enodeb wraparound testing
TRANSCRIPT
Leader in Converged IP Testing
915-2701-01 Rev B August, 2009
eNode B Wraparound Testing a Comprehensive Guide
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Contents1. Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. eNode B Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.1 Advantages of Wraparound Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Considerations for Wraparound Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.1 Radio Interface and S1 Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2 NAS Signaling Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.3 S1-Flex / Radio Network Access Sharing . . . . . . . . . . . . . . . . . . . . 9
3.2.4 Paging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.5. Radio Link Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2.6. Intra E-UTRAN Handover without MME Relocation . . . . . . . . . . . . . 10
3.2.7. Inter-eNode B Handover with MME Relocation . . . . . . . . . . . . . . . 11
3.3 User Plane Traffic Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.1 User Plane Data Stream Transfer . . . . . . . . . . . . . . . . . . 13
3.3.2 eNode B User Plane Traffic . . . . . . . . . . . . . . . . . . . . . . 13
3.3.2 Testing the eNode B User Plane . . . . . . . . . . . . . . . . . . . . . . . .14
3.3.3 Uu Interface Security Keys . . . . . . . . . . . . . . . . . . . . . . 14
4. Ixia’s Approach to eNode B Testing . . . . . . . . . . . . . . . . . . . . . . . . . 164.1 Network Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.2 Uu Radio Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.3 High Capacity LTE UE Simulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.4 Direct Bearer Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.5 Control Plane Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.6 User Plane Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.6.1 Using LTE Test Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.7 LTE Test Development (Scripting) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5. Strengths of Ixia’s Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Executive SummaryIncrease in consumer demand for sophisticated data applications requiring higher band-width is in turn driving the need for an IP-based network architecture accessible by mobile devices. The trend in user expectation is for wireless broadband data to be delivered at the same or higher speeds than traditional wireline data networks.
In response to this growing customer demand for mobile data, the Third Generation Part-nership Project (3GPP) has created Release 8 specifications for a new IP-based network for mobile broadband, the Long Term Evolution (LTE) network. LTE promises true mobile broad-band by delivering peak user data rates of up to 300 Mbps on the downlink and 75 Mbps on the uplink, with user plane latency of less than 5 ms.
The new LTE network is intended to satisfy end user demands, however deploying new technologies presents new testing challenges for network equipment manufacturers (NEM) and service providers. These challenges range from ensuring proper functionality to evalu-ating performance and verifying full interworking with existing network structures. End user experience suffers if products are deployed without an all-encompassing test plan.
Key LTE testing areas of concern include:
� Managing complex user plane and control plane traffic multiplexed on the same connection.
� Interworking of new elements (E-UTRAN and Evolved Packet Core) with existing networks.
� New LTE network elements supporting simultaneous voice, data and video traffic. � Generating high data rates that meet or exceed LTE standards.
The eNode B is the key network element in the new radio access system and packet-based core network known as Evolved Packet System (EPS). A test plan that includes a wrap-around approach is essential for validation of the eNode B.
The purpose of this paper is to provide a technical overview, illustrate the necessity of a wraparound eNode B test strategy and demonstrate how Ixia’s IxCatapult™/LTE test system satisfies this need. Various areas associated with eNode B testing are discussed in detail including:
� Signaling � S1-Flex � Paging � Handover � User plane traffic handling
LTE promises true mobile broadband by delivering peak user data rates of up to 300 Mbps on the downlink and 75 Mbps on the uplink.
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2. BackgroundIn Release 8, the Third Generation Partnership Project (3GPP) standardized Long Term Evo-lution (LTE) technology for a new radio access system with an IP-based core network. LTE defines an efficient air interface, flexible use of radio spectrum and flat IP-based network architecture, with the goal of increasing mobile network performance and cost efficiency.
Compared to the UMTS and GSM architectures that preceded it, LTE reduces the number of network elements and eliminates the circuit-switched domain. The LTE architecture defines the Evolved Packet System (EPS) as a combination of the IP-based core network and LTE access system. The EPS consists of the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and the Evolved Packet Core (EPC) as illustrated in Figure 1. The EPC is divided into a user plane, consisting of the Serving Gateway (S-GW) and the PDN Gateway, and a control plane known as the Mobility Management Entity (MME). The EPC is the LTE core network and the E-UTRAN corresponds to the LTE access network. The E-UTRAN consists of the eNode B (E-UTRAN Node B) network elements.
UTRANUTRANUTRAN
eNode B
eNode B
X2
S1-MME
CDMA2000
Uu
E-UTRAN
MME
Serving
GatewayPDN
Gateway
UE
S1-U
IP-Services
Internet
EPC
EPS
eNode BeNode B
eNode BeNode B
X2
S1-MME
CDMA2000CDMA2000
Uu
E-UTRAN
MME
Serving
GatewayPDN
Gateway
UE
S1-U
IP-ServicesIP-Services
InternetInternet
EPC
EPS
GERANGERANGERAN
Figure 1: E-UTRAN Architecture
The eNode B is the most complex part of the LTE access network, and is a much more complex network element in comparison to counterparts UMTS Node B or GSM BTS as it operates without a central controller (RNC, BSC). The functions of the central controller are now performed by the eNode B, making it a critical component of the new LTE network architecture. Due to a flat LTE architecture, access network testing consolidates the network testing and physical radio interface testing domains.
