Satellite & Space

DAMA (Demand Assigned Multiple Access)

Q: What is the difference between DAMA and PAMA on a satellite link?

PAMA (Pre-Assigned Multiple Access) gives each terminal a fixed, dedicated carrier whether or not it has traffic, so a 50-channel transponder serves at most 50 terminals regardless of activity. DAMA pools the same capacity and assigns a carrier only during an active call or burst, so if terminals are active only 5 to 10 percent of the time, the same transponder can support several hundred users. The trade-off is call-setup latency: DAMA must request and grant a channel before traffic flows, adding 0.5 to 2 seconds of setup time per session, whereas a PAMA channel is always ready. DAMA is therefore preferred for bursty, low-duty-cycle traffic such as voice and tactical messaging, while PAMA suits continuous high-rate trunks.

Q: How much call-setup latency does DAMA add over a GEO satellite?

A demand-assigned channel request must travel from the terminal to the network control center (NCC), which assigns a free carrier and signals the destination terminal before traffic starts. Over a geostationary satellite, each hop incurs about 250 ms of one-way propagation delay (roughly 36,000 km up and down). A typical DAMA reservation handshake involves a request to the NCC, an assignment broadcast, and channel acquisition, totaling two to four satellite hops. Real-world military DAMA systems on UHF (the MIL-STD-188-183 5 kHz and 25 kHz channels) show call-setup times of roughly 0.5 to 2 seconds. After setup, the assigned channel behaves like a normal SCPC link with the usual single-hop 250 ms latency.

Q: Which multiplexing schemes do DAMA controllers assign?

DAMA is a control philosophy layered on top of an underlying access scheme. In FDMA-based DAMA (SCPC/DAMA), the controller assigns a discrete carrier frequency and bandwidth, common in commercial VSAT and Inmarsat networks. In MF-TDMA DAMA, the controller assigns time-slot and carrier-frequency pairs from a burst-time plan, used by iDirect and military WGS terminals to share capacity dynamically. Code-based DAMA over CDMA assigns spreading codes on request. Modern hub-based VSAT systems combine demand assignment for the return (terminal-to-hub) link with a continuous TDM outbound carrier, since the outbound is always busy and gains nothing from on-demand allocation.

/DAH-mah/, Demand Assigned Multiple Access

Used to squeeze far more users out of a single satellite transponder, this channel-access technique hands a terminal a carrier (a frequency slot, a TDMA burst, or a spreading code) only while it actually has traffic, then returns that capacity to the shared pool the instant the call ends. A network control center arbitrates every request, so a pool sized for, say, 50 simultaneous SCPC carriers can serve hundreds of low-duty-cycle terminals. The cost is a call-setup handshake of roughly 0.5 to 2 seconds over a GEO link. DAMA dominates bursty voice and messaging traffic in military UHF SATCOM and commercial VSAT return links, where most terminals are idle most of the time.

Category: Satellite & Space

Setup Latency: 0.5 to 2 s (GEO)

UHF Channels: 5 / 25 kHz (MIL-STD-188-183)

How Demand Assignment Shares a Transponder

Satellite capacity is expensive, and most user traffic is bursty. A tactical voice terminal or a remote VSAT may carry data only 5 to 10 percent of the time, yet a pre-assigned (PAMA) carrier reserves bandwidth around the clock whether the terminal is talking or silent. Demand assignment breaks that waste by treating the transponder as a shared pool. When a terminal has traffic, it sends a short request to a network control center (NCC) over a common signaling channel; the NCC checks for a free resource, assigns a carrier or time-slot, and signals both ends to acquire the channel. When the session ends, the resource returns to the pool for the next requester.

