tdma vs scpc technical note - rev f

8/10/2019 TDMA vs SCPC Technical Note - Rev F

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28 August 2014 TDMA vs. SCPC Page 1


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TDMA vs. SCPC

SatLinkTechnical NoteDoc # 200660


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Background

TDM/TDMAand SCPCare the main alternative technologies for satellite networking in the world today. The modem and

management technologies underlying both approaches have been advancing rapidly in recent years, causing some

confusion as to which technology is better for a given set of networking requirements. This technical note will explain the

important trends and trade-offs.

These two alternative technologies are illustrated inFigure 1 andFigure 2 for a simple star topology network, highlighting

their key elements and configuration differences.

Figure 1: TDM/TDMA Network(Time Division Multiplexing with Time Division Multiple Access)


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Figure 2: SCPC Network(Single Channel Per Carrier)

SCPC uses a separate dedicated carrier to each remote terminal ("VSAT") to receive information from the central site, and

another dedicated carrier for each VSAT to transmit information back to the central site. Both carriers are modulated in

"continuous mode". Usually some non-standard coding techniques are used since SCPC technology was never

standardized.

In contrast, TDM/TDMA technology uses a single high-speed TDM carrier transmitted from the central site or "Hub", from

which many VSATs can receive information. For this TDM forward link, the DVB-S2standard is most commonly used. It is

also the most flexible for multiplexing many concurrent streams of traffic to different sites, and the most efficient with itssupport of Adaptive Coding and Modulation (ACM). ACM dynamically adjusts the modulation and coding on the "virtual

link" to each VSAT individually, as local conditions (e.g., weather, interference) at the VSAT change.

To transmit back to the central site efficiently, the VSATs in a TDM/TDMA network are synchronized, and they transmit

information in "burst mode" within a series of short, scheduled timeslots. Timeslots may be assigned across multiple TDMA

carriers and accessed using "fast frequency hopping". Timeslots are assigned to each VSAT exclusively (i.e., without

contention) based on their current traffic needs. This is called Dynamic TDMA, and it is the most advanced form of

TDM/TDMA. This technology is fully standardized internationally by the DVB group under the DVB-RCSfamily of standards.

TDM/TDMA networks allow all VSATs to dynamically share multiple TDMA carriers, as if they were a single large pool of

bandwidth. Each TDMA carrier groupmay contain dozens of carriers, with up to 32 carriers per carrier group in a SatLink

network. Therefore the "return link" may contain huge amounts of capacity, in aggregate.

In a SatLink TDM/TDMA network the TDMA carriers may operate at widely different symbol rates (e.g., from 500 ksps to 5

Msps and even higher). To determine which VSAT will use which timeslots on which carrier at any moment, SatLink has ACS

(Adaptive Carrier Selection). ACSis applied dynamically for each VSAT, given its local weather conditions, configuration

(e.g., antenna and BUC size) and service policy (e.g., maximum rate requirements). ACS determines what carrier & symbol

rate will work best at the current signal levels of those available in the carrier group.


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In addition, in a SatLink DVB-RCS2("2nd Generation") network, ACM per burstis supported for each VSAT and on all TDMA

carriers in the carrier group. This further optimizes efficiency, throughput, and reliability for each VSAT and greatly

simplifies network operations. Any VSAT can use any MODCOD, on any carrier, if necessary.

DVB-RCS2 SatLink TDM/TDMA networks now surpass SCPC networks not only in efficiency, but also in throughput and link

availability for almost any conceivable network configuration and satellite band (e.g., C, Ku, X, and Ka band).

Total Cost-of-Ownership Comparison

Today, SCPC only makes economic or technical sense for very small networks; that is, those with only a handful of remote

sites and relative low-speed links, where total transponder capacity requirements are low (e.g., just a few MHz) and

opportunities for bandwidth sharing are limited.

For most larger networks (e.g., with >20 sites and many MHz of transponder capacity), the use of TDM/TDMA will result in

much lower operating cost (OPEX) than incurred with SCPC technology. Typically transponder requirements can be reducedby 50% to 80% or more, while also increasingaverage link speeds, application throughputs, and overall link availability for

"free".

The relative financial advantage of TDM/TDMA vs. SCPC is shown inFigure 3.

Figure 3: Number of Sites Required to Justify TDM/TDMA vs. SCPC

TotalCostofOwnership

(OPEX+AmortizedCAPEX)

Number of Sites in the Network

50 100 150 200

In this cost analysis OPEX is the dominant consideration. The reason a TDM/TDMA network is not easily justified when there

are just a few sites is due to the fixed capital cost of the Hub for a TDM/TDMA network, which is higher than the cost of a f ew

SCPC modems.

