=Paper=
{{Paper
|id=Vol-2746/paper4
|storemode=property
|title=Monitoring the Quality of Reference Synchronization Signals on the 4G Network
|pdfUrl=https://ceur-ws.org/Vol-2746/paper4.pdf
|volume=Vol-2746
|authors=Nataliia Fedorova,Yurii Khlaponin,Andrii Tolbatov,Yevgen Havrylko,Serhii Fryz,Oleksandr Drobyk,Roman Odarchenko,Oleh Polihenko
|dblpUrl=https://dblp.org/rec/conf/cpits/FedorovaKTHFDOP20
}}
==Monitoring the Quality of Reference Synchronization Signals on the 4G Network==
https://ceur-ws.org/Vol-2746/paper4.pdf
Monitoring the Quality of Reference Synchronization
Signals on the 4G Network
Nataliia Fedorova1[0000-0002-4548-4198], Yurii Khlaponin2[0000-0002-9287-0817],
Andrii Tolbatov3[0000-0002-9785-9975], Yevgen Havrylko1[0000-0001-9437-3964],
Serhii Fryz4[0000-0002-5263-1790], Oleksandr Drobyk5[0000-0002-9037-6663],
Roman Odarchenko6[0000-0002-7130-1375], and Oleh Polihenko6[0000-0002-2427-4976]
1 National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute,” Ukraine
2 Kyiv National University of Construction and Architecture, Ukraine
3 Sumy National Agrarian University, Ukraine
4 S. Korolev Military Institute, Ukraine
5 State University of Telecommunications, Ukraine
6 National Aviation University, Ukraine
natasha_f@ukr.net
Abstract. The article discusses the transition from TDM networks to networks
with packet data transmission. The evolutionary path that led to the modification
of the stability parameters of synchronization signals during the transition to
packet networks is considered. The main parameters of the stability of
synchronization signals, as well as methods of their measurements and
monitoring in packet networks, are determined. The article proposes a method
for mutual monitoring of distributed primary synchronization devices for their
effective use under the condition of the combined use of embedded and dedicated
networks. The circuit for mutual monitoring of synchronization reference signals
consists of three RTP servers and does not contain the most unreliable element—
the mechanism for switching to the reserve. This scheme maximizes the
efficiency of using PTP servers, each of which is not just in “hot standby,” but is
operational and contributes directly to the stability of the reference signals.
Keywords: Synchronization, Packet Networks, Synchronization Signal,
Stability Parameters, PTP Server, PTP Client, Base Station, Monitoring.
1 Introduction
With the introduction of new technologies, in particular 4G, the issues related to the
section of time-frequency support of communication sessions, as well as the problems
of coordinating the scales of local keepers of exact time in geographically dispersed
telecommunication infrastructure, do not lose their relevance.
Measuring the parameters of stability of synchronization signals is an integral part
of monitoring existing synchronization networks [1, 2]. This rule is also true when
switching to packet networks. The traditional parameters of stability of synchronization
signals have undergone further development when evaluated on Internet Protocol /
Multiprotocol Label Switching (IP/MPLS) networks.
Copyright © 2020 for this paper by its authors.
Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0)
34
With the active transition from one technology to the next generation networks, the
issues of synchronization are facing specialists with increasing force, since the exact
adherence to universal time ultimately translates into an increase in the availability and
quality of services provided [3].
The first attempt to evaluate the stability of synchronization signals in a packet
network was the templates from ITU-T Recommendation G.823 for a “classic” time-
division multiplexing (Time Division Multiplexing, TDM) synchronization network.
Further, a template for packet frequency synchronization was developed ITU-T
Recommendation G.8261.1 [4]. These templates allow you to determine how well the
packet stream passing through the network meets the criteria for the quality of
synchronization of application processes, for example, a client—base station NodeB
(this article discusses synchronization using the PTP protocol (Precision Time Protocol
(IEEE 1588v2) from the PTP server to the PTP—clients NodeB).
The template proposed in [4] is an attempt by the International Telecommunication
Union to use approaches that are applied to measurements of stability parameters of
synchronization signals in Synchronization Digital Hierarchy (Synchronization Digital
Hierarchy, SDH) networks, to packet networks. This pattern is a pretty rough estimate.
