tdma vs scpc technical note - rev d

8/11/2019 TDMA vs SCPC Technical Note - Rev D

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TDMA vs. SCPC Technical Note

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SCPC uses a separate dedicated carrier to each remote terminal ("VSAT") to receiveinformation from the central site, and another dedicated carrier for each VSAT totransmit information back to the central site. Both carriers are modulated in"continuous mode ". Usually some non-standard coding techniques are used sinceSCPC technology was never standardized.

In contrast, TDM/TDMA technology uses a single high-speed TDM carrier transmittedfrom the central site or "Hub", from which many VSATs can receive information. Forthis TDM forward link, the DVB-S2 standard is most commonly used. It is also the mostflexible for multiplexing many concurrent streams of traffic to different sites, and themost efficient with its support of Adaptive Coding and Modulation (ACM). ACMdynamically adjusts the modulation and coding on the "virtual link" to each VSATindividually, 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 aresynchronized, and they transmit information in " burst mode " within a series of short,

scheduled timeslots. Timeslots may be assigned across multiple TDMA carriers andaccessed using " fast frequency hopping ". Timeslots are assigned to each VSATexclusively (i.e., without contention) based on their current traffic needs. This is calledDynamic TDMA , and it is the most advanced form of TDM/TDMA. This technology isfully standardized internationally by the DVB group under the DVB-RCS family ofstandards.

TDM/TDMA networks allow all VSATs to dynamically share multiple TDMA carriers, as ifthey were a single large pool of bandwidth. Each TDMA carrier group may containdozens 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 differentsymbol rates (e.g., from 500 ksps to 5 Msps and even higher). To determine whichVSAT will use which timeslots on which carrier at any moment, SatLink has ACS(Adaptive Carrier Selection). ACS is applied dynamically for each VSAT, given itslocal weather conditions, configuration (e.g., antenna and BUC size) and servicepolicy (e.g., maximum rate requirements). ACS determines what carrier & symbolrate will work best at the current signal levels of those available in the carrier group.

In addition, in a SatLink DVB-RCS2 ("2nd Generation") network, ACM per burst issupported for each VSAT and on all TDMA carriers in the carrier group. This furtheroptimizes efficiency, throughput, and reliability for each VSAT and greatly simplifiesnetwork operations. Any VSAT can use any MODCOD, on any carrier, if necessary.

DVB-RCS2 SatLink TDM/TDMA networks now surpass SCPC networks not only inefficiency, but also in throughput and link availability for almost any conceivablenetwork configuration and satellite band (e.g., C, Ku, X, and Ka band).

2. Total Cost-of-Ownership ComparisonToday, 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 totaltransponder capacity requirements are low (e.g., just a few MHz) and opportunitiesfor bandwidth sharing are limited.


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For most larger networks (e.g., with >20 sites and many MHz of transpondercapacity), the use of TDM/TDMA will result in much lower operating cost (OPEX) thanincurred with SCPC technology. Typically transponder requirements can be reducedby 50% to 80% or more, while also increasing average link speeds , applicationthroughputs , and overall link availability for "free".

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

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

In this cost analysis OPEX is the dominant consideration. The reason a TDM/TDMAnetwork is not easily justified when there are just a few sites is due to the fixed capitalcost of the Hub for a TDM/TDMA network, which is higher than the cost of a fewSCPC modems.

In the range of 20 to 50 sites, the total transponder OPEX, plus details of the networktraffic patterns and various user requirements, must be examined more closely todetermine which technology offers a lower cost of ownership.

Using teleport hub services to support a small number TDM/TDMA VSATs can also shiftthe balance in favor of TDM/TDMA, enabling "virtual private " TDM/TDMA networkswith just a few VSATs that replace SCPC with good savings for network operators.

3. Statistical Multiplexing of IP Traffic Across SitesOne major reason for the ascendency of TDM/TDMA over SCPC in the last decade isthat user traffic in satellite networks is increasingly all IP (Internet Protocol) , regardlessof whether it is data, voice, or video. Most TDM/TDMA networks are optimized forhandling 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 usingbandwidth-on-demand . These efficiency gains can reduce aggregate bandwidth

T o

t a l C o s

t o f O w n e r s

h i p

( O P E X + A m o r t

i z e d

C A P E X )

Number of Sites in the Network

50 100 150 200


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

the peak-to-average load ratios for traffic at each site (Tx and Rx) during thepeak 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 thatare variable throughout the month, the week, and the day. And their loads fluctuateduring the peak hour as well. Thus they all benefit from using bandwidth-on-demandwhich matches actual load requirements second-by-second. This benefit isespecially 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 500kbps or 1 Mbps per site as the maximum rate in a satellite network is no longeracceptable to most user sites (with a few exceptions). User sites in many satellitenetworks now demand peak load receive (Rx) rates of 10 to 40 Mbps or more, andpeak load transmit (Tx) rates of 3 to 20 Mbps, or more. And these rates are only goingto increase in coming years.

