Adaptive Multiplexing Gateway for LMR-based Group ...

accepted for presentation at IEEE Conference on Homeland Security Technologies 2012, Boston, USA

Adaptive Multiplexing Gateway for LMR-based Group Communications over High Latency IP-based Satellite Backhaul Links Sebastian Subik, Brian Niehoefer and Christian Wietfeld Communication Networks Institute TU Dortmund University Dortmund, Germany {sebastian.subik, brian.niehoefer, christian.wietfeld}@tu-dortmund.de Abstract—In this paper, the authors present a proof-of concept demonstrator for an adaptive inter-system-interface gateway between LMR and IP based networks. To underline the flexibility of the proposed solution, a satellite link is utilized to enable the integration of an independent LMR cell or direct connected terminals (MicroSpot) into an existing infrastructure over long distances without the need for dedicated transport networks. This set-up is motivated by both, homeland security operations as well as industrial use-cases. The scenario consists of an isolated communication group which needs to be linked with a core network from a degraded area. To deploy such a system, a prior in depth analysis of the restrictions of the components is performed. These results are used for an overall usability analysis based on measurements with a demonstrator. Main challenge is a high variance in the delay, which inhibit a direct connection between a strictly timed TDMA LMR network and IP-based transport networks. The proposed gateway is based on an adaptive convergence layer multiplexer, which optimizes the LMR data stream for the transmission on a time variant channel like a satellite link. This enables homeland security organizations to operate independent from land based radio networks. Keywords-Convergence; Critical Communication; Group Communication; Multiplexing; Gateway; Satellite Network; TETRA; Inter-System Interface; LMR; PMR

I. I NTRODUCTION Efficient disaster preparation, recovery and response relies on secure information sharing through wireless communication systems. Standardized Land Mobile Radio systems (LMR)1 , such as Terrestrial Trunked Radio (TETRA), have been established in recent years and provide secure voice as well as limited data services [1]. Commercial mobile radio systems provide already high performance broadband services and worldwide coverage with 100% uptime guarantee. In addition, homeland security organizations ask for the same level of service for their duties within public safety operations. To fulfill this requirement, intermediate systems based on gateways between heterogeneous networks (dedicated or commercial) need to close the performance gap between Public Safety Communication Systems (PSC) and commercial available systems. The challenge is to deal with the different link conditions influencing the quality of service (QoS). A PSC gateway 1 also

known as professional mobile radio systems (PMR)

services need to provide solutions for an optimal usage of the interconnected systems without relying on special customization. II. S CENARIO FOR THE S YSTEM S ETUP Throughout this paper, we assume a scenario in which terrestrial radio networks are either damaged or at least have no core-network connection2 . This use case is mainly based on classical public safety and disaster relief operations [2], like shown in Fig. 1. In such a situation LMR terminals are able to switch to direct mode operations (DMO) to ensure adhoc communication without relying on installed infrastructure. For a full range of application, the terminals in the operational area need to be able to be linked with the core network for full communication capabilities. If DMO mode is used for connectivity, a so called MicroSpot is set up at the scene. Additionally, specialized base stations could be applied within the scene to provide the full range of LMR services but they need to be connected to a core network as well. For both interconnection various heterogeneous transport networks can be applied. This could be either long distance links, like satellite networks or other communication networks with a better coverage or availability (fixed or wireless). Another option is to span a wireless mesh network based on unmaned aerial vehicles (UAV) at the operation area. This can be used to provide a better coverage, but still need to be connected to a core network. In most cases IP-based networks are available and could be utilized with a QoS gateway service between different technologies. In this paper the authors analyze and test an IP based satellite link as an interconnection for TDMA based TETRA LMR networks. This paper is structured as followed: After introducing the scenario, the authors give an overview on critical communication via heterogeneous networks. Afterwards a novel adaptive multiplex service for TETRA over IP is proposed, which enables the interconnection of LMR systems via heterogeneous networks. In the end the performance of the proposed solution is evaluated on a real satellite link to ensure the operation within harsh conditions. 2 In

this case, PSC systems work in a fall back stand alone mode

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PTX Convergence Layer for Critical Communication PTX

PTX

TETRAoIP Interface

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PTX

PTX

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3G/4G link DVB-S2 / RCS link TETRA PMR (Trunked & Direct Mode) Emergency Satellite Link Testequipment

Operation Area (no coverage)

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Fig. 1.