There are no qualitative parameters like the parameters of the maximum time interval
error (Maximum Time Interval Error, MTIE) and time deviation (Time Deviation,
TDEV) taking into account the packet transmission medium. Also, it makes no sense
to compare the measurement results in the packet network with the templates of the
“classical” network: since in it the average relative frequency at any node should be
equal to 1 × 10−11 , and in a packet mobile network, the limit value of the relative the
frequencies in the radio segment are two orders of magnitude lower –50 × 10−9 .
It follows from this that when assessing the quality of synchronization signals in a
packet network, it is advisable to abandon the traditional limits adopted in the
documents of the International Telecommunication Union for the “classic”
synchronization network (MTIE diagram) and go to templates that are better suited for
packet networks.
2 Review of the Literature
The properties of isogenies for Weierstrass curves are well studied. Effective methods
for constructing and isogenies properties of promising classes of curves in the Edwards
form are much less known. The Edwards curves with one parameter, defined in [2],
have very attractive advantages for cryptography: fastest exponentiation of a point [2],
completeness and universality of the law of point’s addition, affine coordinates of a
neutral element of a points group, enhanced security against side-channel attacks [2–
5]. 3- and 5-isogenies are considered in previous works [6] and [7].
The programming of group operations is accelerated due to the absence of a singular
point at infinity as a neutral element of an Abelian group of points. The introduction of
the second curve parameter in [8] extended the class of curves in the Edwards form and
gave rise to classes of quadratic and twisted curves with new properties of interest to
cryptographic applications. In this paper, the known results for the 2-isogeny of
complete and quadratic Edwards curves [4, 9] are generalized to the class of twisted
35
Edwards curves [10, 11]. In particular, an analysis of the existing conditions of such
curves over a prime field is given.
3 Moving from TDM to Packet
The peculiarity of the rationing of the joints of the “classical network” of
synchronization according to the norms of the International Telecommunication Union
is that at the top level of the hierarchy a primary source of cesium accuracy class is
placed so that the average frequency of the generating equipment of the network
elements (over a large network interval) is no worse than 1 × 10−11 [5], and allowable
phase wanderings (both short-term and long-term) according to the MTIE diagram [4].
The criteria for assessing the quality of TDM network synchronization signals are
two parameters MTIE and TDEV [6].
In monograph [7], protocols were presented that allows generating a reference
frequency in a reference generator and transmitting it to all points of a packet network
(IP/MPLS).
The question arose—how to evaluate the stability of such a reference frequency?
Due to the presence of the IP / MPLS packet transmission medium and the
corresponding technological protocols, new measurable parameters of the stability of
synchronization signals have been developed [4], which make it possible to assess the
quality of stability in a packet environment.
Measurements in a packet environment are based on the calculation of all data [4]
necessary not only to assess the accuracy of time comparison and to assess frequency
stability but also to assess such network parameters as two-way and one-way packet
delays, as well as packet delay deviation (Packet Delay Variation, PDV). The network
performance is estimated by the TDEV and the minimum deviation of packet time
packet (MinTDEV), calculated based on the PDV measurements from the packet
timestamps relative to the local (reference) time [4]. After completing the PDV dataset
measurements, the MTIE, TDEV, MAFE, FPP, FPC performance metrics are
calculated.
Currently, there is no complete understanding of a clear operational list of these
parameters (in contrast to the “classic” synchronization network TIE, MTIE, TDEV).
At present, the parameter MAFE (Maximum Average Frequency Error) can be
considered the main one for determining the quality of the synchronization signal in the
packet network [8, 9].
In the last couple of years, Ukraine has been intensively implementing 3G
technology on the networks of leading mobile operators. To date, the introduction and
use of networks based on 4G technology have been actively started. A prerequisite for
the operability of these technologies is the provision of synchronization of base stations
in the transport environment of IP/MPLS networks. The most optimal technology for
this is to provide synchronization using the PTP protocol, which was specially
developed for solving synchronization problems in packet networks. Unlike 3G
technology, 4G technology provides for the placement of a larger number of RTR
servers. Also, in the case of 4G, when planning a PTP synchronization network, it is
necessary to take into account the unicast (ITU-T G.8265.1) and multicast (ITU-T
G.8275.1/275.2) operating modes of the equipment.