Higher peak transfer speeds increase the peak-to-average load ratio per site duringthe peak hour, and therefore greatly favor the trend to use TDM/TDMA and replaceSCPC in all forms.

4. Hybrid TDM/SCPC NetworksDue to the increasing dominance of IP traffic, many former SCPC networks havealready been converted to TDM/TDMA. However, some SCPC networks haveconverted 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 in Figure 4.


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Figure 4: TDM/SCPC Hybrid Network

This hybrid configuration is called "TDM/SCPC" for convenience here. If using DVB-S2it gets the full benefits of statistical multiplexing and ACM on the forward link, butthese benefits are non-existent on the return link in this hybrid network . Therefore, thetechnical and business rationales for using the TDM/SCPC hybrid networks are weakat best.

Nonetheless, the TDM/SCPC hybrid configuration is commonly promoted and used incertain types of VSAT networks, in particular in cellular backhaul networks and insome 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 usehigher-rate, more efficient MODCODs.

A belief that SCPC links are better at providing guaranteed capacity and willoperate 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 somepopular TDM/TDMA technologies. For those technologies, the hybrid TDM/SCPCoption is useful and may even be "cost effective" in networks with nearly constantlevels 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-RCS2standards, these conditions do not hold true . In fact, the opposite is true.

The SatLink DVB-RCS2 implementation exceeds or matches the modem performance


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specifications of all but the most expensive SCPC modems with information rates > 24Mbps for Tx.

In terms of total network efficiency, a SatLink DVB-RCS2 return link ( operating in TDMAburst mode ! ) will deliver 2x more in bps/Hz than some popular SCPC options, even

before adding in the benefits of statistical multiplexing with TDMA.

Table 1 compares the return link performance of SatLink DVB-RCS2 TDMA modemsagainst the SCPC return link modem options of two leading suppliers: Comtech EFData and iDirect.

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

Attribute SatLink TDMA(DVB-RCS2)COMTECH

CDM-625 SCPC Modems

iDirect'sSCPC Option for

Evolution Return Link

Technology Used

DVB-RCS2 standard

16-State Turbo Code

(Proprietary)

VersaFEC

(Proprietary)

2D 16-State

MODCODs supportedQPSK 1/3 to 5/68PSK 2/3 to 5/6

16QAM 3/4 to 5/6

BPSK: .488QPSK .533 to .8038QAM .642 to .780

16QAM .731 to .853

QPSK 1/2 to 4/5 (Plus: 8PSK 4/5 & 6/7

but only if symbolrate is >1.5 Msps)

Adaptive Carrier Selection(ACS)

Yes(dynamically selectsbest carrier & symbolrate for each burst )

No (Fixed Carrier)

No (Fixed Carrier)

ACM on Return Link Yes, included

ACM per TDMA Burst (DVB-RCS2 standard)

Yes(Proprietary)

Extra Cost Option No

Minimum Return LinkSymbol Rate 125 ksps 18 ksps

128 ksps (1.5 Msps at 8PSK)

Maximum Return LinkSymbol Rate 8 Msps 12.5 Msps 15 Msps

Maximum Return Link Info Rate (after subtracting TDMA burst overheads

for SatLink DVB-RCS2 modems

24 Mbps at 16QAM 5/6

& 8 Msps 14 Mbps

(in ACM mode)

24 Mbps at QPSK 4/5& 15 Msps

(QPSK required for this max)

Maximum Return Link (Tx) VSATIP Throughput

(after physical layer overheadsw/ 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 required for same bps/Hz & error rates

(after all overheads)

See SatLinkmodem performance

documentation

+/- 0.3 dB vs. SatLink

+/- 0.3dB vs. SatLink

Average Return LinkSpectral Efficiency

(after SatLink's burst mode overheads& required minimum carrier spacing

applied to each modem)

1. 6 to 2.6 bps/Hz with 8PSK & 16QAM at1.12x carrier spacing

1.6 to 2.6 bps/Hz 8PSK & 16QAM at

1.25x carrier spacing

0.8 to 1.3 bps/Hz with QPSK at

1.2x carrier spacingBecause no ACM, see Section 4.2

Link Availability Best (with ACS & ACM)

Good(with ACM)

Poor, or will reduceefficiency & speed

(without ACM)

Latencies & Delays


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using their ACM feature. Their latency could be less if using a different FEC.