System and Layer Overview

III. R EQUIREMENTS FOR C RITICAL C OMMUNICATION VIA H ETEROGENEOUS N ETWORKS Although the requirements are varying, reliable voice group communication is still the most important feature of critical communication systems. With rising data performance in cellular networks, the gap between PSC and commercial systems hamper an effective use of PSC in critical communication scenarios. Due to the long term investments in the public safety sector, a rapid upgrade to state-of-the-art communication technologies is impossible. Instead of developing a new standard for Next Generation Public Safety Communications (NG-PSC), a suitable short-term solution is to rely on existing networks and technologies to enhance the performance. For example 4G networks for enhanced data rates or satellite networks for worldwide and infrastructure independent coverage could be coupled via a gateway service with PSC systems. For this service, a convergence layer is needed to enable a transparent coupling of the different communication technologies. A. Convergence Layer for PMR Communications Service Basis of the proposed convergence layer is the PTX3 protocol [1]. This UDP/IP based protocol enables the use of push group communication in IP based networks. Furthermore it is used as gateway protocol inside of an inter-system-interface for TETRA. Its flexibility guarantees also the utilization of IP clients on various platforms from desktops to hand held devices. As depicted in Fig. 1, PTX is independent from the underlying transport protocols. It provides call handling and group management features without adding overhead to the 3 Push-To-X

http://openptx.org

communication because its standard header size is optimize to fit into an additional one byte header on top of UDP/IP. Another feature of PTX is the realization of advanced communication options like cross layer controlling of gateways or multiplexer as described in Section IV. This is important in critical communication scenarios without full access to lower layer protocols. In dedicated networks, the same protocol can be used for full controlling the lower layers. B. Challenges in Satellite Communications Using satellite technology within critical communication systems always comes along with scientific deliberations concerning the resulting failure safety and gained performance. The dependence on the direct surrounding for line-of-sight connections, potential atmospheric interference and long propagation delays of up to 250ms are the mostly mentioned arguments, neglecting the upcoming advantages especially for the given case of application like independence of any given terrestrial infrastructure, global availability, internationally standardized protocols, commercially available mobile equipment and high data rates. Nevertheless, these conditions are putting limitations on possible inter-working approaches with other terrestrial communication technologies. In Fig. 2 different impact factors to all satellite signals are depicted. Shadowing or diffraction influence the gained performance of the signal as well as the measured Inter-Arrival Time (IAT). Developing a TETRA over satellite service using UDP packets can be seen as a kind of trade-off between QoS and resource management. Thereby, the used bandwidth on demand (BoD) scheme is highly effecting both sides. For enabling a reasonable real-time service, the following issues should be in consideration for the possible schemes [3] • Minimizing jitter

Satellite Free space lost Scattering Refraction Line-of-Sight

Shadowing

Scattering

original signal from the source (microphone) is digitized (sampled and encoded with ACELP4 ) with a sampling rate of fs = 16 23 Hz ⇡ 60ms. This frequency is the slot interval of TETRA TDMA systems [1] and is chosen to guarantee the interoperability between IP and LMR networks. In our scenario the internal parameters of the satellite link are protected by the commercial provider, so the link is modeled as a combination of two delays (⌧channel , ⌧jitter (t)) and two errors rates (packet error and bit error rate). To handle the time variant delay (jitter), the receiver has a receiving buffer before decoding. Inside of the sender, a buffer and the

Diffraction Reflection Source

Receiver Fig. 2.