36
In the process of building a synchronization network using the RTR protocol, such
factors have been identified that ensure the quality and reliability of this network:
planning the placement of RTR servers, ensuring the necessary redundancy (both at the
hardware level of the RTP server and the network level), as well as monitoring
measurements parameters of stability of synchronization signals on IP networks [8].
4 Capabilities of Control Systems of Modern
Synchronization Equipment
Measurements in the packet environment are based on the calculation of all data, which
is necessary not only to assess the accuracy of time reconciliation and frequency
stability but also to estimate parameters in the network such as one-way and two-way
packet delay, as well as Packet Delay Variation and calculations from the obtained data
of additional stability parameters (for example, the most recent parameter MAFE
(Maximum Average Frequency Error) is of greatest interest [4–6]. Measurement of the
parameters of the stability of the synchronization signals will be increasingly
automated, and data processing will be increasingly efficient. The need to monitor
synchronization signals will grow.
On the synchronization networks of Ukrainian operators, there are control systems
for synchronization equipment of various manufacturers, for example, the most
common is synchronization equipment from Microsemi (USA) and Oscillquartz
(Switzerland) with the corresponding control systems TimePictra and SyncView Plus.
In the latest versions of these control systems, it is possible to measure the stability
parameters of the output and input synchronization signals on the network
synchronization equipment using hardware implemented in this equipment and
supported by the corresponding software in TimePictra and SyncView Plus control
systems. That is the measurement ideology that Microsemi proposed in its
TimeAnalyzer 7500 meter smoothly shifted to the synchronization network itself.
Oscilloquartz once produced the OSA5565 SyncTester, which was able to perform
TIE (Time Interval Error) measurements and calculate the classic MTIE (Maximum
Time Interval Error) and TDEV (Time Deviation) parameters. This device is still
sometimes used on networks. However, Oscilloquartz did not make a measuring
instrument for IP network performance, but quite successfully implemented the
measuring function in the SyncView Plus control system. Prerequisites for measuring
the stability of synchronization signals are the presence of appropriate licenses on
network devices and the appropriate software in the SyncView Plus control system.
Microsemi simply “transferred” the functions of the TimeAnalyzer 7500 to network
devices, such as the TP5000 RTP server, the TP500 RTP client, and the TP4100
aircraft. Such measurements also require the appropriate licenses on network devices
and the appropriate software in the TimePictra management system, namely
TimeMonitor. Unfortunately, the disadvantage is that not all older Microsemi devices
support the stability measurement function. Since there are different versions of the
hardware implementation, for example, in the equipment of the RTP server TP5000, it
is not always possible to organize measurements using the control system (even if the
operator has all the software licenses).
37
TimePictra and SyncView Plus control systems allow us to carry out such “internal”
measurements in packet networks, namely measurements of PDV, packet MTIE, packet
TDEV, packet minTDEV. Thus, based on synchronization equipment control systems,
it is possible to create a system of “full-fledged” monitoring of the stability of
synchronization signals, but today, with certain limitations. Thus, such restrictions
include the inability to calculate the MAFE parameter, restrictions on the maximum
time of measurements, and restrictions related to the volume and storage time of the
results.
5 Method for Mutual Monitoring of the Quality of Timing
Reference Signals
Monitoring is the constant monitoring of the quality of reference signals at critical
points of the synchronization network to timely identify the deviation of their
parameters from regulatory requirements.
The monitoring system should not be equated with the Management System for
Synchronization Network, which is a set of ways and means to manage the network to
ensure its maximum efficiency. So, the control system necessarily contains a
monitoring tool.
The modern synchronization network includes two components: a synchronization
network control system and a system for measuring the parameters of synchronization
signals [2, 5].
The branched topology of modern synchronization networks does not allow us to
fully predict their behavior in the event of one or more failures. In the case of further
reconfiguration of the network, loops may be formed in the propagation of
synchronization signals, as a result of which the entire synchronization network may
“degrade” in any area or completely. Real-time monitoring of synchronization signal
parameters is used to combat such undesirable consequences. To do this, the parameters
of the synchronization signals are measured at all sections of such a network, both at
the output of the synchronization equipment and other junctions of the synchronization.
In essence, these measurements are measurements of frequency and parameters of its
stability, that is, frequency measurements [6].