The COMTECH CDM-625 modem is normally used in a traditional SCPC networkwhere both forward and return links are SCPC. COMTECH also has the CDM-840modem, with similar SCPC specs, but which works with a central site DVB-S2 TDMmodem for the forward link in multi-site multiplexed basis (i.e., TDM/SCPC hybrid).

The iDirect SCPC option can be used on any iDirect VSAT in their Evolution productline (with the DVB-S2 forward link) and that VSAT may be part of an iDirectTDM/TDMA network, managed from the same NMS.

Sections below further explain the key points of Table 1 and why SatLink TDMA burstmodems using DVB-RCS2 match or exceed the SCPC modem performance of thesetwo suppliers.

4.1 SatLink's Advanced TDMA MODCOD Support with ACS & ACMSatLink DVB-RCS2 burst modems are the first to provide high-order MODCODs (i.e.,both 8PSK and 16QAM) with the newest, most advanced FEC (Forward ErrorCorrection) technologies using 16-state turbo codes for industry-leading efficiencyand total performance, with carrier spacing of only 1.12x symbol rate.

Furthermore, this is done to work at symbol rates up to 8 Msps , and IP throughputrates up to 24 Mbps on Tx (from VSAT-to-Hub) on TDMA burst mode carriers. Theserates are more than 4x higher than many TDMA burst modems. And they workconcurrently with all TCP acceleration, IP routing, IP packet filtering, and QoSfeatures operating at these same speeds.

SatLink DVB-RCS2 burst modems are also the first and only to support both ACS and

ACM per burst, dynamically adapted to the immediate local conditions at the VSATat these very high speeds.

The combination of all this is powerful and unique in the satellite networking industry.

In addition, SatLink DVB-RCS2 modems have excellent signal-to-noise (SNR)requirements, which puts them slightly lower than the very best SCPC modems for thesame spectrum efficiency (bps/Hz) and bit error rate results. And they have very lowburst overheads.

Therefore even after subtracting burst overheads, SatLink modem efficiency matches

or exceeds that of SCPC modems, in terms of the bps/Hz and Quasi-Error Free (QEF) performance , at all comparable operating points for SNR (C/N or Eb/No.)

(Note: we assume here that the "long-burst" option is used on SatLink's DVB-RCS2modems, which is fair when comparing with SCPC options. The differences in

required SNR vs. these other technologies are typically +/- 0.3 dB, depending on the MODCODs used for comparison. All SatLink efficiency figures in bps/Hz shown haveburst overheads subtracted from their nominal efficiency and adjust for carrier

spacing differences.)

4.2 Lack of ACM Reduces iDirect's Average EfficiencyBecause iDirect has no support for ACM on their SCPC return links, this means it isunlikely they can operate at their most efficient MODCODs (8PSK 4/5 and 6/7). In


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most Ku and Ka band networks, iDirect VSATs will need to use QPSK 1/2 to QPSK 4/5to have sufficient fade margin with typical Ku or Ka band transponders and typicalVSAT 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 thehigher symbol rates normally allowed with iDirect's SCPC option (those from 10Msps to 15 Msps) cannot be used with 8PSK.

QPSK must also be used for symbol rates less than 1.5 Msps. This is unfortunatebecause 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 (requiringSNR levels of 2.2 to 6.0 dB, respectively, during fades). They will have correspondingefficiencies 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 and2.6 bps/Hz, by using 8PSK and 16QAM, or 2x better efficiency than iDirect's SCPC, withtypical 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 ofthe world, greatly increasing average network efficiency across all sites.

Furthermore, due to statistical multiplexing of IP traffic, the required capacity of theTDMA carrier group, in Mbps, is much less than the sum of the required SCPC returnlinks would be (typically 2x to 8x less), depending on traffic patterns across the sites.

4.3 Link Availability Advantages of SatLink with ACS and ACMWhen using ACS & ACM within a TDMA carrier group there are advantages notpossible when using ACM alone on individual SCPC links. This explains why SatLink'soverall link availability will be better than what COMTECH can obtain in anycomparable network. The differences will be most noticeable in Ku and Ka bandnetworks 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 thecarrier group for its burst. When there is severe local fading at a VSAT, simplyreducing the MODCOD with ACM may not be sufficient. Also that reducesbandwidth efficiency.

With ACM and ACS working together in a SatLink network, the MODCOD and thesymbol rate for a VSAT can both be reduced during a severe fade. This allows theVSAT's return link to keep operating during the most severe fade (albeit at a lowerinformation rate). Maintaining return link connectivity during fades is as important asmaintaining forward link connectivity, since all monitoring of the VSAT (including theACM 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 ACSavailable, as with the iDirect SCPC link (and by the way, also true with iDirect TDMA


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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.