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Possible impacts to satellite signals, resulting in a variable IAT

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Enabling QoS Minimizing access delay Referring to [4], other approaches focusing on a more general demonstration of possible opportunities using integrated satellite and terrestrial networks. In our case, we mainly focusing on a more specialized scenario, e.g. disaster relief operations like explained above, by using UDP instead of analyzing possible fairness principles for TCP. If a dedicated public safety satellite link is used, the whole system can be optimized for multimedia traffic [5]. As an example of actual research in interoperability the CHORIST project outlined the design of a multi-technology communication system and identified overall requirements of critical communication in disaster relief scenarios. Although satellite links are a part of the system architecture, the overall delay for voice communication is given with less than 400ms [6]. Regarding to Section III-B the minimal delay on geostationary satellite links is greater than 500ms, which prevent the seamless interconnection to existing QoS enabled systems. Therefore in [7], the authors try to determine an interconnection between TETRA networks over satellite links based on telephony protocols. The analysis of different timeout values shows that the system could not work without changing the underlying protocols. In [8] a TETRA over Satellite system is outlined. The authors propose the use of geostationary satellites, but give no parametrization for the interface. In [9], a universal and complex WISECOM Access Terminal protocol (WAT) is defined. •

⌧

•

IV. A DAPTIVE M ULTIPLEX G ATEWAY FOR TETRA OVER IP S ERVICE As outlined in the previous sections the controlling and optimization of lower layer protocols is necessary in heterogeneous environments. In Fig. 3 a classical model of the voice transmission link is depicted. It is divided into three parts: Sender, transmission channel and receiver. The

and could be utilized. Although not onl could be utilized. A + communication b to ensure QoSand enabled to ensure QoS enabled c Sending between the Buffer different technologies. In this the different techi analyze an IPbetween based Padding satellite link as the analyze TDMA basedSender TETRA.an IP based satel ⌧channel TDMA based TETRA. ⌧jitter (t) ⌧channel

D

(t)

jitter II. C RITICAL COMMUNICATION VIA H PER

BER

II. C

NETWORKS COMMUN

Transmission ChannelRITICAL

Although the requirements are changN D group communication is still the most im Drain Although the requirem A critical communication systems. With the Receiving Buffer Receiver group communication is mance in cellular networks, the perform critical communication sy themof and PMR networks hamper an effec Fig. 3. Model Transmission Link mance in cellular networ communication scenarios. addition of extra padding Due bytes to are the included thePMR model. them and networks long in term investments in Both manipulations of the original stream are used to study communication scenarios sector, a rapid upgrade to state-of-the-a the behavior of the transmission channel. In addition, satellite Due terms technologies is impossible. Along suitable channels are mostly suspect of negative aspectstolikethe high ofthedeveloping new upgrade standard packet error rates (PER).instead Aiming on integration satellite sector, aofarapid channels within TETRAtion networks, a PER of up to 10% would Public Safety Communications is t technologies is impossib be acceptable without decreasing the QoE5 . Hence, we use networks andinstead technologies to enhancea of indeveloping this value as limit for successful connection build-ups our coupling networks f measurements setups. Examples aretion Public4G Safety Comm rates or satellite networks for worldwide If the underlying transmission networknetworks or link is not and config-technolog urable, the possible userindependent intervention iscoverage. limited. As described Examples are coupling in Section III-B, channelsFor of commercial satellite systems this solution, a convergence layer are not optimized for minimizing the jitter ratesincoupling ortransmission satellite network able a transparent of the differe streams6 . In most cases an internal sending buffer is used independent coverage. technologies. to prevent short burst on the up-link. This buffer lead to the

For this solution, a co ⌧jitter (t), especially in the depicted scenario. A. Convergence layer forincrease PMR communi a transparent couplin If the model in Fig. 3 is assumed to beable correct, an of the packet size would force the system to bypass all packets technologies. TETRA with PTX Interface, structure, right to the up-link, which will minimize the jitter. An optimal from [2] behavior is assumed at the maximum transfer rate. In this point A. Convergence layer for all potentially buffers should be neutralized. B. Satellite communication - Brian

4 Algebraic

TETRA with PTX Inte critic from [2]