The defining engineering trade-off is latency for efficiency. Every demand-assigned session pays a setup penalty: the request, the assignment, and the channel acquisition each cross the geostationary arc, where one-way propagation is about 250 ms over roughly 36,000 km of path. Military UHF DAMA systems built to MIL-STD-188-183 (the 5 kHz and 25 kHz channelization of the 240 to 318 MHz band) typically show 0.5 to 2 second call-setup times. Once the channel is granted, it behaves like an ordinary single-carrier-per-channel link with the usual single-hop delay, so the penalty is paid only once per session, not per packet.

DAMA is best understood as a control layer rather than a waveform. It rides on top of CDMA, FDMA, or TDMA: a frequency-division controller hands out discrete carriers (SCPC/DAMA), an MF-TDMA controller hands out carrier-and-timeslot pairs from a burst-time plan, and a code-division controller hands out spreading codes. The dimensioning math is classic teletraffic engineering, so a DAMA pool is sized with the Erlang B formula to hold blocking probability below a target during the busy hour.

DAMA Channel Sizing and Latency Equations

Erlang B blocking (pool of N channels, offered load A erlangs):
B(N, A) = (A^N / N!) / ∑_k=0^N (A^k / k!)

Offered traffic from M terminals:
A ≈ M × ρ_util (erlangs), where ρ_util = per-terminal activity factor

GEO call-setup latency (n satellite hops):
T_setup ≈ n × t_hop + T_proc, t_hop ≈ 250 ms

Example: M = 400 terminals at ρ_util = 0.08 → A = 32 erlangs. To hold B ≈ 1%, Erlang B needs N ≈ 43 channels, so 43 carriers serve 400 users. With n = 3 hops and T_proc ≈ 0.5 s, T_setup ≈ 1.25 s.

DAMA Versus Fixed and Random Access

Access Scheme	Allocation	Capacity Efficiency	Setup Latency	Best Traffic Type
DAMA	On request, per session	High (5 to 10× PAMA for bursty users)	0.5 to 2 s (GEO)	Bursty voice, tactical messaging
PAMA / SCPC fixed	Dedicated, permanent	Low at low duty cycle	None (always on)	Continuous trunks, high-rate data
MF-TDMA (DAMA-controlled)	Carrier + time-slot, dynamic	Very high, fine granularity	0.5 to 2 s	Mesh VSAT, mixed-rate networks
Random access (Aloha/CRDSA)	Contention, no reservation	Moderate (collisions limit it)	None (no handshake)	Tiny signaling, IoT bursts
Fixed TDMA	Pre-planned slot per terminal	Low if slots idle	None after sync	Predictable periodic data

Common Questions

Frequently Asked Questions

What is the difference between DAMA and PAMA on a satellite link?

PAMA gives every terminal a permanent dedicated carrier, so a 50-channel transponder serves at most 50 terminals regardless of activity. DAMA pools that capacity and assigns a carrier only during an active session; if terminals are busy just 5 to 10 percent of the time, the same transponder supports several hundred users. The cost is a 0.5 to 2 second call-setup handshake, so DAMA suits bursty voice and messaging while PAMA suits continuous trunks.

How much call-setup latency does DAMA add over a GEO satellite?

The request reaches the network control center, which assigns a free carrier and signals the far terminal before traffic flows. Each GEO hop adds about 250 ms of one-way delay over roughly 36,000 km. A reservation handshake spans two to four hops, so military UHF DAMA on MIL-STD-188-183 5 kHz and 25 kHz channels shows 0.5 to 2 second setup. After that the channel behaves like a normal SCPC link at single-hop 250 ms latency.

Which multiplexing schemes do DAMA controllers assign?

DAMA is a control layer over an underlying access scheme. FDMA-based DAMA (SCPC/DAMA) assigns a discrete carrier frequency and bandwidth, common in Inmarsat and VSAT networks. MF-TDMA DAMA assigns carrier-and-timeslot pairs from a burst-time plan, used by iDirect and military WGS terminals. CDMA-based DAMA assigns spreading codes on request. Hub VSAT systems usually demand-assign only the return link, keeping a continuous TDM outbound.

Related Terms

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