4.4 SatLink TDM/TDMA Delivers Lower Latencies than SCPCTDMA return link carriers in a SatLink network using DVB-RCS2 standards deliver lowerlatency than SCPC links. This is accomplished in three ways:

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

Using relatively small FEC block sizes (=burst sizes) compared to SCPCmodems. For example, only 540 symbols for the "small burst" and 1620 symbolsfor the "long burst" vs. 4,000 symbols for the FEC blocks used by COMTECH'sVersaFEC .

Using advanced methods for assigning bandwidth-on-demand so that thereare no delays for receiving capacity assignments once activity has begun.

The relatively low information rates common on many SCPC carriers (e.g., 500 kbps to2 Mbps) used to avoid excessive amounts of dedicated capacity to a single site areoften 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 havemuch lower latency than each of many SCPC forward link carriers operating lowersymbol rates and low information rates.

5. Use of DVB-S2 for "SCPC" Return LinkSome suppliers (e.g., Gilat) are now using DVB-S2 TDM carriers as their "SCPC " returnlink option, within the context of the hybrid TDM/SCPC configuration of Figure 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 proprietarySCPC option). The weaknesses are:

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

It is more expensive for each site, and requires one dedicated DVB-S2receiver 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 useeither of two modes.

When compared to SatLink's implementation of DVB-RCS2 for TDMA (i.e., burstmode), the efficiency of DVB-S2 modems are very similar. They are only slightly


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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 comparingsimilar 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 ratesdynamically.

And, of course, as with all SCPC options on the return link, there is no ability to gainfrom statistical multiplexing of traffic across multiple transmitting sites.

6. Advantages of SatLink TDM/TDMA for Cellular BackhaulGiven the results above, the advantages of using a SatLink TDM/TDMA network forcellular backhaul applications (vs. a TDM/SCPC network) can be significant.

Most of the advantage will come from statistical multiplexing the return link trafficacross multiple transmitting sites. With the rapid growth of cellular data, even returnlink 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 beingcarried over IP with variable rate codecs, plus silence suppression in certain regionsof the world.

This means the statistical multiplexing gains of using TDMA for return link traffic mayeasily 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. Thusforward 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 QoSand traffic engineering features to assure each cell site receives its necessarycapacity and quality for delay sensitive voice and video traffic during rain fades orother forms of interference or congestion.

7. Option for an "SCPC-like" Dedicated Carrier in SatLinkSometimes in a customer network, a special technical or security reason may existthat requires a dedicated return link carrier for a given VSAT site. Even though SatLinkdoes not support an SCPC option in "continuous mode", it is simple to assign a TDMAcarrier 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. InSatLink this is called the " SCPC-like " mode.

As we explained above there is no penalty from using burst mode transmissions forcreating this "SCPC-like" mode in a SatLink network. And it still allows that VSAT to use


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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 onimmediate needs) is not usually a good network design strategy. For example, VSATtransmissions could be impaired if interference or jamming at some frequency affects

its assigned carrier, or if it needs to operate at a higher or lower symbol rate totransmit at higher speed or improve its SNR during rain fades.

Therefore, no efficiency, speed, or reliability benefits are gained from assigning adedicated return link carrier to a specific VSAT in a SatLink network.

8. Justification for Conversion from SCPC to TDM/TDMAWe saw in Figure 3, earlier, that the total cost of ownership will usually favor usingTDM/TDMA over SCPC for networks with 20 sites or more. Likewise the conversionfrom an old ("fully depreciated") SCPC network can usually be justified when thenumber 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 onSCPC equipment and software features.

Sometimes financial justifications can be found for a TDM/TDMA network upgradeeven if the network is composed of only 5 sites. The degree of savings will depend onthe 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. temperatevs. 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


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www.emc-corp.net Emerging Markets Communications | 777 Brickell Avenue, Suite 1150 | Miami, FL 33131, USAT +1 305 539 1358 | FAX +1 786 425 0679

All specifications and features subject to change without notice. SatLink and the EMC logo are a registered trademarks of Emerging Markets Communications, LLC.

Document # 200660; Revision D - 030314 2014 12

documentation of all assumptions, to help them evaluate whether SCPC orTDM/TDMA will deliver better performance relative to their network requirements,and how both the OPEX and CAPEX for the network will compare under eachalternative. It is worth noting that networks with more than 50 sites easily justifyTDM/TDMA except in rare situations.

tdma vs scpc technical note - rev d

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