PacketNotificationWPMC2011-Vweb

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this paper was published at WPMC 2011(The 14th International Symposium on Wireless Personal Multimedia Communications, 3-6 october 2013, Brest, France)

Improving LTE/EPC signalling for sporadic data with

a control-plane based transmission procedure

Izuru SatoNetwork System Laboratories

Fujitsu Laboratories Limited

Kawasaki, Japan

[email protected]

Ahmed Bouabdallah, Xavier LagrangeInstitut TELECOM ; TELECOM Bretagne

Université européenne de Bretagne

Rennes, France

{firstname.name}@telecom-bretagne.eu

Abstract — The architecture and the protocols of LTE/EPC were

designed to deliver high transmission rates with QoS

requirements. In order to be able to guarantee different levels of

QoS each transmission is based on a bearer, which need to be set

up within the network. It then requires to store contexts in the

various entities of the network and to set up several tunnels or

connections both in the user and the control planes. We considerthe Short Message Service (SMS), which is a very simple but

popular service and analyze the number of signaling messages

that are exchanged within the core network and the access

network to transmit one data message. We then propose a new

procedure based on a set of simple messages to transport such

isolated messages only in the control plane. We show that this

procedure may generate additional signaling in some cases but is

efficient as soon as the proportion of sporadic traffic is not

negligible. This procedure is generic and may be used for any

type of sporadic traffic, which will be increased in the next few

years with the deployment of wirelessly connected smart devices.

Keywords: LTE/EPC, SMS, IMS, paging, idle mode, low bit

rate transmission

I. INTRODUCTION

The Long Term Evolution / Evolved Packet Core(LTE/EPC) may be defined as a generic IP access network. Asopposed to previous networks like GSM that provides a largeset of integrated services, LTE/EPC deliberately provides IPpacket transmission only. The EPC network is seen as an IP-CAN (IP Connectivity Access Network) to external packet datanetworks (PDN) or overlay networks like IMS (IP MultimediaSystem).

Other objectives of LTE/EPC were to provide hightransmission rates with quality of service (QoS) guarantees and

low latencies [1]. Before making any transmission in LTE/EPCit is necessary to set up a bearer. The bearer may be seen as apath between two end-points that is made of several tunnelsthrough the network. A QoS level is associated to a givenbearer. Due to the always-on feature, a default bearer is alwaysset on. However, in order to avoid excessive resource usage thebearer in the access network is only set up when some traffic isreally sent. When a terminal needs to send or receive data,several tunnels or connections both in the user and the controlplanes should then be set up. Due to the generic property of

LTE/EPC the same procedure is used for any type of service:web access, voice call and Short Message Service (SMS).

The SMS is a text based communication which is popularand available on virtually any mobile terminals. A surveyreports that 2.052 trillion SMS messages has been sent and

received annually [2]. In 2G and 3G networks SMS messagesare sent using control messages sent over Control Plane. InLTE, SMS messages are sent using IP packets containing SIPmessage as its payload.

Releasing and acquiring radio access network resourcesrequires several messages exchanged between nodescomprising the operator network. This overhead is not an issuewhen a service sends or receives many packets before the radioaccess resources are released. Music file downloading is anexample of such service. When the data sent or received by aservice is short and sent sporadically, the overhead will be anissue. One example of such service is the SMS in LTE.

This paper proposes a framework to reduce network

resource usage by using only the control plane to send shortsporadic user data such as SMS messages. It is organized asfollows. In sections 2 and 3 the realization of SMS in LTE isexplained. In section 4 new packet notification messages torealize our proposal are presented. Section 5 includes ananalysis of the signaling load. Section 6 concludes the paper.

II. SMS SERVICE OVER LTE/EPC

A. SMS basics

SMS can fundamentally be seen as a push service providingan asynchronous limited-size text transfer between two mobileterminals. An intermediate server (the SMS Center) allows to

split up the global transfer into two temporally uncorrelatedoperations, namely the submission of the text and its delivery.These two ones are acknowledged through reports indicatingtheir respective success or failure, bringing by this way someglobal reliability to the service. Last but not least, sending orreceiving an SMS necessitates very few network resources andcan be concurrently executed with a current voicecommunication or data session.