As a rule, in practice they usually talk about frequency instability (Frequency
Instability) and adhere to the principle: to reduce long-term instability, you should
increase the accuracy. This principle is based on the conclusion that long-term changes
in frequency are due to various internal and external destabilizing factors, which are the
cause of systematic deviations [5]. By minimizing and controlling them, we improve
both accuracy and long-term stability. Accuracy cannot be better than long-term
stability. For example, in several nodes of the asynchronous communication network,
it is not so much the accuracy of the frequency that matters, but the well-coordinated
stability over time.
There are requirements regarding time. If the course of the local clock is to be
consistent with the nominal time scale from the remote source, certain means of regular
time corrections and adjustments to the local clock frequency should be provided.
Synchronize frequency—means to adjust the frequency of the generator so that it is the
same for all clocks, synchronize time—synchronize the clock readings with the nominal
38
time scale (usually UTC Coordinated Universal Time) and, synchronize clock
(synchronize clock)—means to synchronize both frequency and time. The purpose of
remote time reconciliation is to calculate the divergence of the scales.
If the control systems of the synchronization network of different manufacturers of
synchronization equipment today can not provide “full” monitoring, then there is a need
for dedicated monitoring systems for synchronization signals and the relevance of such
monitoring in IP networks increases significantly.
In [5, 6], a scheme for monitoring signals using the RTP protocol was proposed, and
a slightly modified scheme for the NTP protocol was proposed in [7, 8]. The essence
of this scheme was to reconcile/measure signals from three sources to determine the
emergency and efficient switching to the reserve. Also, this monitoring option can be
used to effectively reconcile timelines (which, incidentally, is currently not allowed by
any NTP server management system).
The scheme (Fig. 1) for mutual monitoring of the quality of reference signals on the
4G network can include two local sources of synchronization signals: one based on a
GPS receiver, the second based on a local RTP1 server, which is connected to the
calibration circuit. Also, an averaging circuit can be included in the specified scheme,
which serves to preprocess the reference signals received from the remote PTP2 and
PTP3, before these signals are fed to the network of 4G base stations.
Fig. 1. Monitoring the quality of PTP/NTP reference signals.
The calibration signal is used in the averaging circuit to generate the LOC control
signal. The stability of the resulting signal after such processing turns out to be no worse
than the short-term stability of the local crystal oscillator, during the average time of
the stability of the local РТР1 and the long-term stability of the GPS receiver [8, 9].
In the presence of several RTR servers on the network, the selection of the best
quality is achieved using a phase-locked loop (PLL) with multiple inputs, which is
digitally controlled by a tuned oscillator. In such a system, one of the input signals from
the crystal oscillator ensures stability over short measurement intervals. The local GPS
receiver, and in the event of an emergency, one or two remote RTR servers, contributes
39
to the stability of the resulting output signal over the intervals of average operating
time. In such a phase-locked loop system with many inputs, the stability of the resulting
signal at the output turns out to be no worse than the stability of any of the existing
sources, and they all serve to adjust the output signal.
A local quartz tunable TG generator with better short-term stability of frequency f1
is connected directly to the output of the control loop. The signals from the GPS and
the local PTP server are used to fine-tune the signal through a calibration circuit that
generates the first reference signal at f10 for the closed-loop averaging circuit.
The closed control loop considered in this work includes two digital integrators, a
weighting summation circuit. The main reference signal comes from the calibration
circuit to the summation weight circuit. Signals with good average time stability are fed
to the inputs of digital integrators and are used as the second and third reference signals
with frequencies f2 and f3, respectively.
The time constants of the calibration circuit and control loops are chosen such that
the prevailing influence of each of the reference signals is selective and generally
maximizes the overall stability of the output signal.
The synchronism condition in such a network for each node is:
t
A f n f1 dt B1 f1 f10 C
df1
A b1no 0
n 0 dt n
where
A is a constant transmission coefficient;
B1 is a constant coefficient characterizing the inertia of the calibration circuit;
C is a constant coefficient that determines the rate of change in the frequency of the
tunable generator of the generator;
b1no is the initial state of the digital integrator.
In digital integrators of a two-input control loop, it is preferable to use the relative
frequency calculation:
t
f 2 f1 dt
0
rather than phase and time, since the frequency error is limited, so rounding and
overflow errors are avoided when calculating the relative frequency. For the integral of
the frequency error to be equal to the time error up to a constant, the frequency error
should be calculated at zero dead time.
