This article summarizes the origins of CDMA technology and the introduction of 3G versions such as CDMA2000 1X and CDMA2000 1x EV-DO. An overview of the network architecture is presented with detailed explanations of the role of each component and interface in the network and protocol testing to change the needs of the network. The article ends with a discussion of some of the technical issues that may arise in CDMA networks and some suggested solutions.
Detect and solve some common problems in CDMA2000 1X networks
All the features and capabilities of today's 3G mobile broadband are geared towards a complex system with multiple modes, nodes, elements, interfaces and protocols. Problems that may arise from hardware as well as software. As a mobile Internet connection becomes widespread, the challenge of maintaining continuous data interactions will require more potential new monitoring solutions and procedures. Now, let's examine some of the common problems that can occur in CDMA2000 1X networks.
Incidents during Mobile Data Packet Setup, Mobile IP Packet Data Initialization and Enrollment Setup
In order to achieve packet data services, mobile devices register with the wireless network on the A1 interface and then to the packet network on the A10 / A11 interface. The mobile sends an "Origination Message" to the BS containing the packet data service option. This leads to the allocation of traffic channels, the establishment of connection A10, the establishment of link layer (PPP) and also in case of mobile IP used by the terminal.
The user data flow can now be traversed by the A10 connection packaged within the GRE frames. The PCF registers periodically with the PDSN selected via the A11-Registration Request before the expiration of the A10 connection's expiration.
Figure 2: Setting up a CDMA2000 1X mobile data call
A successful call setup script is illustrated in Figure 2. The standard message sequence diagram outlines a series of steps that are summarized in items 1 through 12 below. Note that this explanation ignores BTS's BTS transmissions, instead focusing only on protocol functions that begin with "Origination dialogue" between the mobile device and the BSC.
1. To subscribe to the packet data service, the mobile device sends an "Origination Message" via the "Access Channel" to the BSS.
2. The BS acknowledges receipt of the "Origination Message" and returns a "Base Station Ack Order" to the mobile device.
3. The BS develops a "CM Service Request" message and sends this message to the MSC.
4. The MSC sends an "Assignment Request" message to the BSS requesting the allocation of radio resources. No terrestrial channel between MSC and BS is allocated to the packet data call.
5. BS and mobile device implement radio resource installation procedures. The PCF confirms that there is no A10 connection associated with this mobile device and selects a PDSN for that data call.
6. PCF sends a "A11-Registration Request" message to the selected PDSN.
7. "A11-Registration Request" is validated and PDSN accepts this connection by returning a "A11-Registration Reply" message. Both PDSN and PCF create a binding protocol for the A10 connection.
8. After the radio link and the A10 connection are installed, the BS sends an "Assignment Complete" message to the MSC.
9. The mobile device and the PDSN establish a link layer (PPP) connection and then perform the MIP (Mobile IP) registration procedure through that link layer connection.
10. After completing the MIP registration, the mobile device can send and receive data by framing the GRE via the A10 connection.
11. PCF periodically sends a "A11-Registration Request" message to register for the A10 connection.
12. For a valid "A11-Registration Request", the PDSN returns a "A11-Registration Reply" message. At this point, both PDSN and PCF update the A10 connection binding record.
This relatively complex process can be the source of a number of issues that affect service and quality. A rigorous monitoring plan, including simultaneous monitoring of the A1 and A10 / A11 interfaces, is the best way to detect and fix errors as soon as possible. Here a multi-interface call tracking application is particularly effective by detecting the path and grouping all the procedures involved in the operation of a single subscriber in a CDMA network, even when the gateway Continuous processing for multiple interfaces.
During the call setup process, each error in any element or procedure can prevent the remaining steps. For example, suppose the MSC does not respond to the CM Service Request (Step 3 in Figure 2) sent from the BSC / PCF via the A1 interface. That is sometimes due to intrinsic MSC issues. If it obstructs the completion of the "CM Service Request", BSC / PCF can not allocate radio resources to the mobile station and thus further prevent connection setup. Users can not find it to make a data call-a service for those who have paid premiums.