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B.

Embedding SMS in the IMS messaging service

The SMS service can be easily embedded in the IMSmessaging service [3], if one accepts to loose its asynchronousdimension. To understand why, let us briefly recall how thislater works. The messaging service defined in the IMS reliescompletely on the capabilities of the SIP protocol [4,5]. Thebasic approach called page mode messaging [3] allows a one-shot text transfer through the MESSAGE message, which can

convey a text in its body with the general 1500-byte SIP sizemessages limitation.

In order to illustrate how SMS over LTE/EPC works whenit is embedded in the IMS messaging service we consider twousers Alice and Bob and a scenario in which Alice sends anSMS to Bob. The same example is kept in following sections.Alice has an ordinary IMS terminal and Bob has a mobile UE(User Equipment). We assume that the UE of Bob is attachedto the LTE/EPC and that both Bob and Alice are registered asIMS users. The SIP URI of Bob is SIP:Bob@op. Alicecomposes her text and sends it as usual. For sake of simplicity,neither Alice nor Bob gets a value-added service in its profile.MESSAGE obeys the classical IMS routing rules going

through the S- and P- proxys associated respectively to Aliceand Bob (see Figure 1). Note that the one-hop 100 Trying isnot mandatory [4,5]. The success of the receipt of theMESSAGE message is confirmed by the 200 OK response.

Figure 1. Message Sequence Chart of the IMS page mode messaging

When the addressee is currently registered in, the pagemode way offers a behavior completely similar to our plain oldSMS service. In this case, the messaging service can betransparently substituted to SMS, without requiring to the userssome changes of their habits. When the recipient is howeverunregistered, the “store-and-forward” ability of the SMSservice, is not ensured by the messaging service. The directconsequence is that the sender has to submit the text at a latertime, again and again until its addressee is finally registered.This fundamental push feature can be easily compensated inthe IMS, through an AS based service which will have to storeeach MESSAGE message sent to an unregistered user anddeliver it when the user is registered. Finally to reach with IMSthe same functional level as the classical SMS, the operatormust systematically and transparently associate the previousfeature to the page mode messaging service.

This approach can be fruitful when restricted to IMSnetworks or more generally to SIP-based ones. But forinteroperability with “legacy” networks, a dedicated gatewaythe IP-SM-GW has been introduced in IMS since Release 7[6,7]. Acting as an application server from the IMS side, itbehaves like an SMS Center on the circuit side. This gatewaycan also interact with the HSS/HLR through Diameter andMAP protocols and the charging servers entities of the operator

[6,7].

C. Pros and cons of this embedding

We can quote that the general SIP-based messaging servicegoes far beyond SMS in several directions. The first extensionconcerns the ability to exchange messages of any size [4,5] orcontaining not only text but any MIME type object. The secondone is devoted to the definition of a session mode allowingtext-based dialogue as exemplified in the instant messagingservice [3]. On the other side, as we have seen, the page modemessaging between pure IMS terminals loses the SMSfundamental push property. Moreover in the general case,using a reliable transport protocol like TCP, is costly from the

point of network resources. One way consists in limiting thesize of the transported text and using UDP transport in order tominimize the network load to one IP packet per MESSAGEmessage. The new drawback which must be however quoted isthe complete unreliability of the service.

III. SMS TRANSFER THROUGHT THE LTE/EPC NETWORK

In this section, we describe the message exchange withinthe LTE/EPC that is generated when a SMS is transmitted. Wefirst give a reminder on the ECM (EPC Connection Mana-gement) layer.