The smaller the coefficient A, the greater the inertia of the system. Since in the
conditions of trouble-free operation, a local PTP server (PTP1) is selected then the
remote servers PTP2 and PTP3. And, the more B1, the greater the inertia of the system.
Therefore, the time constant τ1 of the control loop, including the averaging circuit and
the TG, is equal to:
B1
1
A
40
If С 0, then not only the frequency control of the synchronization device is described,
but also the intensity of its frequency change df1/dt. If C = 0, then the equation reflects
the drift of the synchronization device frequency (fn – f10) from its calibrated nominal
value f10.
Also, during monitoring, the values of the MAFE parameter (n0) can be calculated,
which will give a complete picture of the quality of signals from three RTP servers [10].
The scheme shown in Figure 1 is used to pre-process the reference signals received
from remote PTP2 and PTP3 sources before these signals are presented to the network.
The calibration signal is used in the averaging circuit to generate a control signal for
the local adjustable generator (designated as “TO” in the circuit). The stability of the
resulting signal after such processing is not worse than the short-term stability of the
local quartz oscillator “TO,” the average hourly stability of the local PTP1, and the
long-term stability of the GPS receiver or receiver of any other GNSS (Global
Navigation Satellite System).
It should be noted that measurements of PDV values are possible here for both NTP
and PTP signals. And further calculations of such stability parameters as MAFE.
If there are several NTP or RTR servers on the network, the choice of the best quality
is achieved with the help of a multi-pass system of phase auto-tuning of the PLL
frequency with digital control of the tuned generator. In such a system, one of the input
signals from the quartz generator “TO” provides stability at short intervals of
measurement. The local GPS receiver contributes to the stability of the resulting output
signal at average hourly intervals, and in case of its failure—one or two remote NTP or
RTP servers. In such a high-pass phase-locked loop system, the stability of the resulting
output signal is no worse than the stability of any of the operating sources, and they all
serve to adjust the output signal [9].
The local quartz tuning generator “TO” with the best short-term frequency stability
is connected directly to the output of the control loop. The GPS signals and the local
NTP or RTP server are used to adjust the signal through the calibration circuit, which
forms the first reference signal for the closed-loop averaging circuit. The calibration
time constants and control loops are selected so that the predominant influence of each
of the reference signals is selective and generally maximizes the stability and accurate
timestamp of the output signal.
Fig. 2–4 shows the results of measurements obtained by carrying out measurements
on the network of the company PrAT “Kyivstar.” Fig. 2 shows the PDV (t) graph, which
is used to calculate the MAFE (τ) parameter according to [4]. The graph shown in Fig.
3 gives an idea of the behavior of the relative frequency over time. Fig. 4 shows the
sync packet delay distribution function.
41
Fig. 2. PDV parameter measurements.
Fig. 3. Relative frequency over time.
Fig. 4. Sync packet delay distribution function.
42
By organizing such monitoring on an ongoing basis, it becomes possible to reserve the
reference signals of 4G base stations in a certain section of the network from three RTR
servers. It is noteworthy that in case of emergencies, switching to the reserve is avoided
at the maximum signal quality [11].
The proposed scheme, in addition to solving the accident rate of the device and the
formation of the resulting stable signal to the network, makes it possible to measure the
stability parameters of synchronization signals from three sources, to match phase
synchronization from three sources. The question arises of how to manage
measurements at a distance and where to process and then save the measurement
results. Measurements at these three points of the network are possible if there are
measuring instruments in them, or use the capabilities of the control system of the
network synchronization equipment at these points. In both cases, the measurement
results are accumulated directly on the measuring devices or network equipment.
Subsequently, these results should be sent to a centralized server. In existing
synchronization networks management systems such as SyncView Plus and
TimePictra, this functionality does not exist and is not expected. That is, it is necessary
to additionally develop software for analysis (for example, calculating the MAFE
parameter [5]) and storing measurement data.
The problem that arises in the case of implementing this scheme is the processing of
the obtained measurement results and their analysis and storage. With the current state
of information databases and approved ITU-T Recommendations [10–12, 15–21], this
task does not cause significant difficulties. But it is also necessary to present an
algorithm for interaction between the three signal sources and the central server.