The eNode B interconnects with other eNode Bs over the X2 interface and to the EPC over the S1 interface. The S1 interface is composed of the S1-MME control plane interface to the MME and the S1-U user plane interface to the Serving Gateway. The Uu interface defines the radio interface between the eNode B and user equipment (UE).
The eNode B terminates the RRC, PDCP, RLC, MAC and physical layers of the Uu interface. It also terminates Access Stratum (AS) signaling between the E-UTRAN and EPC on the S1 interface as well as intra-E-UTRAN signaling between eNode Bs on the X2 interface.
The eNode B is the most complex part of the LTE network.
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3. eNode B TestingTo fully test the eNode B, both network and air interfaces must be simulated. There is a criti-cal need for coordinated eNode B testing that supports the Uu, X2, and S1 interfaces.
3.1 Advantages of Wraparound Testing
Many eNode B functions and procedures can only be verified using wraparound testing. At layer 3, signaling between the UE and eNode B on the Uu interface is tightly coupled with intra-E-UTRAN signaling on the X2 interface and signaling between the eNode B and the EPC on the S1 interface. This tight coupling makes testing any one interface in isolation difficult, if not impossible. The preferred approach is to use an integrated set of wrap-around test procedures that simultaneously exercise and coordinate all eNode B interfaces.
Wraparound testing provides a complete testing solution for each eNode B interface by:
•Exercising each eNode B interface.
•Measuring response time.
•Recording eNode B response details.
•Determining resultant test actions in conjunction with the configured test script.
Wraparound testing verifies eNode B user plane connections by:
•Applying a variety of realistic user plane traffic flows against each interface.
•Coordinating each user plane flow according to the signaling exchanged with the eNode B.
•Verifying the content of the user plane traffic flows sent by the eNode B.
•Exercising control of user plane frames at both source and sink.
•Measuring the quality of service (QoS) and quality of experience (QoE) delivered for each traffic flow.
3.2 Considerations for Wraparound Testing
Wraparound testing of the eNode B is illustrated by taking a closer look at a few of the basic functions and procedures that are required of an eNode B.
3.2.1 Radio Interface and S1 Signaling
When addressing the Uu and the S1 interfaces that connect the e-UTRAN to the UE and EPC, testing the full RRC and S1-AP protocol stacks are required:
Over the Uu interface, the RRC signaling protocol:
•Terminates in the UE and eNode B.
•Controls UE sessions and radio bearers.
On the S1 interface, the S1-AP protocol:
•Terminates in the eNode B on the E-UTRAN side.
•Terminates in the MME on the EPC side.
•Controls UE sessions and EPS bearers.
Wraparound testing of eNode B signaling:
•Coordinates signaling on the S1 and Uu interfaces to verify the tightly coupled relationship between RRC and S1-AP signaling.
•Initiates procedures from both interfaces (S1 or Uu) and verifies that the eNode B takes
Many eNode B functions and procedures can only be verified using wraparound testing.
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appropriate action on each side.
•Provides a single source of control to enable simulation of racing conditions between interfaces.
•Applies load to characterize how the eNode B handles surges of signaling messages on one or more interfaces.
UE E-UTRAN
Uu
EPC
S1-MME
S1-APSCTP
IP
MSMS
eNode BeNode B MME
MME
PDCPRLCMACPHY
RRCNAS
PDCPRLCMACPHY
RRC S1-APSCTP
IP
NAS
COORDINATION
UE E-UTRAN
Uu
EPC
S1-MME
S1-APSCTP
IP
MSMS
eNode BeNode B MME
MME
PDCPRLCMACPHY
RRCNAS
PDCPRLCMACPHY
RRC S1-APSCTP
IP
NAS
UE E-UTRAN
Uu
EPC
S1-MME
S1-APSCTP
IP
S1-APSCTP
IP
MSMS
eNode BeNode B MME
MME
PDCPRLCMACPHY
RRCNAS
PDCPRLCMACPHY
RRCNAS
PDCPRLCMACPHY
RRCPDCPRLCMACPHY
RRC S1-APSCTP
IP
NASS1-APSCTP
IP
NAS
COORDINATION
Figure 2: LTE Control Plane (S1 and Uu)
3.2.2 NAS Signaling Transport
NAS messaging supports EPS mobility management and session management procedures between the UE and MME. NAS consists of two protocols:
•EPS Mobility Management (EMM) protocol provides procedures for mobility control as well as security for NAS protocols
•EPS Session Management (ESM) protocol provides control of EPS bearer contexts and, together with Uu Access Stratum signaling, control of user plane bearers
NAS messages are subject to the encryption and integrity checks of the encapsulating PDCP protocol on the Uu interface, but appear unencrypted on the S1-MME interface. Wraparound eNode B testing checks NAS signaling transport by:
•Verifying NAS message transfer
•Determining the underlying eNode B latency for signaling transfer
•Verifying signaling encryption (downlink) and decryption (uplink)
To fully test the eNode B, all air and network interfaces must be simulated.