In the range of 20 to 50 sites, the total transponder OPEX, plus details of the network traffic patterns and various user

requirements, must be examined more closely to determine which technology offers a lower cost of ownership.


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Using teleport hub services to support a small number TDM/TDMA VSATs can also shift the balance in favor of TDM/TDMA,

enabling "virtual private" TDM/TDMA networks with just a few VSATs that replace SCPC with good savings for network

operators.

Statistical Multiplexing of IP Traffic Across Sites

One major reason for the ascendency of TDM/TDMA over SCPC in the last decade is that user traffic in satellite networks is

increasingly all IP (Internet Protocol), regardless of whether it is data, voice, or video. Most TDM/TDMA networks are

optimized for handling IP traffic of all types with excellent QoS. This is especially true of SatLink.

In particular, interactive IP data traffic (e.g., web browsing, email, downloads) is notoriously bursty. This results in huge

statistical multiplexing gains from using bandwidth-on-demand. These efficiency gains can reduce aggregate bandwidth

needs, and therefore transponder OPEX, by 10x or more, depending on two key traffic pattern attributes:

the peak-to-average load ratios for traffic at each site (Tx and Rx) during the peak hour, and

the variability in the timing of the peak hour, across the various sites.

Even video, audio, and voice traffic have peak load timings and magnitudes that are variable throughout the month, the

week, and the day. And their loads fluctuate during the peak hour as well. Thus they all benefit from using bandwidth-on-

demand which matches actual load requirements second-by-second. This benefit is especially applicable when using the

most efficient new codecs for these traditional "real-time" media types, when transporting them over IP links.

Furthermore, getting the maximum feasible peak speed for each VSAT site has become very important for all types of IP

applications and media. Having just 500 kbps or 1 Mbps per site as the maximum rate in a satellite network is no longer

acceptable to most user sites (with a few exceptions). User sites in many satellite networks now demand peak load receive

(Rx) rates of 10 to 40 Mbps or more, and peak load transmit (Tx) rates of 3 to 20 Mbps, or more. And these rates are only

going to increase in coming years.

Higher peak transfer speeds increase the peak-to-average load ratio per site during the peak hour, and therefore greatly

favor the trend to use TDM/TDMA and replace SCPC in all forms.

Hybrid TDM/SCPC Networks

Due to the increasing dominance of IP traffic, many former SCPC networks have already been converted to TDM/TDMA.

However, some SCPC networks have converted only "half-way", whereby a DVB-S2 TDM carrier is used on the forward link,

but SCPC links are used for return link communications. This is illustrated inFigure 4.


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Figure 4: TDM/SCPC Hybrid Network

This hybrid configuration is called "TDM/SCPC" for convenience here. If using DVB-S2 it gets the full benefits of statistical

multiplexing and ACM on the forward link, but these benefits are non-existent on the return link in this hybrid network .

Therefore, the technical and business rationales for using the TDM/SCPC hybrid networks are weak at best.

Nonetheless, the TDM/SCPC hybrid configuration is commonly promoted and used in certain types of VSAT networks, in

particular in cellular backhaulnetworks and in some other types of networks where fast access to large amounts of capacity

for the return link (upstream) traffic must be guaranteed.

There are three possible reasons for the continued use of this form of SCPC:

A belief that SCPC ("continuous mode" ) will provide better modem efficiency (in bps/Hz) than TDMA burst mode

due to lower overhead and ability to use higher-rate, more efficient MODCODs.

A belief that SCPC links are better at providing guaranteed capacity and will operate more reliably against rain

fades, interference, or congestion.

A belief that SCPC links will provide lower latency or less total delay.

A belief that SCPC links can be operated at a higher speed, when necessary, for any or all sites within the satellite

transponder footprint.

These beliefs (or some of them) are true with respect to the limitations of some popular TDM/TDMA technologies. For thosetechnologies, the hybrid TDM/SCPC option is useful and may even be "cost effective" in networks with nearly constant levels

of traffic in the peak hour at each site, a consistent peak hour time each day.

However, in comparison to SatLink TDM/TDMA networks using the DVB-RCS2 standards, these conditions do not hold

true. In fact, the opposite is true.

The SatLink DVB-RCS2 implementation exceeds or matches the modem performance specifications of all but the most

expensive SCPC modems with information rates > 24 Mbps for Tx.


