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Aloha,slotted Aloha,Selective Reject Aloha,DA-TDMA protocol for VSAT


This page on Aloha protocol vs Slotted Aloha protocol vs Selective Reject Aloha protocol vs DA-TDMA mentions difference between Aloha, Slotted Aloha and Selective Reject Aloha, DA-TDMA protocol used in VSAT systems. Refer satellite tutorial covering following sub topics:
Main Satellite Tutorial Page   About satellite   Bands   Parts   Orbits   Services   Types   capacity allocations   network configurations   Applications   Remote sensing  


As we know VSAT network consists of a Hub station and number of VSAT stations or terminals. VSAT network works on TDM/TDMA based protocol mechanism. Refer TDMA basics➤➤ and TDM basics➤➤ for more information on TDMA and TDM respectively.

There are two types of links viz. Hub to VSAT (known as outbound link) and VSAT to Hub (known as inbound link). Before analyzing these protocols, we recommend to study article on VSAT system basics.

Depensing on the traffic capacity, all the packets are multiplexed and transmitted on single carrier from HUB to all the different VSATs in the network. Respective VSATs will check the address and confirms whether the transmission is intended for them or not. If the address matches their own address, then VSAT will decode the information carried in the packet.

Depending upon size of the network, there will be one or more inbound carriers. When VSAT-1 say wants to communicate with VSAT-2, VSAT-1 sends the data to Hub, Hub will relay the data to VSAT-2 on second hop.

Protocols used in VSAT are referred as random or contention protocols. This is due to the fact that there is no central provision or control which determines which VSAT will transmit and when. This lack of central control in VSAT network keeps inbound capacity open for contention among VSAT terminals.

Each VSAT terminal transmits data as packets at random time intervals and contends with peer VSAT terminals for capacity on inbound carrier links. The contention protocols typically used in VSAT are described as follows:
• ALOHA Protocol
• Slotted ALOHA Protocol
• Selective Reject ALOHA Protocol
• Demand Assignment TDMA With slotted ALOHA reservation access

ALOHA Protocol used in VSAT

Aloha Protocol

Figure-1 depicts operation of ALOHA Protocol. Here MSG 1(i.e. message 1 is from VSAT-1), MSG 2 is from VSAT-2 and MSG 3 is from VSAT-3 and so on. This protocol is the simplest of contention technique used in VSAT network.

• Here whenever there is data to be send, packet is formed and transmitted by VSAT to Hub.

• VSAT (say VSAT-1) will wait for ACK from Hub. If everything is fine, ACK will be received within the time as expected (i.e. equal to two times round trip delay)

• However another say VSAT-2 transmits at the same time instant, it will results into collision. In this situation Hub will ignore the corrupted packets and will not transmit any ACKs.

• When VSAT-1 does not receive any ACK, it re-transmits the packet after some random delay. Here retransmission will create any additional packet loading.

Figure shows MSG-2 and MSG-3 are collided and hence their transmissions are re-scheduled after random time delay such that they will not collide.

Slotted Aloha protocol,S-Aloha

Slotted Aloha Protocol

Figure-2 depicts operation of Slotted ALOHA Protocol. It is also known by S-Aloha. This VSAT protocol type inserts dedicated time slots in the inbound carrier. This will reduce likelihood of collision and also improves throughput efficiency.

In this slotted ALOHA protocol; each VSATs synchronize themselves with the master clock transmitted in the outbound carrier by the Hub. This way all the VSATs in the network create their own time slots. This synchronization will help VSATs to transmit information in dedicated chunks of time slots rather than cyclic way.

In this type of protocol, each VSAT creates fixed length packet and transmit only at the beginning of the respective time slot.

As we know DTE (Data Terminating Equipment) such as modem/mux will originate data. This data is assembled by the VSAT in the form of fixed size packet. After the packet construction, VSAT buffers its packet till the start of next time slot arrives and then transmits it. As shown MSG-2 and MSG-1 are transmitted at the beginning of their dedicated slot markers.

This concept of insertion of slots reduces probability of packet collisions obsered in pure ALOHA protocol above. Hence SLotted ALOHA doubles the maximum carrier throughput of pure Aloha to about 36%.

Selective Reject Aloha protocol-SREJ Aloha

Selective Reject Aloha Protocol

Figure-3 depicts operation of Selective Reject ALOHA Protocol. It is also known by SREJ Aloha. It is a non-slotted and random access type of VSAT protocol. It achieves the throughput equivalent to SLotted Aloha without the need of synchronization.

The Slotted Aloha protocol formats a packet in the form of sub-packets. Each subpackets will consist of header, acquisition preamble, data and trailer part. This protocol exploits the fact that packet collisions occur only in some portion of the entire packet and hence the retransmission should be carried out for the collided portion rather than the entire packet. The division of packet into subpackets will simply this task and hence only subpackets which have been collided will be retransmitted rather than the entire packet.

As shown in the figure-3, MSG-1 and MSG-2 are the transmissions of VSAT-1 and VSAT-2. They have collided and during collision parts (4,5) of VSAT-1 and parts (1,2) get corrupted. Hence in this Selective Reject Aloha (SREJ ALOHA) only parts (4,5) of MSG-1 and parts (1,2) of MSG-2 are retransmitted.

Slotted Aloha protocol will have throughput in the range of 30%. It works fine for variable length message scenarios.

VSAT DA-TDMA protocol

DA-TDMA Protocol

Figure-4 depicts operation of DA-TDMA (Demand Assigned TDMA) Protocol. TDMA is the short form of Time Division Multiple Access. This VSAT protocol allows the VSAT to transmit in a conflict free manner by reserving its period for transmission. The rest of the VSATs can use remaining in-bound slots ans carries. The advantages of DA-TDMA protocol is high throughput and lower delay. The performance of this protocol is guaranteed in high traffic load conditions.

This DA-TDMA protocol takes one of the two modes (S-Aloha and SREJ Aloha) based on size of the packets.

If the information from DTE fits within the packet and slot size of inbound carrier, then network will use Slotted Aloha.

If the information from DTE is lengthy, VSAT prepares packet which requests capacity reservation and transmit the packet using S-Aloha protocol. Once this packet is received by the Hub station, it reserves several time slots dedicatedly to the requesting VSAT demanding reservation. In addition Hub informs allthe VSATs in the network about the reservation made and hence other VSATs will not contend during this reserved period for the capacity requirements of their own. The rest of the VSATs can use remaining in-bound slots as well as carriers. The same have been explained in figure-4.

Following table mentions comparison of Aloha, Slotted Aloha, Selective Reject Aloha(SREJ Aloha) and DA-TDMA protocols with respect to their use in VSAT.


VSAT Protocol Technique Max. throughput Typical Delay Application Remarks(if any)
Aloha 13 to 18 % <0.5 Sec Variable length messages Timing not required
Slotted Aloha 25 to 36 % <0.5 Sec Fixed length messages
Selective Reject Aloha 20 to 30% <0.5 Sec variable length mesages capacity competitive with slotted Aloha
DA-TDMA 60 to 80 % < 2 Sec variable length messages Generally attractive for long messages (batch data, voice)

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