at the same time, you'll have the same sorts of problems you would with any other system. In this case ALOHAnet invented a very clever solution. After sending any packet the nodes listened to see if their own message was sent back to them by a central hub. If they got their message back, they could move on to their next packet.
If instead they never got their packet back, that would mean that something prevented it from arriving at the hub – like a collision with another node's packet. In this case they simply waited a random time and tried again. Since each node picked a random time to wait, one of them would be the first to re-try, and the other nodes would then see that the channel was in use when they tried. Under most circumstances this would avoid collisions.This sort of collision avoidance system has the advantage of allowing any one node to use the entire network's capability if no one else is using it. It also requires no "setup"; anyone can be hooked up and start talking without any additional information like the frequency or time slot to use.
On the downside, if the network gets busy the number of collisions can rise dramatically to the point where every packet will collide. For ALOHAnet the maximum channel utilisation was around 18%, and any attempts to drive the network over this would simply increase collisions, and the overall data throughput would actually decrease, a phenomenon known as congestion collapse.
With Slotted Aloha, a centralised clock sent out small clock tick packets to the outlying stations. Outlying stations were only allowed to send their packets immediately after receiving a clock tick. If there is only one station with a packet to send, this guarantees that there will never be a collision for that packet. On the other hand if there are two stations with packets to send, this algorithm guarantees that there will be a collision, and the whole of the slot period up to the next clock tick is wasted. With some mathematics, it is possible to demonstrate that this protocol does improve the overall channel utilisation, by reducing the probability of collisions by a half.
The ALOHAnet itself was run using 9600 baud radio modems across Hawaii. The system uses two 100 kHz "channels" (slices of frequency), one known as the broadcast channel at 413.475 MHz, and the other the random access channel at 407.350 MHz. The network was a star, with a single central computer (a HP 2100) at the university receiving all messages on the random access channel, and then re-broadcasting them to all of the nodes on the broadcast frequency. This setup reduced the number of collisions possible, there could be no collisions at all on the broadcast frequency, for what that was worth. Later upgrades added repeaters that also acted as hubs, greatly increasing the area and total capability of the network.
Reservation ALOHAReservation ALOHA, or R-ALOHA, is a channel access method for wirelesstransmission which allows uncoordinated users to share a common transmission resource. Reservation ALOHA (and its parent scheme, Slotted ALOHA) is a schema or rule set for the division of transmission resources over fixed time increments, also known as slots. It is via this rule set or schema that, if followed, allows the bandwidth users to cooperatively utilize a shared transmission resource—in this case, it is the allocation of transmission time.
Reservation ALOHA is an effort to improve the efficiency of Slotted ALOHA. The improvements with Reservation ALOHA are markedly shorter delays and ability to efficiently support higher levels of utilization. As a contrast of efficiency, simulations have shown that Reservation ALOHA exhibits less delay at 80% utilization than Slotted ALOHA at 20-36% utilization.
The chief difference between Slotted ALOHA and Reservation ALOHA is that with Slotted ALOHA, any slot is available for utilization without regards to prior usage. Under Reservation ALOHA's contention-based reservation schema, the slot is temporarily considered "owned" by the station that successfully used it. Also with Reservation ALOHA, once the station has completed its transmission, it simply stops sending data. As a rule, idle slots are considered available to all stations that may then implicitly reserve (utilize) the slot on a contention basis.
Single channel per carrierSingle channel per carrier (SCPC) refers to using a single signal at a given frequency and bandwidth. Most often, this is used on broadcast satellites to indicate that radio stations are not multiplexed as subcarriers onto a single videocarrier, but instead independently share a transponder. It may also be used on other communications satellites, or occasionally on non-satellite transmissions.
In an SCPC system, satellite bandwidth is dedicated to a single source. This makes sense if it is being used for something like satellite radio, which broadcasts continuously. Another very common application is voice, where a small amount of fixed bandwidth is required. However, it does not make sense for burst transmissions like satellite internet access or telemetry, since a customer would have to pay for the satellite bandwidth even when they were not using it.
Where multiple access is concerned, SCPC is essentially FDMA. Some applications use SCPC instead of TDMA, because they require guaranteed, unrestricted bandwidth. As satellite TDMA technology improves however, the applications for SCPC are becoming more limited.
Advantages- simple and reliable technology- low-cost equipment- any bandwidth (up to a full transponder) - usually 64 kbit/s to 30 Mbit/s- easy to add additional receive sites (earth stations)
Disadvantages- inefficient use of satellite bandwidth for burst transmissions, typically encountered with packet data transmission- usually requires on-site control- when used in remote locations, the transmitting dish must be protected- a dish which is moved out of alignment can result in fines as high as $1,100 per minute (as of 2003) from the satellite operator
MCPCWith multiple channels per carrier (MCPC), several subcarriers are combined into a single bitstream before being modulated onto a carrier transmitted from a single location to one or more remote sites. This uses time-division multiplexing (TDM). It is a retronym of sorts, as it was the only way radio networks were transmitted ("piggybacked" on television networks) until SCPC.In digital radio and digital television, an ensemble or other multiplex or multichannel stations can be considered MCPC, though the term is generally only applied to satellites.The major disadvantage of MCPC is that all of the signals must be sent to a single place first, then combined for retransmission — a major reason for using SCPC instead.
