CDMA network technologies: A decade of development and challenges - Part 2

The article briefly describes 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 structure is presented with detailed explanations of the role of each component and interface in the network and protocol testing to change according to the needs of the network. The article will conclude with a discussion of some technical issues that may appear in CDMA networks and some proposed solutions.

Discover and solve some common problems in CDMA2000 1X networks

All the features and capabilities of modern 3G mobile networks are geared towards a complex system with multiple modes, network nodes, elements, interfaces and protocols. Problems can arise from hardware and software. When a mobile Internet connection becomes popular, the challenge of maintaining continuous data interactions will require new potential solutions and monitoring procedures. Now, we will examine some common problems that may appear in CDMA2000 1X networks.

Problem while Mobile Packet Data Call Settings, Mobile Initialization and Registration

To achieve packet data services, the mobile device performs registration with the wireless network on interface A1 and then with the packet network on the interface A10 / A11. The mobile device sends an 'Origination Message' to the BS containing the packet data service option. This results in traffic channel allocation, A10 connection establishment, link layer setup (PPP) and even in the case of mobile IP used by the terminal.

User data traffic can now go through the A10 connection encapsulated within GRE frames. The PCF periodically registers with the selected PDSN by sending an A11-Registration Request A11 notification before the expiration of the A10 connection expires.
CDMA network technologies: A decade of development and challenges - Part 2 Picture 1
Figure 2: Set 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 sections 1 through 12 below. Note that this explanation ignores BTS radio transmission / reception activities, instead concentrating only on the protocol functions that begin with the Origination dialogue between the mobile device and the BSC.

1. To register packet data service, the mobile device sends an 'Origination Message' via the 'Access Channel' to the BSS.
2. The confirmation doctor received the 'Origination Message' above and returned a 'Base Station Ack Order' to the mobile device.
3. The BS builds 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 (terrestrial) channel between MSC and BS is allocated for packet data call.
5. BS and mobile device perform 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. The PCF sends an 'A11-Registration Request' message (requires registration A11) to the selected PDSN.
7. 'A11-Registration Request' is validated and PDSN accepts this connection by returning a 'A11-Registration Reply' message (accepting the registration). Both PDSN and PCF create a binding record for the A10 connection.
8. After the radio link and A10 connection are installed, the BS sends a 'Assignment Complete' message to the MSC.
9. Mobile device and PDSN establish link layer (PPP) connection and then perform MIP (Mobile IP) registration procedures 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 an 'A11-Registration Request' message to register for the A10 connection.
12. For a valid 'A11-Registration Request', PDSN returns a 'A11- Registration Reply' message. And now both PDSN and PCF update the A10 connection binding record.

This relatively complex process may be the source of some problems affecting service and quality. A rigorous monitoring plan includes simultaneous monitoring of A1 and A10 / A11 interfaces as the best way to detect and correct errors as soon as possible. Here a multi-interface call trace application is particularly effective by detecting the path and group of all procedures related to the operation of each single subscriber in a CDMA network, even if the Continuous processing for multiple interfaces.

Within the call setup process, each error in any element or procedural step can prevent the remaining steps. For example, suppose that MSC does not respond to the 'CM Service Request' service request (Step 3 in Figure 2) sent from BSC / PCF via A1 interface. That is sometimes due to internal MSC issues. If it interferes with the completion of the 'CM Service Request', the BSC / PCF cannot allocate radio resources to the mobile station and thus prevent the connection establishment. Users cannot find it to make a data call — a service for those who have paid premiums.

Before a specific timer expires, PCF periodically sends a 'A11- Registration Request' message (Step 11) to refresh the registration for the A10 connection. In order for an 'A11- Registration Request' to take effect, PDSN returns an 'A11-Registration Reply' message (Step 12). Here again, internal problems in PDSN may be the cause of a later reply or never. As a result, the process of establishing or maintaining a connection cannot continue. The user again cannot make a data call.

In both cases a protocol programmer connected to the A1 and A10 / A11 interfaces can assist to find the problem. The call tracker application can distinguish the origin of notifications and detect any failure to respond. This will be easier to locate each MSC and PDSN separately in these examples. Not effective when transferring user data packets.

Frequently in CDMA2000 network, TCP packets have small window sizes. That means end-to-end TCP connections are not stable. The more TCP packets on the network are lost and unconfirmed, the smaller the window size, resulting in more TCP connections being 'broken' and must be reset. The small TCP window size is due to the soft start mechanism (soft-start) built into the TCP protocol.

The characteristics of the problem must be specified, which is necessary to obtain TCP / IP user-level packets circulating in the GRE tunnels on the A10 interface. With the application of different types of protocol filters and with increasing levels of detail, it is possible to isolate points that cause a shortened TCP packet window size.