Code Excited Linear Prediction Using satellite technology within 5 Quality-of-Experience [1] 6 special links are available systems comes along with but not focus always of this research

scien concerning the failure safety B. resulting Satellite communicatio formance. Whereby the given depende Using satellite technolo sourrounding for Line-of-Sight connect systems always comes a mospheric interferences and potentially

concerning the resulting

TABLE I T ESTBED PARAMETER

Thus the structure of the information is fixed to a sampling frequency as well as packet size, only few options are left to optimize the channels behavior: 1) Buffering a number of n packets to build bigger packets (jumbo packet) 2) Increasing the size of the packets by adding padding bytes 3) Building bigger packets by multiplexing different voice streams into one intermediate transmission stream Enumeration 1) is a suitable solution but will lead to an increase of the overall delay and therefore limit the maximum packet size P S. In addition only multiple of the packet size are possible values which limits the fine-tuning. The 2) option is flexible, will not increase the delay and enables bytesize alignment. The disadvantage is the huge decrease of the effective data rate. PTX Adaptive Convergence Multiplex-Gateway Sender

Frequency Datarate (up/down) Satellite Position ESLT position Inter-arrival time Packet size

12 GHz 128 kbit/s / 1 Mbit/s (shared) geostationary at 23, 5 E 51 310 N, 7 280 E 60ms 60 Byte

buffer’s size. The surveillance of the packets inter arrival time at the inverse multiplexer prevent overload situations. V. P ERFORMANCE EVALUATION The performance analysis focuses on the correlation of the jitter of a voice stream with the actual packet size controlled by an adaptive multiplex gateway service as described in Section IV. To ensure the usability, the round trip time (RTT) of a call setup is measured within the same conditions. All tests where run at least 50 times to reduce the impact of external or environmental influences. The talk-bursts within the pushto-talk voice streaming have a minimum length of 10 seconds.

+

A. Measurement Setup - Emergency Satellite Link Test equipment (ESLT)

Sending Buffer

Sender

Padding PTX-Gateway-Control

Fig. 4.

Adaptive Multiplexer

To take advantage of the described option while minimizing the negative effects, the authors propose the use of an adaptive multiplexer as depicted in Fig. 4. The multiplexer constructs lager packets consisting of different voice streams without increasing the delay nor the overhead. If not enough traffic is available, missing bytes need to be added as a padding again. In [10] a comparable system is described and the positive effects in terms of decreased overhead is outlined. The proposed multiplexer need to be installed as a gateway service in the overall system architecture on both sides of the transport network’s inter-system-interfaces. The additional PTX signaling and controlling interface enables an adaptive change of sending parameter to adopt the performance to different channel conditions. This feedback loop important to avoid overload which will lead to an increase of the packet error rate. The additional signaling information can be send within unused slots inside of the multiplexed stream or as an overhead withing a given maximum error range. A controlled overload could also be utilized for increasing the capacity of the transport link. Previous work [1] showed that ACELP encoded voice is still understandable up to a packet error rate of 10%. If the multiplexing-gateway detects an overload, it could spread the error between the different streams to ensure a uniform distribution of the failure within the streams. On the client side, the use of the gateway is transparent within the convergence layer and will only be noticed through a decrease of the measured jitter and the resulting receiver

To analyze the subjective and impartial performance and possible occurring effects to the our proposed solution, the authors established a so-called Emergency Satellite Link Testequipment (ESLT) which is deployed on a mobile hand cart including energy supply, a WiFi-Router (802.11n) and an Antenna Control System for automatically satellite searching and antenna positioning. The elements are shown in Fig. 1. B. Jitter Analysis 250 2200

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Within voice streams, the variation of the IAT should be as small as possible. In the assumed system, packets of 60 Bytes are send every 60 ms. As depicted in Fig. 5 the IAT mean of the voice stream is constant at the original level, but with a

high jitter. The 2 standard derivation shows that an incoming buffer need to be able to deals with IAT up to 250ms. In Section IV it is posited that the system will run within the best performance at a 100% traffic load. To identify this operating point (OP), the packet size is increased. Our measurements show an optimal OP at a packet size of 840 Bytes. This lead to an effective upload data rate of 112 kbit/s. For packet sizes greater than this OP, an increase of the IAT can be seen. This increase is in fact an increasing PER caused by an overload of the link. Compared to the original packet size, the jitter could be reduced about 11 times. In Fig. 6 a 1 Clock: 60ms