Figure 2. Tunnels in LTE/EPC when the mobile is in ECM-idle state

A. User plane of an UE

Two connection management states called the LTE/EPCConnection Management (ECM) states are defined in the LTE.They are ECM-IDLE and ECM-CONNECTED. A UE inECM-IDLE mode does not have a connection to MME. Thereis no User Plane bearers established, which is used to send andreceive packets to and from IP-CANs. In ECM-CONNECTEDmode, there is a connection between the UE and the MME.When there are User Plane bearers established, the UE isalways in ECM-CONNECTED state.


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Delivering an SMS message to an ECM-IDLE mode UE

A SIP message sent from P-CSCF is an IP packet having anIP address of Bob’s UE as its destination address. Such IPpacket will be routed to Bob’s P-GW. The P-GW encapsulatesand sends the packet to Bob’s UE via S-GW and eNodeB thatBob’s UE is associated with.

Figure 3. Tunnels in LTE/EPC when the mobile is in ECM-connected state

Radio resources and access bearers between eNodeB and S-

GW are released when a UE is not sending or receiving anydata for certain time. The UE’s connection mode will be idlemode. The network activates the UE when there are downlinkpackets sent to the UE by initiating the Network InitiatedService Request procedure.

When a SIP message is sent from P-CSCF to Bob’s UE,which is in idle mode, an IP packet containing the SIP messageis routed to the P-GW and sent to the S-GW as a GTPv1-Uencapsulated packet.

Because Bob’s UE is in idle mode in this scenario, the S-GW does not have a bearer to Bob. The S-GW buffers thepacket and starts Network Initiated Service Request Procedureby sending a GTPv2-C Downlink Data Notification Message to

the MME. The MME sends S1-AP Paging message to theeNodeBs in the Tracking Areas where the UE has registeredfrom. The requested eNodeBs page the UE in its areas.

The UE paged from its eNodeB responds to the MME bysending Service Request message. If Bob could not respond ina certain time, a timer on the MME expires and triggers theMME to send a GTPv2-C Downlink Data Notification Rejectmessage to S-GW. The S-GW deletes the buffered packetwhen it receives the GTPv2-C Downlink Data NotificationReject message from MME. S-CSCF has a timer and resendsSIP message to Bob each time the timer expires. This maytrigger another paging for Bob.

The MME signals the eNodeB and the S-GW to establish abearer to the UE. The S-GW sends the buffered SIP message toBob through eNodeB after Network Initiated Service RequestProcedure re-establishes radio bearers to Bob.

When Bob receives the SIP message, it may return a "100trying" message, which is not shown in Figure 4. Bob thenreturns a "200 ok" message back to Alice. These messages aresent uplink using the user plane bearer. After certain timepassed, eNodeB may release the established radio bearers usingS1-Release procedure.

Figure 4. Message Sequence Chart of SMS in the LTE/EPC network

IV. PROPOSAL OF A PACKET NOTIFICATION SERVICE

As seen in the previous section, transmitting data inLTE/EPC is only possible when the mobile is in ECM-

connected state. Switching the mobile from ECM_idle state toECM_connected state generates several messages. We thenpropose to add the possibility to transmit a data packet in thecontrol plane as shown in Figure 5 when there is isolated data.

Several additional messages should be defined. On the S11interface between the S-GW and the MME, a pair of new GTP-C messages is proposed, namely IP-Packet-Notification and IP-Packet-Notification-Complete. These messages include thecontrol tunnel identification like any other GTP-C packet andencapsulate an IP Packet.


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Figure 5. Transport of an IP Packet in the control plane

On the S1-MME interface, an equivalent pair of messagesis proposed, namely Packet-Notification and Packet-Notification-complete. Four new RRC messages are necessaryon the radio interface: a specific paging message that indicatesthat only one packet has to be transmitted on the downlink andone packet on the uplink, a Ready-for-Notification messagesent by the UE to fulfill the 3-step handshake of the LTEaccess procedure (see section 18.3 of [1]), two messages RRCPacket Notification and Packet Notification Complete that arethe equivalent of the packet notification messages on the S11and the S1-MME interface.