Routing loops of user packets in the core network

Tunnel routing loops are another class of CDMA2000 network problems that can reduce service quality for subscribers. The problem is caused by the misconfiguration in the PDSN routers. It can be detected by acquiring IP traffic on the PH interface (see Figure 1 of part 1).

To understand tunnel routing loops, imagine a subscriber surfing the Web (WWW) with a laptop connected to a CDMA2000 mobile device. Packets directed to a specific HTTP proxy are routed (after passing PCF) from PDSN / FA to HA (Home Agent) [2].

With some incorrect internal configuration, packets for port 80 WWW are not 'de-tuned' by HA [2]. Instead, they are sent back to PDSN / FA. As a result, many packets move on the same network segment with the same packet ID, wasting bandwidth and not reaching the desired destination. In addition, for each repeated hop, a packet travels between PDSN / FA and HA nodes [2], the 'IP Time To Live' field (IP TTL) 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. The lost packets must be played back, resulting in excessive cost of retransmitting the packet and reducing throughput.

As in the previous example, the solution to using protocol filtering has the effect of 'capturing' IP packets on the PH interface. With the end-to-end browsing of data captured by applying incremental increments of filtering, it is able to recognize packets periodically and solve this problem.

Duplication of IP traffic

PDSN configuration problems can generate a variety of types in tunnel loops. A common problem involves the logical IP addresses of PDSN with more than one physical MAC address. If that happens, there is more than one hardware card with the same IP address. All traffic sent to this IP address enters two different hardware entities and receives feedback from both. This results in doubling the total IP traffic associated with a single IP address on this segment. Once again, protocol filtering capabilities are needed to effectively troubleshoot. A protocol analyzer must 'hold' IP packets moving to a specific IP destination address via the PH interface. Browsing all the data and using filtering to narrow down the query (question), the nature of the problem (duplicated, duplicated) is soon obvious.

Routing issues in the core network

Sometimes internal intrinsic problems can cause PDSN routers 'offline' and go back to 'online' after a period of time. This can happen frequently and continuously in the CDMA2000 core data network. When a router is 'online', its routing table is not optimized. It takes time for the built-in OSPF (Open Shortest Path First) routing algorithm to find the best path to route packets depending on the routers that are ready next to them. 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 it to filters in OSPF routing messages, changes in the designated router and changes In routers adjacent to a router can be identified easily. The use of smart and detailed filtering capabilities based on OSPF messages and information elements within messages to identify routing problems on an IP network become a viable task.

Epilogue

CDMA infrastructure is being expanded and certainly creates the foundation for the wide penetration of CDMA networks. CDMA2000 and other 3G technologies provide telecommunications with packet switching capabilities, plus a host of new services and a lot of complexity during implementation.

Troubleshooting activities now require an understanding of both traditional 'telecom' concepts related to circuit switching and new 'datacom' concepts related to packet switching. Network operators and maintenance personnel must always improve their processes to deal with complex new troubleshooting challenges, from misconfiguration problems to duplicated and problematic IP addresses. another topic. Protocol analysis tools can play a bigger role than ever before to maintain an effective network. Features such as multi-interface call tracking and protocol filtering will become the standard for maintenance work.

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: Sub-system Base Station
BSSAP: BSS Application Part
BTS: Base Transmission System
CDMA: Code-Division Multiple Access
CdmaOne: 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
IOS4.0: Inter-operability 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 D-Interface protocols for CDMA
IS-95 (Interim Standard 95): Defines U-Interface protocols 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
PH: Interface PDSN to the Home Agent
PPP: Point-to-Point Protocol
PSTN: Public Switched Telephone Network
P.S0001: Specification for Wireless IP based protocols
RAN: Radio Access Network
RP: 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 empower users to access the network and store usage statistics of subscribers for billing and billing.

[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 to be transferred to the handheld transceiver. It also preserves user registration, sends again packets to PDSN and can optionally through the 'tunnels' safely to PDSN. Finally, HA supports user distribution from AAA and (optionally again) assigning a 'home' address.

[3] 'hand-off': In a cellular telephone network, hand-off is a transition for a given user of signal transmission from a base station (BS) to the next base station. by geography when users move. In an ideal cellular telephone network, each end-user or modem device (hardware of the subscriber) is always within the scope of the base station. Area covered by each base station is defined as a 'cell'.

[4] 1xRTT1: A 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 intentionally changed. That leads to a bandwidth greater than the signal if its frequency does not change. Most spread-spectrum signals use a digital scheme called 'frequency hopping'. The transmitter frequency changes abruptly, many times per second. Between the 'hop' (short jump) the transmitter frequency is stable. The length of time the transmitter remains on a given frequency between the "hop" is the stop time (dwell).