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In terms of total network efficiency, a SatLink DVB-RCS2 return link (operating in TDMA burst mode !) will deliver 2x more in

bps/Hz than some popularSCPCoptions, even before adding in the benefits of statistical multiplexing with TDMA.

Table 1compares the return link performance of SatLink DVB-RCS2 TDMA modems against the SCPC return link modem

options of two leading suppliers: Comtech EF Data and iDirect.

Table 1: SatLink TDMA vs. SCPC Modems (for Return Links Only)

AttributeSatLink TDMA

(DVB-RCS2)

COMTECH

CDM-625 SCPC

Modems

iDirect's

SCPC Option for

Evolution

Return Link

Technology Used

DVB-RCS2 standard

16-State Turbo Code

(Proprietary)

VersaFEC

(Proprietary)

2D 16-State

MODCODs supported

QPSK 1/3 to 5/6

8PSK 2/3 to 5/616QAM 3/4 to 5/6

BPSK: .488

QPSK .533 to .803

8QAM .642 to .78016QAM .731 to .853

QPSK 1/2 to 4/5

(Plus: 8PSK 4/5 & 6/7

but only if symbol rate is

>1.5 Msps)

Adaptive Carrier Selection

(ACS)

Yes

(dynamically selects

best carrier & symbol

rate for each burst)

No

(Fixed Carrier)

No(Fixed Carrier)

ACM on Return LinkYes, included

ACM per TDMA Burst(DVB-RCS2 standard)

Yes(Proprietary)

Extra Cost OptionNo

Minimum Return Link

Symbol Rate125 ksps 18 ksps

128 ksps

(1.5 Msps at 8PSK)

Maximum Return Link

Symbol Rate8 Msps 12.5 Msps 15 Msps

Maximum Return Link Info Rate(after subtracting TDMA burst overheads

for SatLink DVB-RCS2 modems

24 Mbpsat 16QAM 5/6

& 8 Msps

14 Mbps

(in ACM mode)

24 Mbpsat QPSK 4/5

& 15 Msps(QPSK required for this max)

Maximum Return Link (Tx) VSAT

IP Throughput(after physical layer overheads

w/ specified equipment)

24 Mbps

(in ACM mode)

14 Mbps

(in ACM mode)

15.6 Mbps(for X5 router)

20 Mbps(for 8000 series)

Return Link SNR requiredfor same bps/Hz & error rates

(after all overheads)

See SatLinkmodem performance

documentation

+/- 0.3 dBvs. SatLink

+/- 0.3dBvs. SatLink

Average Return Link

Spectral Efficiency(after SatLink's burst mode overheads

& required minimum carrier s pacing

applied to each modem)

1. 6 to 2.6 bps/Hzwith 8PSK & 16QAM at

1.12x carrier spacing

1.6 to 2.6 bps/Hz8PSK & 16QAM at


0.8 to 1.3 bps/Hz

with QPSK at


Because no ACM,

see Section0


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AttributeSatLink TDMA

(DVB-RCS2)

COMTECH

CDM-625 SCPC

Modems

iDirect's

SCPC Option for

Evolution

Link Availability Best(with ACS & ACM)

Good(with ACM)

Poor, or will reduceefficiency & speed

(without ACM)

Latencies & Delays


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Lack of ACM Reduces iDirect's Average Efficiency

Because iDirect has no support for ACM on their SCPC return links, this means it is unlikely they can operate at their most

efficient MODCODs (8PSK 4/5 and 6/7). In most Ku and Ka band networks, iDirect VSATs will need to use QPSK 1/2 to QPSK

4/5 to have sufficient fade margin with typical Ku or Ka band transponders and typical VSAT configurations (0.98 to 1.8

meter antennas with 2W to 4W BUCs), to get 99.7% availability or better (unless operating in a desert).

Also iDirect requires using QPSK in two important and common scenarios:

QPSK must be used to get the maximum data throughput rates at the VSAT (because 8PSK is very "processing

intensive" for them). This means that the higher symbol rates normally allowed with iDirect's SCPC option (those

from 10 Msps to 15 Msps) cannot be used with 8PSK.

QPSK must also be used for symbol rates less than 1.5 Msps. This is unfortunate because SCPC links often need

lower symbol rates than 1.5 Mbps.

Therefore iDirect SCPC links will use QSPK 1/2 to QPSK 4/5 most commonly (requiring SNR levels of 2.2 to 6.0 dB,

respectively, during fades). They will have corresponding efficiencies of 0.8 to 1.3 bps/Hz, according to iDirect modem

documentation.