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3.2.3 S1-Flex / Radio Network Access Sharing
S1-Flex enables the E-UTRAN to support network redundancy and load sharing. Under S1-Flex, a single eNode B can be connected to multiple MMEs and S-GWs, which may be associated with multiple PLMNs as shown in Figure 3.
Uu S1
eNode
B
EPC Simulation
UE Simulation
UE 1 (PLMN-A)
TEST CONTROL
MME/S-GW
PLMN-A
MME/S-GW
S1
S1
S1
Uu
PLMN-B
MME/S-GW
MME/S-GW
UE 1 (PLMN-B)
Figure 3: S1-Flex/Radio Network Access Sharing
Wraparound testing of S1-Flex implementation verifies that the eNode B:
•Routes new EPC session requests to the MME entity identified by the temporary UE identity that it receives over the Uu interface.
•Supports up to 6 PLMNs.
•Broadcasts system information for each supported PLMN over the Uu interface.
•Routes UE paging responses and EPC session requests to the proper S1-MME interface in accordance with the PLMN indicated by the response.
3.2.4 Paging
The EPC uses S1 signaling to indicate to the eNode B the identity of UEs to be paged. The eNode B stores the paging information, and incorporates it into paging messages that it broadcasts over the Uu interface.
Wraparound testing of paging:
•Provides identities of UEs to be paged to the eNode B over the S1 interface.
•Verifies paging messages broadcast by the eNode B over the Uu interface.
•Sends UE paging response messages on the Uu interface in accordance with the test objective.
S1-Flex enables the E-UTRAN to support network redundancy and load sharing.
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3.2.5 Radio Link Failure
Reliability in the network is often verified by generating error conditions to ensure that the system properly handles these without further degradation. The eNode B implements a phased recovery procedure to recover from radio link failure on the Uu interface.
Wraparound testing of radio link failure:
•Triggers the eNode B phased recovery procedure by simulating a radio link failure over an active Uu signaling connection.
•Returns a handover failure result over an X2 or S1 interface to a source eNode B undergoing handover to directly trigger the second stage of the radio link recovery procedure.
•Simulates RRC signaling failure over the Uu interface to trigger the eNode B to idle the RRC connection and close the S1-AP connection to the EPC.
•Simulates radio link failure on the Uu interface to verify the eNode B phased recovery procedure resulting in either connection recovery or connection release.
3.2.6 Intra E-UTRAN Handover without MME Relocation
Handovers occur for a number of reasons, for instance to maintain a call when the UE moves from one cell into another. Testing of these handover/handoff mechanisms in various scenarios is necessary to validate proper call handling.
Wraparound testing provides a complete test of eNode B intra E-UTRAN handover ca-pabilities, testing the eNode B as both the source eNode B and the target eNode B. The X2 interface transports signaling and user data between eNode Bs during intra-E-UTRAN handover without MME relocation.
As shown in Figure 4, testing intra-E-UTRAN handover without MME relocation involves UE simulation, E-UTRAN simulation, and EPC simulation elements that support the Uu, X2, S1-MME and S1-U interfaces.
eNode B
EPC SimulationUE Simulation
UE
TEST CONTROL
MME
S-GW
S1-MME
S1-U
Uu
X2
eNodeB
E-UTRAN Simulation
eNode B
EPC SimulationUE Simulation
UE
TEST CONTROL
MME
S-GW
S1-MME
S1-U
Uu
X2
eNodeB
E-UTRAN Simulation
Figure 4: Intra-E-UTRAN Handover without MME Relocation
Wraparound testing provides a complete test of eNode B intra E-UTRAN handover capabilities.
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Wraparound testing of the source eNode B involves the following steps:
•Establish a signaling connection and associated user bearers between the wraparound tester and the eNode B over the Uu and S1 interfaces.
•Send an appropriate RRC measurement report message over the Uu interface to trigger intra E-UTRAN handover.
•Simulate the target eNode B over the X2 interface.
•Simulate the EPC over the S1 interface.
•Send downlink PDUs over the S1-U interface until the source eNode B transfers PDCP status to the target eNode B.
Wraparound testing of the target eNode B involves the following steps:
•Send a handover request over the X2 interface to trigger the target eNode B handover procedure.
•Transfer active user bearers to the target eNode B and forward user bearer traffic over the X2 interface.
•Utilize information received from the target eNode B over the X2 interface to synchronize the radio connection and confirm the handover over the Uu interface.
•Open bearer paths to the eNode B and send downlink user plane PDUs on the S1-U interface while continuing to forward user data PDUs to the eNode B over the X2 interface.
Both target and source eNode B wraparound testing of intra-E-UTRAN handover without MME relocation require the coordination of both signaling and user data flows over the S1, X2, and Uu interfaces.
3.2.7 Inter-eNode B Handover with MME Relocation
Wraparound testing provides a coordinated way to test the eNode B’s ability to act as source or target eNode B during an inter-eNode B handover with MME relocation. Inter-eNode B handover with MME relocation, as opposed to handover without MME reloca-tion, relies on S1 signaling rather than X2 signaling to handle inter-Node B communication.