Before a specific timer expires, PCF periodically sends an "A11- Registration Request" message (Step 11) to refresh the register for the A10 connection. For an "A11-Registration Request" to take effect, the PDSN returns a "A11-Registration Reply" message (Step 12). Here again, internal problems in the PDSN may be the cause of it's later reply later or never. As a result, the process of establishing or maintaining a connection can not continue. Users again can not make a data call.
In both cases a protocol programmer connected to the A1 and A10 / A11 interfaces can assist in finding the problem. The call trace application can differentiate the origin of the messages and detect any failures to deal with. This would be easier to locate in individual MSCs and PDSNs in these examples. It is not effective to transmit user data packets.
Frequent in the CDMA2000 network, TCP packets have small window sizes. That means end-to-end TCP connections are unstable. The more TCP packets on the network are lost and unrecognized, the smaller the window size, the more TCP connections are "broken" and must be reset. The small TCP window size is due to the soft-start mechanism built into the TCP protocol.
The nature of the problem must be specified, which is necessary to obtain TCP / IP user level traffic flowing in the GRE tunnels on the A10 interface. By applying different types of protocol filters and with increasing levels of granularity, it is possible to isolate the point that causes the TCP packet window size to be shortened.
The routing loop of user packets in the core network
The tunnel router loop is another layer of CDMA2000 network problems that can degrade service quality for subscribers. The problem is caused by misconfiguration in the PDSN routers. It can be detected by capturing IP traffic on the P-H interface (see Figure 1 of Part 1).
To understand tunnel routing loops, imagine a web surfing (WWW) client with a laptop connected to a CDMA2000 mobile. Packets directed to a specific HTTP proxy are routed (after passing through the PCF) from the PDSN / FA to the HA (Home Agent) [2].
With some incorrect internal configurations, packets for port 80 WWW are not "de-tunnelled" by HA [2]. Instead, they are sent back to the PDSN / FA. As a result, many packets travel on the same network segment with the same packet ID, It wastes bandwidth and does not reach the desired destination. Also, for each repeated hop, a packet moves between the PDSN / FA and HA nodes [2], the "IP Time To Live" field decreases by one unit. If the packet is stuck in a router loop, the TTL eventually drops to "0" and the packet is removed from the network nodes. Packets that are "lost" should be retransmitted, resulting in an overhead of over-re-transmission and downsizing.
As in the previous example, the solution to use protocol filtering is to "capture" IP packets on the P-H interface. By browsing from the beginning to the end of the captured data by applying incremental filtering, it is possible to recognize packets periodically and solve this problem.
Duplicate IP traffic
PDSN configuration issues can generate many types of variations in tunnel loops. A common issue relates to logical IP addresses of PDSNs with more than one physical MAC address. If that happens, it means more than one hardware card has the same IP address. All traffic sent to this IP address will go to two different hardware entities and receive feedback from both. This results in doubling the total IP traffic associated with a single IP address on this segment. Again, protocol filtering capabilities are needed to handle the problem effectively. A protocol analyzer must "hold" IP packets moving to a specific IP destination address via the P-H interface. Browsing the entire data and using filters to narrow down the query, the nature of the problem (duplicated address, duplication) is soon evident.
Routing issues in the core network
Sometimes internal problems can cause offline PDSN routers (offline) and return online (online) after a period of time. That can happen frequently and continuously in the CDMA2000 core data network. When an "online" router, its routing table is not optimized. It takes time for the Open Short Path Path (OSPF) built-in algorithm to find the best route to route packets depending on which routers are available next. Until the routing table is optimized, there will be a decrease in service quality.
By "capturing" IP packets on the PH interface with a protocol analyzer and applying them to filters in OSPF routing messages, changes in the designated router and changes in the router next to a router can be easily identified. The use of intelligent and detailed filtering based on OSPF messages and information elements within messages to identify routing problems on an IP network becomes a viable task.
Epilogue
CDMA infrastructure is expanding and will definitely provide the foundation for widespread penetration of CDMA networks. CDMA2000 and other 3G technologies bring telecom packet switching capabilities, plus a wealth of new services and complexities in the implementation process.