0.9 0.8 0.7 CDF

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VI. C ONCLUSION In this paper, the authors present a novel solution for minimizing the negative impact of heterogeneous transport networks on critical communication. These transport networks often provide no interface for dedicated voice traffic. With the implementation of the proposed adaptive multiplexer as a gateway service, an optimal operating point (OP) could be determined. Within this OP the jitter of voice transmission could be nearly eliminated. The use of a PTX-protocol based signaling interface enables a feedback loop for avoiding system overload situations. The proposed solution is not limited to the described scenario but can be applied in multiple constellations. This is an important requirement for a module within the next generation public safety communication system. The system setup can also be evolved to use 4G technologies. They will provide additional features such as higher data rates to the responders without loosing the backwards compatibility. ACKNOWLEDGMENT

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This work was funded by the German Federal Ministry of Education and Research (BMBF) in the project ANCHORS (13N12204). Additionally, the authors want to thank Mrs. Lisa Underberg for her support during the incurrence of this contribution.

0.4 0.3 0.2

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60 IAT [ms] Fig. 6.

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IAT Analysis for different packet sizes

detailed CDF analysis of the IAT is presented. It can be seen that at the OP the real IAT is close to an ideal value of 60ms. Especially in contrast to a smaller packet sizes, an optimal behavior of the system could be enforced. 1

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In Fig. 7, the resulting delay is depicted. The round trip time (RTT) of the call setup procedure increases with the packet size. This can be tolerated because delay has a smaller impact on the quality of voice transmissions than jitter. Furthermore, signaling packets in PTX are optimized for packet size.

R EFERENCES [1] S. Subik and C. Wietfeld, “Integrated PMR-Broadband-IP Network for Secure Realtime Multimedia Information Sharing,” in Technologies for Homeland Security (HST), 2011 IEEE International Conference on. IEEE, November 2011. [2] S. Subik, S. Rohde, T. Weber, and C. Wietfeld, “SPIDER: Enabling Interoperable Information Sharing between Public Institutions for Efficient Disaster Recovery and Response,” in IEEE International Conference on Technologies for Homeland Security. Waltham, MA, USA: IEEE, 2010. [3] H. Skinnemoen, A. Vermesan, A. Iuoras, G. Adams, and X. Lobao, “VoIP over DVB-RCS with QoS and Bandwidth on Demand,” Wireless Communications, IEEE, vol. 12, no. 5, pp. 46 – 53, oct. 2005. [4] C. Roseti, M. Luglio, and F. Zampognaro, “Analysis and Performance Evaluation of a Burst-Based TCP for Satellite DVB RCS Links,” Networking, IEEE/ACM Transactions on, vol. 18, no. 3, pp. 911 –921, june 2010. [5] P. Pace, G. Aloi, and S. Marano, “Multimedia traffic admission schemes comparison for satellite systems,” in Vehicular Technology Conference, 2004. VTC2004-Fall. 2004 IEEE 60th, vol. 6, sept. 2004, pp. 4022 – 4026 Vol. 6. [6] A. Durantini, M. Petracca, F. Vatalaro, A. Civardi, and F. Ananasso, “Integration of broadband wireless technologies and pmr systems for professional communications,” mar. 2008, pp. 84 –89. [7] R. Novak, “Viability of ISI-based TETRA over Satellite,” WTOC, vol. 7, pp. 765–775, July 2008. [8] E. Re, M. Ruggieri, and G. Guidotti, “Integration of TETRA with Satellites,” in Aerospace Conference, 2008 IEEE, march 2008, pp. 1 –8. [9] M. Berioli, N. Courville, and M. Werner, “Integrating Satellite and Terrestrial Technologies for Emergency Communications: the WISECOM Project,” in QShine 2007 Workshop: Satellite/Terrestrial Interworking, ser. IWSTI ’07. New York, NY, USA: ACM, 2007, pp. 2:1–2:6. [10] J. Saldana, J. Fernandez-Navajas, J. Ruiz-Mas, J. Aznar, E. Viruete, and L. Casadesus, “First Person Shooters: Can a Smarter Network Save Bandwidth without Annoying the Players?” Communications Magazine, IEEE, vol. 49, no. 11, pp. 190 –198, november 2011.