The eNode B needs to be tested as both as the source and the target of the handover. In addition, the handover procedure differs depending upon whether or not a direct path is available between the source and target eNode Bs. When a direct path is available, inter-eNode B user plane PDUs are forwarded over the X2 interface; otherwise, indirect path forwarding transfers inter-eNode B user plane PDUs over the S1 interface through the EPC.
As shown in Figure 5, when indirect path forwarding is configured, testing of inter-eNode B handover with MME relocation involves UE simulation and EPC simulation elements sup-porting the Uu, S1-MME and S1-U interfaces.
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eNode
B
EPC Simulation
UE
TEST CONTROL
MME
S-GW
S1-MME
S1-U
Uu
S-GW
UE Simulation
KeNB -KeNB-UP-enc,KeNB-RRC-int,KeNB-RRC-enc
MME�
Figure 5: Inter-eNode B Handover with MME Relocation and Indirect Transfer
Testing of inter-eNode B handover with MME relocation involves simulation of the UE, E-UTRAN and EPC elements that support the Uu, X2, S1-MME and S1-U interfaces. This direct path forwarding configuration is shown in Figure 6.
eNode B
EPC SimulationUE Simulation
UE
TEST CONTROL
MME
S-GW
S1-MME
S1-U
Uu
MME
X2
eNodeB
E-UTRAN Simulation
S1-MME
eNode B
EPC SimulationUE Simulation
UE
TEST CONTROL
MME
S-GW
S1-MME
S1-U
Uu
MME
X2
eNodeB
E-UTRAN Simulation
S1-MME
Figure 6: Inter-eNode B Handover with MME Relocation and Direct Transfer
Wraparound testing of the target eNode B with MME relocation:
•Sends a handover request to the eNode B over the S1-MME interface to trigger the handover procedure.
•Transfers user bearers to the target eNode B over S1-MME interface.
•Forwards user plane traffic to the eNode B over the S1 or X2 interface, depending upon whether direct or indirect transfer is to be simulated.
Testing of inter-eNode B handover with MME relocation involves simulation of the UE, E-UTRAN and EPC elements.
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•Synchronizes the radio connection and confirms the handover over the Uu interface.
•Sends new downlink PDUs to the eNode B over the S1-U interface while continuing to forward PDUs sourced from the simulated source eNode B over the X2 or S1-U interfaces.
Wraparound testing provides a coordinated test of both signaling and user data flows over the S1 and Uu interfaces, and, in the case of direct transfer, of user data flows over the X2 interface.
3.3 User Plane Traffic Characteristics
A major selling point of LTE is its capability to offer cellular user data rates comparable to those provided by fixed network broadband services. In order to properly verify user plane traffic several dimensions need to be addressed. These dimensions include but are not limited to validation of multiple media streams per UE, quality of service (Qos), packet analysis, and security.
3.3.1 User Plane Data Stream Transfer
Figure 7 shows the LTE user plane protocol stacks in the context of wraparound eNode B testing.
UE
KeNB
KeNB-UP-enc,
KeNB-RRC-int,
KeNB-RRC-enc
E-UTRAN
Uu S1-U
eNode B
EPC
COORDINATION
Context
Server
Application
IP
GTP-U
UDP
IP
GTP-U
UDP
IP
Application
IP
PDCP
RLC
MAC
PHY
PDCP
RLC
MAC
PHY
S-GW
�
Figure 7: LTE User Plane (S1 and Uu)
The eNode B terminates the PDCP, RLC, MAC and PHY protocol layers of the Uu interface user plane. The eNode B also terminates the GTP-U protocol used to tunnel user plane PDUs over the S1-U and X2 interfaces. On the Uu interface, the eNode B schedules and transmits downlink user plane data received from the Serving Gateway (S-GW) onto com-mon radio channels and allocates the uplink radio resources that enable each UE to send data to the EPC. The PDCP protocol layer of the radio interface protocol stack, imple-mented in both eNode B and UE, is responsible for the compression and encryption of user plane data over the Uu interface. The eNode B forwards unacknowledged and unsent data PDUs on the X2 or S1 interfaces as part of various handover procedures.
3.3.2 eNode B User Plane Traffic
LTE delivers all dedicated bearer services to a UE over a single dedicated packet connec-tion between the UE and the EPC. Because of the increased data throughput, improved network performance, and reduced latency requirements of LTE, stress and load test are fundamental aspects of any eNode B wraparound testing. Also, the great variety of user services carried by the bearers that make up a user’s packet connection makes simulation
A major selling point of LTE is its capability to offer cellular user data rates comparable to those provided by fixed network broadband services.
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of a variety of packet service flows over the user plane an essential feature of eNode B wraparound testing.
VoIP service is very sensitive to delay and imposes strict latency requirements on the net-work, including the eNode B. Delays in voice traffic create transmission gaps that may be heard by the listener. Packet loss can severely impact voice quality. Retransmission of voice packets, unlike data packets, is not a realistic option because of stringent latency require-ments.