Troubleshooting activities now require an understanding of both the traditional "telecom" concepts associated with circuit switching and the new "datacom" concepts associated with packet switching. Network operators and maintenance personnel must constantly improve their processes to cope with complex new troubleshooting challenges, from misconfigurations to duplicated IP addresses and multiple another one. Protocol analysis tools can play a greater role than ever before to maintain an effective network. Features such as multi-interface call and protocol filtering will become the norm for maintenance.
Nguyen Hoang Linh
Acronyms
Abis: Communication line from BTS to BSC
AMPS: Advanced Mobile Phone System
ANSI-41: American National Standard Institute
ATM: Asynchronous Transfer Mode
BSC: Base Station Controller
BSS: Base Station Sub-system
BSSAP: BSS Application Part
BTS: Base Transmission System
CDMA: Code-Division Multiple Access
CDMA: CDMA for 2G
DHCP: Dynamic Host Configuration Protocol
FA: Foreign Agent
FR: Frame Relay
FW: Fire Wall
GRE: Generic Routing Encapsulation
GSM: Global System for Mobile Communication
HA: Home Agent
HDLC: High-level Data Link Control
HLR: Home Location Register
IKE: Internet Key Exchange
Interoperability Specification Version 4.0, see IS2001
IP: Internet Protocol
IPv4: IP Version 4
IPv6: IP Version 6
IPsec: IP Security
IS2001: Interim Standard 2001, defines protocols for interfaces A1, A7, A9, A11 for CDMA.
IS-41e: Interim Standard 41, defines the protocols for D-Interface (D-Interface) for CDMA
IS-95 (Interim Standard 95): Defines the protocols for the U-Interface (U-Interface) for CDMA
MAC: Medium Access Control
MIP: Mobile IP
MS: Mobile Station
MSC: Mobile Switching Center
PCF: Packet Control Function
PDSN: Packet Data Serving Node
P-H: PDSN interface to Home Agent
PPP: Point-to-Point Protocol
PSTN: Public Switched Telephone Network
P.S0001: Specification for Wireless IP based protocols
RAN: Radio Access Network
R-P: RAN to PDSN
SDU: Signal Data Unit
TCP: Transmission Control Protocol
TIA: Telecommunications Industry Association
U: Air interface between MS and BTS
UDP: User Datagram Protocol
VLR: Visitor Location Register
1xRTT: 1x chip rate of 1.2288 Mcps for Radio Transmission Technology
3GPP2: 3rd Generation Partnership Project 2
Note
1 AAA-Server (Authentication, Authorization and Accounting server) is used to authenticate and authorize users to access the network and store usage statistics for billing and invoicing.
[2] HA (Home Agent): HA supports smooth data roaming into other networks that support 1xRTT. The HA provides an "anchor" IP address for the mobile device and forwards any "mobile-bound" traffic to the appropriate network for delivery to the handset. It also preserves the user registry, sends the packet again to the PDSN and can optionally pass through the tunnel securely to the PDSN. Finally, the HA supports dynamic user allocation from AAA and (optional again) assigns a "home" address.
[3] "hand-off": In a cellular telephone network, a hand-off is a transition for a given user of transmitting a signal from a base station to a next base station. Geographically, as the user moves. In an ideal cellular telephone network, each end-user telephone device or modem (subscriber hardware) is always within range of the base station. The area covered by each base station is defined as a "cell".
[4] 1xRTT1: A 1xRTT1 network provides a 1x chip speed of 1.2288 Mc / s for Radio Transmission Technology.
[5] "spread-spectrum" is a form of radio communication in which the frequency of the transmitted signal is deliberately altered. That leads to a larger signal bandwidth if its frequency does not change. Most "spread-spectrum" signals use a digital scheme called "frequency hopping." The frequency of the transmitter changes abruptly, many times per second. Between the "hop" (short jump) generator frequency is stable. The duration of the transmitter on a given frequency between the "hop" is the dwell time.
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