The procedure works as illustrated in Figure 6. If an IPpacket is sent by the corresponding node in the packet networkto the UE, it is transmitted by the PDN GW to the S-GW on thedefault bearer, which is always kept active as it is defined inthe 3GPP recommendations. If the UE is in ECM-connectedstate, the packet is forwarded to the UE through the S1-Uinterface. If the UE is in ECM-idle state, the S-GW starts atimer T1 for a very short duration (several milliseconds). Ifanother packet is received before T1 expires the ordinaryNetwork-Triggered-Service-Request procedure (see 5.3.4.3 of[8]) is used. If no other packet is received then the IP packet istransmitted on the control plane through the MME and asecond timer T2 is started. Note that the maximal value of T2is larger than T1 and is typically several seconds. If a packet isreceived before T2 ends then the packet is buffered and theNetwork Triggered service Request procedure is used. In thatcase, additional control messages are generated. If no otherpackets are received, the UE stays in ECM-idle mode and nocontext is established in the S-GW: no GTP-U tunnel is set upbetween the S-GW and the eNodeB.

In the packet notification procedure, the IP packet istransmitted in the control plane by the S-GW to the MME. Asthe location of the UE is not known at the cell level by theMME, a paging message should be sent on all the cells of the

list of tracking areas given to the mobile during the lastlocation procedure. What is proposed is just to transport the IPpacket together with the paging messages to the concernedeNodeB (see Figure 5). The IP packet is cancelled by eNodeBsthat have no answers to their paging message and is kept onlyby the eNodeB where the mobile is. The standard accessprocedure is used on the radio interface and the IP packet isonly transmitted after the 3-step handshake once all possiblecontentions are solved. An IP packet can be encapsulated in thePacket-Notification-Complete message sent by the UE. Itallows an end-to-end answer at the IP level.

Figure 6. The packet notification procedure

The proposed procedure is generic and does not require anydeep inspection of the content of the GTP-U message by the S-GW. Timer T1 is optional and Timer T2 may be very simplebecause no specific action is required when T2 expires. When aGTP-U packet is received and if the UE is in ECM-idle state,the S-GW has just to store the current time and to set up a flag.If another packet is received and if the flag is activated, the S-GW has just to compare the current time and the stored time. Ifthe difference is larger than the T2 value, then the packet isassumed to be isolated and the proposed packet notification isused. If the difference is smaller then the standard serviceactivation procedure is used. Furthermore, T2 does not requirea fine tuning: it can be over dimensioned. Let us assume it hasan infinite value, then the standard LTE/EPC service activationis always used except for the very first packet.

V.

SIGNALLING LOAD ANALYSIS

Very high data rates are available in the core network.Hence, the total amount of signaling exchanged within thenetwork is no more an issue as we can assume quasi-infinitelink capacity. However, as nodes are able to manage a largenumber of subscribers, the processing limitation can be thebottleneck. We then base our analysis on the number ofmessages received by each node. Any message concerning aparticular subscriber that is received by a node triggers an

interruption and that node should look for the context of theconcerned user.

We consider 2 types of traffic: web sessions and SMS. Let

α be the SMS traffic proportion defined as the number of SMStransmissions divided by the total number of both SMStransmissions and web sessions. We compare the cost (i.e. thenumber of messages received by a given node) for the standard

procedure and for the packet notification when α increases.

An SMS session consists of one downlink packetcontaining the SMS Message and one uplink message


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containing a 200 OK message. The traffic model of websessions is taken from [9] for non-real time services at 2048kbit/s and is defined as follows (see Figure 7). A sessionconsists of a sequence of several packet calls, where N pc =5 isthe average number of packet calls per session. During a packetcall, on average 25 packets are generated. The time betweenpacket calls is the reading time and has a mean of 412 seconds.These parameters are geometrically distributed. As LTE/EPC

was designed to be very reactive, we assume that both the UEand the MME switch from MME-connected to MME-idle statebetween two successive packet calls. Average inter-arrival timebetween packets within a packet call is exponentiallydistributed with a mean value Dd =1.95 ms.