VoIP generally makes use of SIP signaling. Within LTE, SIP signaling is transported over a user plane traffic flow through the eNode B and EPC. Although SIP signaling is not processed by either the eNode B or EPC, bearer channels carrying SIP signaling flows are generally marked with a signaling QoS attribute. This allows the signaling flow to be accorded a relatively high priority among the user plane traffic flows transported over the user connection. The eNode B implements prioritization algorithms over the Uu interface for all user plane flows, including those transporting SIP signaling.
With LTE, various types of video services, including broadcast video, video-on-demand (VoD), and video conferencing, become available to the UE. In addition to high band-width, delivery of high quality video services requires minimization of packet loss, jitter, and packet mis-insertion. In the case of video, the loss of even a single packet may be noticeable to viewers.
Data services, compared to voice and video services, is tolerant of latency, jitter, and loss. Generally run over TCP, packet loss can be remedied by retransmission. For data services, throughput is the most important traffic characteristic to be measured.
3.3.2 Testing the eNode B User Plane
Testing the eNode B involves loading the system to capacity. The number of UEs connected to the system, as well as the volume of user plane data flowing through the system, can be raised to meet or exceed the limits of the eNode B. Wraparound eNode B test systems sup-port user plane testing with capabilities that:
•Allow a real world mix of voice, video and data to be incremented until the eNode B’s capacity is exceeded.
•Provide measurements that verify that voice is prioritized over video, and that video is prioritized over data.
•Isolate and accurately measure the performance characteristics of the eNode B itself, information that is of interest to both service providers and NEMs.
•Effectively evaluate the QoS delivered by the eNode B ; assess the ability of the eNode B to maintain its QoS commitments for each stream (GBR and MBR) and each session (AMBR) when loaded with multiple streams of various classes of service, and the proportions of voice, data, and video traffic varied.
•Evaluate the QoE provided by the eNode B ; take PESQ measurements of audio streams at the simulated UEs and at the simulated S-GW.
3.3.3 Uu Interface Security Keys
The EPC delivers the KeNB key to the eNode B over the S1 interface when an RRC connec-tion is established. The eNode B and UE independently derive Uu interface keys for the RRC and User Plane, KUP-enc, KRRC-int and KRRC-enc, from KeNB. The derived key material is used for the integrity checking and encryption of RRC message frames, and the encryption of user plane frames, on the Uu interface.
VoIP service is very sensitive to delay and imposes strict latency requirements on the network, including the eNode B.
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In wraparound testing, the simulated EPC passes KeNB to the eNode B over the S1 interface, and the simulated UE is provided its own copy of the Access Stratum keys derived from KeNB. The flow of security keys is shown in Figure 8.
Coordination of the Uu and S1 interfaces allows wraparound testing to verify that the eNode B:
•Properly ciphers and deciphers both signaling and user plane PDCP PDUs.
•Provides correct integrity protection and integrity verification of signaling PDCP PDUs.
TEST CONTROL
KeNB �
KeNB-UP-enc, KeNB-RRC-int, KeNB-RRC-enc
Uu
S1-MME
eNode BEPC Simulation
S1-AP
UE Simulation
UE 1
LTE Session
KeNB
RRC
PDCP
RRC
PDCP
PDCP
UP
PDCP
UP
K -
K -K
S1-AP
KeNB
KeNB �KeNB-UP-enc, KeNB-RRC-int, KeNB-RRC-enc
K UP-encK
K
TEST CONTROL
KeNB �
KeNB-UP-enc, KeNB-RRC-int, KeNB-RRC-enc
TEST CONTROL
KeNB �
KUP-enc, KRRC-int, KRRC-enc
Uu
S1-MME
eNode BEPC Simulation
S1-AP
UE Simulation
UE 1
LTE Session
KeNB
RRC
PDCP
RRC
PDCP
RRC
PDCP
RRC
PDCP
PDCP
UP
PDCP
UP
PDCP
UP
PDCP
UP
KUP enc
KRRC intKRRC-enc
S1-AP
KeNB
KeNB�KUP-enc, KRRC-int, KRRC-enc
K UP-encKRRC-int
KRRC-enc
Figure 8: Uu Interface Security Key Usage
During intra-E-UTRAN mobility procedures:
•Key material passes over the X2 interface either to (target) or from (source) the eNode B.
•New Uu interface security keys are derived by the target eNode B from the key material that it receives.
•HFN (Hyper Frame Number) and the PDCP SN (Sequence Number) for control and RLC-UM user bearers are reset by the target eNode B.
•Values of HFN and PDCP SN for RLC-AM user bearers received over the X2 interface are maintained by the target eNode B.
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4. Ixia’s Approach to eNode B TestingIxia’s approach to eNode B testing is to address the complexities and intricacies of all its functionality in a comprehensive LTE testing solution. The Ixia LTE solution is comprised of an IxCatapult, a t600 peripheral for patent pending LTE UE Simulation, a complete library of LTE protocol encoder/decoders and state machines, along with an array of test tools and an optional set of prepackaged LTE Test Applications.