In contrast, SatLink's modem efficiency in Ku and Ka band will average from 1.6 and 2.6 bps/Hz, by using 8PSK and 16QAM,

or 2x better efficiency than iDirect's SCPC,with typical transponders and VSAT configurations. This is because the use of

ACM allows "clear sky" link budgets (or nearly so) to prevail for >90% of the time in most regions of the world, greatly

increasing average network efficiency across all sites.

Furthermore, due to statistical multiplexing of IP traffic, the required capacity of the TDMA carrier group, in Mbps, is much

less than the sum of the required SCPC return links would be (typically 2x to 8x less), depending on traffic patterns across the

sites.

Link Availability Advantages of SatLink with ACS and ACM

When using ACS & ACM within a TDMA carrier group there are advantages not possible when using ACM alone on individual

SCPC links. This explains why SatLink's overall link availability will be better than what COMTECH can obtain in any

comparable network. The differences will be most noticeable in Ku and Ka band networks where rain fades can be large.

With ACS in a SatLink network, it is possible for the Hub to dynamically change the carrier symbol rate used by a VSAT. This

is done by selecting a different carrier in the carrier group for its burst. When there is severe local fading at a VSAT, simply

reducing the MODCOD with ACM may not be sufficient. Also that reduces bandwidth efficiency.

With ACM and ACS working together in a SatLink network, the MODCOD and the symbol rate for a VSAT can both be

reduced during a severe fade. This allows the VSAT's return link to keep operating during the most severe fade (albeit at alower information rate). Maintaining return link connectivity during fades is as important as maintaining forward link

connectivity, since all monitoring of the VSAT (including the ACM control feedback for the forward link) relies upon the

return link.

The lowest link availability, though, will tend to occur when there is no ACM or ACS available, as with the iDirect SCPC link

(and by the way, also true with iDirect TDMA links). In those cases, increased link availability can be obtained only by setting

the "fixed" MODCOD to be used all the time and the symbol rate as low as possible, which hurts speed, throughput, and

efficiency.


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SatLink TDM/TDMA Delivers Lower Latencies than SCPC

TDMA return link carriers in a SatLink network using DVB-RCS2 standards deliver lower latency than SCPC links. This is

accomplished in three ways:

Being able to operate at high information rates, up to 24 Mbps, to reduce serialization delays.

Using relatively small FEC block sizes (=burst sizes) compared to SCPC modems. For example, only 540 symbols for

the "small burst" and 1620 symbols for the "long burst" vs. 4,000 symbols for the FEC blocks used by COMTECH's

VersaFEC.

Using advanced methods for assigning bandwidth-on-demand so that there are no delays for receiving capacity

assignments once activity has begun.

The relatively low information rates common on many SCPC carriers (e.g., 500 kbps to 2 Mbps) used to avoid excessive

amounts of dedicated capacity to a single site are often a large source of delays from an end-user perspective when

uploading emails, photos, or other large data options.

The same is true for the TDM Forward Link carrier in a TDM/TDMA network. It will have much lower latency than each of

many SCPC forward link carriers operating lower symbol rates and low information rates.

Use of DVB-S2 for "SCPC" Return Link

Some suppliers (e.g., Gilat) are now using DVB-S2 TDM carriers as their "SCPC " return link option, within the context of the

hybrid TDM/SCPC configuration ofFigure 4.

Using DVB-S2 in this way has some strong points and also creates some issues.

The strengths are that it is very efficient and supports ACM (unlike iDirect's proprietary SCPC option). The weaknesses are:

It will have a high latency very low symbol rate carriers (e.g., < 500 ksps) even when using the short frames option.

It is more expensive for each site, and requires one dedicated DVB-S2 receiver at the Hub site for each remote site

using the SCPC option.

It is not feasible to switch rapidly between DVB-S2 (continuous mode carriers) and a TDMA burst mode carriers in

the event that site needs the option to use either of two modes.

When compared to SatLink's implementation of DVB-RCS2 for TDMA (i.e., burst mode), the efficiency of DVB-S2 modems

are very similar. They are only slightly better (less than 0.5 dB of SNR difference for the same bps/Hz, in the higher rateMODCODs). Even the overhead differences are small (a few %) when comparing similar FEC frame sizes. And because it is

not possible to use "frequency hopping" among DVB-S2 carriers there is no option for using ACS to adjust symbol rates

dynamically.