Figure 7. Signaling for a web session

Sending an SMS message or initiating a packet call, to anidle terminal requires signaling to allocating the User planeresources, and releasing them after sending and receiving themessages. From figure 4, we see that an MME receives 3signaling messages to allocate radio resources (Service Requestprocedure) and 3 signaling messages to release those radioresources (S1-Release procedure). An S-GW receives 4 and 2signaling messages for Service Request and S1-Releaseprocedures By introducing the Packet Notification messages

proposed in this paper, the number of signaling messages toMME and S-GW are reduces to 2 messages for each entity(deduced from figure 6). No additional signaling is necessary,because a UE in idle mode stays in idle mode. The cost forService Request procedure, S1 Release procedure and PacketNotification procedure is denoted by mS , m R and mP respectively and is shown in Table I.

Without introducing Packet Notification, the cost per onesession can be represented as follows:

( )( )( ) RS PC mm N Cost ++−= α α 1

When Packet Notification is introduced, the cost can berepresented as follows:

( ) ( ) ( )( ) PPC RS PC m p N mm N Cost α α α +−++−= 11 where p=exp(-T1 / Dd ) is the probability that second packet

in a packet call in a web session arrives after time T1 is passed.If we do not use T1 timer, T1 is 0, so p=1. If we assumeT1=5ms, then p= 0.077.

Using these formulae, we can calculate average messagesthat SGW and MME receive. Signaling message load (i.e. cost)for the standard based solution which does not introduce

Packet Notification is 30-25α. If Packet Notification is

introduced and T1 is set to 0, the cost is 40-38α. If T1=5ms,

the cost is 30.77-28.77 α. The breakeven points of these

solutions are α=0.77 and α=0.20 respectively. See Figure 8 forcomparison of these solutions.

TABLE I. SIGNALLING COST ON MME AND SGW (NUMEBER OFRECEIVED MESSAGES FOR EACH ELEMENTARY PROCEDURE)

MME SGW

Service Request S m 3 4

Release Rm 3 2

Packet NotificationPm 2 2

Figure 8. Signaling message load

VI.

CONCLUSION

In this paper, we analyzed signaling messages that areexchanged for a simple but popular service like SMS. Weproposed a procedure to transmit isolated packets in the controlplane and to avoid to set up bearers and then to release them.This procedure is generic as it does not rely on any inspectionof higher layer protocol data units. It can be advantageouslyused for SMS but also for any type of sporadic data.

The focus has been made since the 90s on providing higher

and higher data rates to users. However, with the developmentof smart devices it is anticipated that a huge number ofterminals (perhaps several trillions) will be connected to awireless network, each terminal generating a very smallamount of data. Such an evolution will modify the paradigm ofwireless networks and requires to make network architectureand protocols evolve.

REFERENCES

[1] E. Dahlman, S. Parkvall, J. Sköld and P. Beming, 3G evolution: HSPAand LTE for mobile broadband, 2nd edition, Elsevier, 2008)

[2] CTIA-The Wireless Association, “Semi-Annual Survey Results”, March22, 2011. http://www.ctia.org/media/press/body.cfm/prid/2062.

[3] 3GPP TS 24.247 V8.5.0 (2010-12), Messaging service using the IPMultimedia (IM) Core Network Subsystem.

[4] IETF RFC 3261 “SIP: Session Initiation Protocol”, march 2002.

[5] 3GPP TS 24.229, V8.14.0 (2010-12) IP Multimedia Call ControlProtocol based on SIP and SDP.

[6] 3GPP TS 23.204, V8.5.0 (2010-03) Support of Short Message Service(SMS) over generic 3GPP Internet Protocol (IP) access

[7] 3GPP TS 24.341, V8.5.0 (2010-12), Support of SMS over IP networks

[8] 3GPP TS 23.401 V8.10.0 (2010-06), Evolved Universal TerrestrialRadio Access Network (E-UTRAN) access.

[9] 3GPP TR 30.03U V3.2.0 (1998- 04), Selection procedures for the choiceof radio transmission technologies of the UMTS.

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