The IxCatapult/LTE test system is at the core of Ixia’s eNode B wraparound configuration. IxCatapult provides a user interface from which all elements of wraparound testing are controlled and monitored. IxCatapult provides the physical network interfaces that simulate neighboring eNode Bs as well as the EPC. Each interface comes with complete control and user plane protocol stacks. The Ixia eNode B test environment is illustrated in Figure 9.
eNodeB
Under Test
S1-MME & S1-U
eNodeB(s)
(DCT2000)
Content Servers
(MBMS, Voice,
Data, Video)
UE Simulator
(UE Radio Device)
UE-Control
…
EPC
(DCT2000)
…
…
X2
Uu
Test
Control
(SBC)
UE Control
(DCT2000)
EPC Control
E-UTRAN Control
Content Servers
(MBMS, Voice,
Data, Video)
…
… …
eNodeB
Under Test
S1-MME & S1-U
eNodeB(s)
IxCatapult
Content Servers
(MBMS, Voice,
Data, Video)
UE Simulator
(UE Radio Device)
UE-Control
…
EPC
(IxCatapult)
…
…
X2
Uu
Test
Control
(SBC)
UE Control
(IxCatapult)
EPC Control
E-UTRAN Control
Content Servers
(MBMS, Voice,
Data, Video)
…
… …
Figure 9: Ixia eNode B Wraparound Testing Environment
4.1 Network Interfaces
IxCatapult/LTE simulates neighboring eNode Bs (X2 interface) as well as the EPC (S1-MME and S1-U interfaces), and is able to coordinate control plane and user data streams on all network and UE interfaces. IxCatapult is able to control call initiation, trigger UE attach and detach, and simulate UE roaming and handover. IxCatapult, moreover, provides inte-grated log collection and analysis over all tested interfaces.
4.2 Uu Radio Interface
Ixia supports various options when addressing the Uu radio interface. Ixia’s patent pend-ing, LTE UE Simulator uses the t600 peripheral together with the IxCatapult platform to simulate user equipment traffic in order to load an eNode B to capacity. Other options in-clude several third-party LTE UE simulator devices that work with Ixia’s IxCatapult platform.
Ixia integrates its UE upper layers with specialized devices that maximize user control over the radio physical layer and the lower Uu interface protocol layer. In a single test platform, the Ixia IxCatapult controls the PDCP, RRC, and NAS protocols of the UE. The LTE UE Simu-lator controls the RLC, MAC, and PHY layers.
The IxCatapult supports higher level call scripting that sends and receives call control or script agents through an Ethernet connection to the UE Simulator.
Ixia’s patent-pending LTE UE test platform simulates user equipment traffic to load an eNode B to capacity.
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The Uu interface may be connected using either native radio or CPRI wired interface. UE configuration, logs, and call sequence control are consolidated with those of other simu-lated network elements within IxCatapult.
4.3 High Capacity LTE UE Simulator
Ixia provides an integrated high capacity LTE multi-UE Simulator that enables IxCatapult to simulate up to 400 UEs per sector, or up to 2400 UEs overall. Compared to other vendors’ solutions, Ixia’s LTE UE simulation has been designed from the ground up as a cost effective load tester. The multi-UE simulation system allows up to 6 eNodeB sectors to be stressed in a coordinated manner and supports a maximum data throughput of up to 300 Mbps in the downlink and up to 75 Mbps in the uplink per user.
4.4 Direct Bearer Interface
In addition to standard radio interfaces, the Direct Bearer Interface (also known as radio bypass) is supported. Direct Bearer provides a PDCP over UDP/IP Ethernet interface that allows Uu RRC signaling and user bearer PDUs to be exchanged between the simulated UE and the eNode B under test. Direct Bearer Interface is implemented on IxCatapult, control-ling the UE simulation, and allowing the upper layers of the eNode B Uu interface protocol stack to be tested independently of the radio interface.
4.5 Control Plane Assessment
The IxCatapult/LTE provides a comprehensive testing platform to test the Uu, S1-MME, and X2 control plane interfaces. Complete encoding and decoding support is provided for the RRC and NAS signaling protocols on the Uu interface, for the S1-AP signaling protocol on the S1-MME interface, and for the X2 signaling protocol on the X2 interface.
Complete state machine implementations are provided for all transport layers. A message editor provides the user with complete flexibility to edit signaling messages for each sup-ported protocol. Scripted state machines, developed using a C-like programming language (DPCL) or an SDL-like drag-and-drop graphical programming tool (CATTgen), control the behavior of the simulated UE, eNode Bs and EPC. Ixia simplifies test script development and maintenance by providing procedure libraries for the RRC, NAS, X2-AP and S1-AP protocols. Procedure libraries are libraries of building block scripts for some of the higher layer protocols (S1-AP, RRC and NAS). These libraries relieve the test engineer from the complex and difficult scripting tasks.
Ixia’s wraparound testing system includes a testing control function that coordinates RRC signaling on the Uu interface with S1-AP and X2-AP signaling on the S1-MME and X2 interfaces, respectively. Ixia’s system readily verifies the tight coupling that exists between the signaling protocols, and provides the level of control needed to test corner cases and racing conditions. Once the testing scenarios are developed they can be executed in either functional test or load test mode. Ixia’s Load Test Application (LTA) product provides a ready-to-use eNode B wraparound load testing application.