According to this algorithm, a decision on the operability of each of the three servers
will be made according to the majority rule. This is also a pure software task. Here it is
advisable to use the same concept as in the case of developing the hardware part of the
monitoring system, namely the majority rule for evaluating three devices. This will
make it possible to more dynamically control the measurement process itself—to
configure measurement tasks, to evaluate the serviceability of the measuring equipment
even during measurements. Partially, in this case, the capabilities of synchronization
network management systems can be used, but nothing prevents you from doing it
separately—all communication protocols over IP are open, and security will be
preserved by the closedness of internal industrial IP networks of telecom operators [2,
10, 11, 15–21].
In [12] the very prototype of the algorithm for such interaction is given. The
evaluation of three NTP servers relative to the fourth was taken as a basis. The direct
analogy with three source devices and a fourth central server. For clarity of
demonstration of the prototype, public NTP servers on the public Internet (by the
project www.pool.ntp.org) are used. It is problematic to use PTP servers for this since
it is necessary to use the hardware resource of the central server (in general, RTP servers
in the public version are rarely found). The prototype is implemented in the Python
programming language 3.6.4 (the listing of the prototype code is given in Section 2 of
Monograph [12], if necessary, the authors can provide the algorithm code upon request,
since the volume of our publication does not allow placing it here). By the central
server, NTP was selected in Kyiv from the same project www.pool.ntp.org, namely at
www.time.in.ua. This is a Stratum 1 primary server based on GPS receivers.
43
The three servers under investigation are the selected three remote NTP servers
according to the www.pool.ntp.org project in different parts of the world—South
America, Asia, and Europe (Fig. 5).
Fig. 5. Scheme of organization of monitoring according to the project www.pool.ntp.org.
The program initiates a connection with all four NTP servers according to the data
entered by the user—how many measurements need to be taken and after what time.
Then it compares the time from three remote servers about the reference server in Kyiv.
Calculates the delay, and displays the measurement results—date and time from each
in several formats and the delay relative to the reference server. If you accumulate
enough measurements, then the delay should be up to 3 s (which is a lot even for the
public Internet, but it all depends on the quality of the Internet at the place where the
measurement is made). When all the cycles that the user-specified when entering the
number of measurements have been completed, the program stops.
This prototype demonstrates the monitoring capabilities of the majority rule. If
enough data is accumulated, you can create a graph, or fill out datasets for later analysis.
On real networks of telecom operators with working NTP servers, the delays will be
already millisecond, and the accuracy will be at the microsecond level [1, 5–7].
It should be noted that the increase in the efficiency of the proposed circuit is directly
related to the assessment of the stability of the reference signals, which depends on the
parameters of the carrier frequency. In turn, the solution of scientific problems on the
assessment of the carrier frequency of these signals involves the choice of the
assessment parameter and the methodology for their determination. As such a technique
in work for signals that are transmitted in burst mode, it is proposed to use the maximum
likelihood rule using a sliding fast Fourier [13]. In this case, the reference signal
synchronization system itself can be improved by the open-loop synthesis method,
which is described in sufficient detail in [14].
44
6 Conclusions
To apply existing experience in perspective and overcome misconceptions based on
incorrect assumptions, each company must realize its needs and develop its
measurement techniques, justify norms, and limit ratios.
A mutual monitoring scheme is proposed, which consists of three RTR servers and
does not contain the most unreliable element—a reserve switching mechanism. The
considered scheme is the most optimal for building synchronization networks for 4G
technology. This scheme maximizes the efficiency of using PTP servers, each of which
is not just in “hot standby,” but is operational and contributes directly to the stability of
the reference signals.
To illustrate the operation of the proposed scheme, a software prototype of the
interaction between the three NTP nodes and the central server was developed, which
is presented in the final part of section 2 of the Monograph [9]. The NTP protocol was
chosen to be available on the public Internet. The prototype is implemented in the
Python programming language version 3.6.4.
The sources used in the proposed monitoring scheme may not necessarily be
primary, and there may be significantly more than one such scheme on the network
(especially a large network). Thus, IP networks can implement a full-fledged system
for monitoring the parameters of synchronization signals.
The main parameters of the stability of synchronization signals are given for a
qualitative assessment of the monitoring of the synchronization network. The main
operational stability parameter is MAFE. With it, you can quickly assess the signal
quality. The indicator has masks for two modes of MPLS-network operation (up to 5
and up to 10 RTP signal re-reception by routers).
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