Aloha, Slotted Aloha, Selective Reject Aloha, and DA-TDMA Protocols for VSAT

This page compares Aloha, Slotted Aloha, Selective Reject Aloha, and DA-TDMA protocols, all used in VSAT (Very Small Aperture Terminal) systems.

As we know, a VSAT network consists of a hub station and multiple VSAT stations (or terminals). VSAT networks typically operate using TDM/TDMA-based protocol mechanisms.

There are two types of communication links:

• Hub to VSAT (outbound link)
• VSAT to Hub (inbound link)

Depending on traffic demands, packets are multiplexed and transmitted on a single carrier from the Hub to all the VSATs in the network. Each VSAT checks the address and confirms if the transmission is intended for it. If the address matches, the VSAT decodes the information. Depending on network size, there may be one or more inbound carriers.

When VSAT-1 wants to communicate with VSAT-2, it sends data to the Hub. The Hub then relays the data to VSAT-2 on the second hop.

Protocols used in VSAT networks are often called random or contention protocols. This is because there’s no central authority controlling which VSAT transmits and when. This lack of central control allows inbound capacity to be open for contention among VSAT terminals. Each VSAT transmits data packets at random intervals, competing with other VSATs for capacity on inbound carrier links.

The contention protocols commonly used in VSAT networks are:

• ALOHA Protocol
• Slotted ALOHA Protocol
• Selective Reject ALOHA Protocol
• Demand Assignment TDMA (DA-TDMA)

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.

The ALOHA protocol is the simplest contention technique used in VSAT networks.

• When a VSAT has data to send, it forms a packet and transmits it to the Hub.
• The VSAT (e.g., VSAT-1) waits for an ACK (acknowledgment) from the Hub. If all goes well, the ACK should arrive within a time equal to twice the round-trip delay.
• If another VSAT (e.g., VSAT-2) transmits at the same time, a collision occurs. The Hub ignores the corrupted packets and doesn’t send any ACKs.
• When VSAT-1 doesn’t receive an ACK, it retransmits the packet after a random delay. This retransmission adds to the packet load.

As shown in the figure, MSG-2 and MSG-3 collide, so their transmissions are rescheduled after a random time delay to avoid future collisions.

Slotted ALOHA Protocol (S-Aloha)

Figure-2 depicts operation of Slotted ALOHA Protocol.

The Slotted ALOHA protocol, also known as S-Aloha, introduces dedicated time slots in the inbound carrier. This reduces the likelihood of collisions and improves throughput efficiency.

In this protocol, each VSAT synchronizes with the master clock transmitted in the outbound carrier by the Hub. This way, all VSATs in the network create their own time slots. This synchronization helps VSATs transmit information in dedicated chunks of time slots rather than cyclically.

Each VSAT creates a fixed-length packet and transmits only at the beginning of its assigned time slot. Data Terminating Equipment (DTE), such as a modem/mux, originates the data, which is then assembled by the VSAT into a fixed-size packet. After packet construction, the VSAT buffers the packet until the start of the next time slot and then transmits it.

As shown, MSG-2 and MSG-1 are transmitted at the beginning of their dedicated slot markers. This insertion of slots reduces the probability of packet collisions compared to the pure ALOHA protocol.

Slotted ALOHA doubles the maximum carrier throughput of pure Aloha to about 36%.

Selective Reject ALOHA Protocol (SREJ Aloha)

Figure-3 depicts operation of Selective Reject ALOHA Protocol.

The Selective Reject ALOHA protocol, also known as SREJ Aloha, is a non-slotted, random access VSAT protocol. It achieves a throughput equivalent to Slotted Aloha without needing synchronization.

The Slotted Aloha protocol formats a packet as sub-packets, each consisting of a header, acquisition preamble, data, and trailer. This protocol takes advantage of the fact that packet collisions often affect only portions of the entire packet. Therefore, only the collided portion should be retransmitted, rather than the entire packet. Dividing the packet into sub-packets simplifies this task.

As shown, MSG-1 and MSG-2, transmitted by VSAT-1 and VSAT-2, respectively, collide. During the collision, parts (4, 5) of VSAT-1 and parts (1, 2) of VSAT-2 are corrupted. Therefore, in SREJ Aloha, only parts (4, 5) of MSG-1 and parts (1, 2) of MSG-2 are retransmitted.

SREJ Aloha has a throughput in the range of 30% and works well for variable-length message scenarios.

Demand Assigned TDMA (DA-TDMA) Protocol

Figure-4 depicts operation of DA-TDMA (Demand Assigned TDMA) Protocol.

DA-TDMA is a VSAT protocol that allows VSATs to transmit in a conflict-free manner by reserving time periods for transmission. Other VSATs can then use the remaining inbound slots and carriers. DA-TDMA offers high throughput and lower delay, guaranteeing performance even under high traffic load conditions.

DA-TDMA operates in one of two modes (S-Aloha or SREJ Aloha) based on packet size.

If the information from the DTE fits within the packet and slot size of the inbound carrier, the network uses Slotted Aloha. If the information is lengthy, the VSAT prepares a packet requesting capacity reservation and transmits it using S-Aloha.

Once the Hub station receives this packet, it reserves several time slots specifically for the requesting VSAT. The Hub then informs all VSATs in the network about the reservation, so other VSATs don’t contend for capacity during the reserved period. The remaining VSATs can use the remaining inbound slots and carriers.

Comparison Table