4.6 User Plane Assessment
IxCatapult/LTE, in conjunction with several LTE tools, provides an end-to-end test environment for the generation and analysis of various types of user plane data. These tools include Traf-ficGenerator,X•Stream,BERT,PESQandIPPass-through.
The Traffic Generator produces voice and video (AMR/RTP and MPEG4/RTP) traffic ac-cording to various codec definitions into multiple individual bearer streams. Users can either select from pre-coded frame samples or supply voice or video files in a supported format (.wav, .mpg) for conversion to a coded stream. Bearer traffic is inserted into the
IxCatapult/LTE provides a comprehensive testing platform for Uu, S1-MME and X2 control plane interfaces.
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user specified channels with the data profile, packet size and rate defined by the original sample. It also provides throughput assessment.
X•StreamgenerateshighlevelsoffixedframepackettrafficontheS1andX2interfaces.It generates tens of thousands of bearer connections on a single IxCatapult Gigabit Ether-net interface (aggregate of 3.5+ Gbps per interface). Control and configuration of traffic patterns are possible through the user interfaces. Statistical displays are provided showing connection counts, packet transmit and receive rates.
The IP Pass-through tool allows IP traffic streams generated from and directed to standalone application servers and clients to load the eNode B bearer channels on the Uu, S1-U, and X2 interfaces.
Voice/audio testing includes support for AMR WB/NB, G.711, G.723, G.726 and G.729 codecs. Video traffic compliant with the H.261, H.263, MPEG-2 and MPEG-4 standards can be generated.
The IxCatapult/LTE wraparound test system provides a packet level assessment of the QoS of the eNode B user plane streams. Ixia’s system measures end-to-end RTP packet loss, jitter and round trip delay in both uplink and downlink directions. A BERT (Bit Error Rate Test) Analyzer measures the quality of transmission over the air interface (Uu) at the MAC layer. Ixia’s system validates eNode B PDCP compression and ciphering of user data PDUs using test data streams generated from the UE and the EPC simulators. The ability of the eNode B to maintain quality of service commitments for each stream (GBR and MBR) and each user (AMBR) can be assessed while the eNode B is loaded with multiple streams of various classes of service, and with various proportions of voice, data, and video traffic.
Some of the different types of loading that can be delivered by the IxCatapult/LTE system include:
•Testing under a background load.
•Stress testing with increasing load rates to determine failure points.
•Full traffic mixes under load conditions.
In addition, IxCatapult/LTE system evaluates the Quality of Experience (QoE) of the eNode B. The system takes PESQ measurements of audio streams at both the simulated UE and at the simulated Serving Gateway.
4.6.1 Using LTE Test Tools
This section presents how IxCatapult LTE tools can be used to assess the user plane capa-bilities of an eNode B by complementing the IxCatapult platforms and protocols encoder/decoders. The tester sets up an LTE session from the UE Simulator to the EPC Simulator through the eNode B under test. In addition to the default best effort bearer, the UE Simu-lation establishes several dedicated bearers, including a signaling user bearer to carry SIP messages, a GBR bearer to carry AMR VoIP voice, a second GBR bearer to carry an MPEG4 video stream, and a second best effort user bearer to handle an HTTP client com-municating with an HTTP server. Figure 10 illustrates the LTE user plane testing scenario described in this section.
The IxCatapult/LTE wraparound test system provides a packet level assessment of the Qos of the eNode B user plane streams.
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FTP
Client
FTP
Client
HTTP
Client
HTTP
Client
UE Simulation
Uu S1-U
eNode BEPC Simulation
PESQ
FTP
Server
FTP
Server
VoIP TrafficGen
SIP Network
Script
SIP Client
Script
IP
Pass-Through
VoIP TrafficGen
FTP
Script
UE 1
TEST CONTROL
IP
Pass-Through
HTTP
Server
HTTP
ServerLTE Session
FTP
Client
FTP
Client
HTTP
Client
HTTP
Client
UE Simulation
Uu S1-U
eNode BEPC Simulation
PESQ
FTP
Server
FTP
Server
VoIP TrafficGen
SIP Network
Script
SIP Client
Script
IP
Pass-Through
VoIP TrafficGen
FTP
Script
UE 1
TEST CONTROL
IP
Pass-Through
HTTP
Server
HTTP
ServerLTE Session
Figure 10: Example User Plane Testing Scenario
The simulated UE features a SIP client script, which sets up a SIP signaling connection through the signaling user bearer to a simulated network SIP entity located in the simulated EPC. The SIP client establishes and tears down SIP VoIP calls at regular intervals, while the SIP network script located in the EPC Simulation coordinates S1 and SIP signaling to alter-nately establish and tear down a VoIP GBR bearer connection associated with the SIP call.
When the VoIP call is active, the Traffic Generator tool sends real AMR/RTP/UDP/IP/GTP/UDP/IP traffic downstream on the S1-U interface and real AMR/RTP/UDP/IP/PDCP traffic upstream on the Uu interface over the GBR bearer associated with the SIP call. PESQ mea-surements taken at 8 KHz sample rates at both the simulated UE and simulated EPC can be viewed from the Test Control entity. Test Control is the high level controlling script or test application for the wraparound test solution.