And, of course, as with all SCPC options on the return link, there is no ability to gain from statistical multiplexing of traffic

across multiple transmitting sites.


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Advantages of SatLink TDM/TDMA for Cellular Backhaul

Given the results above, the advantages of using a SatLink TDM/TDMA network for cellular backhaul applications (vs. a

TDM/SCPC network) can be significant.

Most of the advantage will come from statistical multiplexing the return link traffic across multiple transmitting sites. With

the rapid growth of cellular data, even return link traffic is rapidly becoming mostly data traffic(e.g., for email, photo

uploads, video uploads, etc.), with a smaller and smaller share as cellular voice traffic.

Even the voice traffic in cellular networks, given the advance of 3G and 4G, is being carried over IP with variable rate codecs,

plus silence suppression in certain regions of the world.

This means the statistical multiplexing gains of using TDMA for return link traffic may easily exceed 2x for a satellite network

with just 5 or more cell sites. That means 50% less transponder capacity is required for the return link capacity.

SatLink networks also offer many efficiency advantages for the DVB-S2 forward link, with MODCODs up to 32APSK (not

offered by iDirect, nor on the COMTECH CDM-625), which become feasible when investing in large remote site antennas.

Thus forward link capacities can reach well over 150 Mbps using only 45 Msps.

The ACM feature on SatLink DVB-S2 carriers also provides the most advanced QoS and traffic engineering features to

assure each cell site receives its necessary capacity and quality for delay sensitive voice and video traffic during rain fades or

other forms of interference or congestion.

Option for an "SCPC-like" Dedicated Carrier in SatLink

Sometimes in a customer network, a special technical or security reason may exist that requires a dedicated return link

carrier for a given VSAT site. Even though SatLink does not support an SCPC option in "continuous mode", it is simple to

assign a TDMA carrier to a given SatLink VSAT for all its return link transmissions.

This is done easily in a SatLink network via a configuration in the SatLink NMS. However, the VSAT will still use burst mode

transmissions on this dedicated carrier. In SatLink this is called the "SCPC-like" mode.

As we explained above there is no penalty from using burst mode transmissions for creating this "SCPC-like" mode in a

SatLink network. And it still allows that VSAT to use ACM per burst, and all the advanced QoS features of SatLink.

Of course, preventing a VSAT from using ACS (i.e., hopping across carriers based on immediate needs) is not usually a good

network design strategy. For example, VSAT transmissions could be impaired if interference or jamming at some frequencyaffects its assigned carrier, or if it needs to operate at a higher or lower symbol rate to transmit at higher speed or improve

its SNR during rain fades.

Therefore, no efficiency, speed, or reliability benefits are gained from assigning a dedicated return link carrier to a specific

VSAT in a SatLink network.


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Justification for Conversion from SCPC to TDM/TDMA

We saw inFigure 3,earlier, that the total cost of ownership will usually favor using TDM/TDMA over SCPC for networks with

20 sites or more. Likewise the conversion from an old ("fully depreciated") SCPC network can usually be justified when the

number of sites is growing to 20 or 30 or more.

Conversion to TDM/TDMA is driven by the combination of following:

Increased business efficiency, revenue, and value from higher reliability, speeds, throughputs, and more total

network capacity.

OPEX savings from reduced transponder capacity requirements.

OPEX savings from reduced operations staff.

OPEX savings from reduced hardware and software maintenance fees on SCPC equipment and software features.

Sometimes financial justifications can be found for a TDM/TDMA network upgrade even if the network is composed of only

5 sites. The degree of savings will depend on the following:

Average load per site (Gigabytes per hour) in the peak hour for Tx and Rx.

Degree of randomness in peak hour timing across sites.

Mix of traffic types (data, voice, video streaming, video conferencing).

Star topology only, or star and mesh topology needs.

Link availability requirements during worst month.

Satellite band(s) to be used (e.g., C, Ku, X, or Ka band).

Geography of the network (e.g., how many sites within tropical vs. temperate vs. arid environments) or sites at low

look angles to the satellite.

How widely scattered remote sites will be, and whether there will be diverse VSAT antenna sizes, mobile VSATs, or

other special site requirements.

EMC will assist any potential customer with a fair and detailed analysis, including documentation of all assumptions, to help

them evaluate whether SCPC or TDM/TDMA will deliver better performance relative to their network requirements, and

how both the OPEX and CAPEX for the network will compare under each alternative. It is worth noting that networks with

more than 50 sites easily justify TDM/TDMA except in rare situations.

tdma vs scpc technical note - rev f

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