After the EPC Simulator establishes the second GBR connection using S1 signaling, both the UE and EPC Simulators use the Traffic Generator tool to send MPEG4 traffic at about 2 Mbps. In the uplink, MPEG4/RTP/UDP/IP/PDCP traffic is sent to the eNode B over the Uu interface, and, in the downlink, MPEG4/RTP/UDP/PDCP traffic is sent to the eNode B over the S1 interface. Application of the RTCP protocol to the video and VoIP RTP streams enables the tester to determine the jitter, packet loss, and latency associated with the video and VoIP GBR streams.
The default best effort user bearer is used to connect an FTP client situated on the UE side of the eNode B with an FTP server located on the EPC side of the eNode B. The second dedicated non-signaling best effort user bearer is used to connect an HTTP client situated on the UE side of the eNode B with an HTTP server located on the EPC side of the eNode B. The IP Pass-Through tool within the UE Simulator passes packets between the remote clients and their respective Uu interface bearers. Similarly, within the EPC Simulator, another instance of the IP Pass-Through tool maps uplink and downlink traffic to and from FTP and HTTP servers to their respective bearers on the S1 interface. The two best effort user bearers fill the Uu connection in the uplink and downlink directions with low priority background traffic.
4.7 LTE Test Development (Scripting)
Ixia has developed a set of LTE Load Test Applications that assist users in configuring and executing test scenarios. These applications are prepackaged, easy to run and designed to eliminate the need for labor-intensive script development effort.
Ixia has developed a set of LTE Load Test Applications.
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5. Strengths of Ixia’s ApproachIxia’s wraparound test system provides a comprehensive eNode B testing environment as illustrated in Figure 11. The system simulates LTE UEs, adjacent eNode Bs, and the EPC. The IxCatapult/LTE system provides a complete protocol stack for each eNode B inter-face, including the Uu, X2, S1-MME and S1-U interfaces. IxCatapult offers a range of test configurations, including options for UE functional and load testing. When combined with Ixia’s high-capacity LTE UE Platform, IxCatapult performs heavy load wraparound testing of eNode B by simulating hundreds of UEs over multiple sectors.
The system’s EPC component simulates a complete core network, and supports inter-RAT and inter-MME handover scenarios. Over the X2 interface, the IxCatapult/LTE system simu-lates one or more adjacent eNode Bs.
Key strengths of the Ixia wraparound test system include:
•Choice of UE simulation devices.
•Flexible programmable platform.
•Signaling protocol validation.
•User plane traffic testing.
•Performance testing.
•S1-Flex simulation.
•Handover testing.
•Negative testing.
•Full load and stress testing capabilities.
•Prepackaged LTE Load Test Applications eliminate the need for test scripting.
IxCatapult/LTE provides a complete protocol stack for each eNode B interface.
X2
S1-C
S1-U
eNodeB(s)Simulated
UE-Ctrl: PDCP-RLC Interface, Configuration, Logging
UE-Ctrl
Radio
Or
CPRI
LTE UE Simulated
(L2/L1)
DCT2000 m500 Chassis
t600 UE Chassis
IP Pass-through
IP Pass-through
Content Servers(MBMS, Voice,
Data, Video)
Content Servers(MBMS, Voice,
Data, Video)
LTE UE Simulated
(L3 + APP)
eNodeB(DUT)
MMEs/SAE GWsSimulated
X2
S1-C
S1-U
eNodeB(s)Simulated
UE-Ctrl: PDCP-
UE-Ctrl
Radio
Or
CPRI
UuLTE UE
Simulated(L2/L1)
IxCatapult m500 Chassis
t600 MicroTCA
IP Pass-through
IP Pass-through
Content Servers(MBMS, Voice,
Data, Video)
Content Servers(MBMS, Voice,
Data, Video)
Content Servers(MBMS, Voice,
Data, Video)
LTE UE Simulated
(L3 + APP)
eNodeB(DUT)
eNodeB(DUT)
MMEs/SAE GWsSimulated
MMEs/ S-GWsSimulated
Figure 11. Ixia’s Wraparound Test System
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6. Conclusions3GPP has defined the LTE technology as a means to increase mobile network performance, provide an efficient air interface, and evolve to an all IP-based network architecture. The eNode B is a critical network element of LTE, and is responsible for control of the radio interface with the UE as well as bearer setup between the UE and EPC.
The eNode B completely terminates the Access Stratum of the Uu radio interface to the UE and controls radio interface resources. The eNode B plays a central role in LTE UE mobility, including intra-eNode B handover, inter-eNode B handover, and inter-RAT handover. The concentration of E-UTRAN functionality into the eNode B calls for a wraparound testing that coordinates control and user plane activity on both radio and network interfaces.
Ixia’s IxCatapult/LTE system including the high-capacity LTE UE Platform for high load, pro-vides a complete eNode B wraparound testing solution that encompasses the Uu, S1 and X2 interfaces. This eNode B test configuration offers protocol simulation for each interface, generates realistic media stream traffic over user plane and supports a complete set of QoS and QoE measurement tools for media streams. Ixia provides the most cost effective, complete and flexible eNode B testing product on the market today.
Ixia provides the most cost effective, complete and flexible eNode B testing product on the market today.
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