Advanced Packet Data Testing with Linux
What is your 56K modem doing while you ponder the sophisticated on-line articles at Linux Journal's web site? Nothing. The majority of the time you are connected to the Internet, your modem sits idle. However, your phone call ties up at least one dedicated circuit in a telephone network. The unused capacity and potential revenue of this circuit's bandwidth is lost forever.
Telephone company operators and manufacturers recognize that forming data paths by switching fixed-capacity circuits is inefficient. Interactive data communication is transmitted in bursts. You pull down a web page and whimsically move on to the next, unconcerned about the circuit bandwidth you tie up.
The same problem exists in cellular systems today. Most of the advanced second-generation digital cellular systems reserve a channel intended for voice transmission when making data connections. This scarce and wasted bandwidth, bought or leased from the governments of the world, is a very expensive commodity to lose while customers read web pages.
If you only knew what was happening beneath that sleek, stylish mobile phone you carry in your pocket. You would be amazed at what it takes to deliver a simple dial tone—not to mention conferencing your uncle in on a forwarded call from your grandmother while crossing the border from Italy into France, all the while downloading the latest Linux kernel.
If you are traveling from Italy to France, it is likely that your mobile phone is a Global System for Mobile communications (GSM) cellular telephone. GSM is a worldwide second-generation digital cellular standard. GSM offers many voice and short message services such as calling line ID, call barring, call forwarding and advice of charge. As shown in Figure 1, GSM's radio frequency (RF) interface is comprised of 200KHz bandwidth carriers divided into eight time slots. A time slot is one channel which can carry digitally compressed voice at 13Kbps. On a data call, this same time slot can carry up to 14.4Kbps of circuit-switched data.
The GSM standards body recognized the inefficiency inherent in circuit-switched data and added a service called General Packet Radio Service (GPRS) to the GSM standards. GPRS is a packet overlay of the GSM network. Packet-switched technology efficiency greatly exceeds circuit-switched technology efficiency for carrying and delivering data.
In a packet-switched cellular network, the mobile telephone shares the RF packet channels with the other mobiles in range of a cell site. Rather than reserving a specific time slot or channel on one of the base station's carriers for the entire session, the GPRS mobile uses one or more packet channels to transmit or receive data in bursts. Once the mobile finishes with that bandwidth, it releases the packet channels, making them available for other mobiles. A GPRS mobile reaches a peak data throughput of 120Kbps while a circuit-switched GSM mobile has a maximum throughput of 14.4Kbps for a data connection.
Nortel Networks is committed to providing integrated voice and data solutions with the highest level of quality. As we introduce new technologies such as GPRS, the challenges for a GPRS developer begin. How does a developer verify that the interface to the protocol layer he just completed works correctly? How does the developer complete an end-to-end call when there are no mobiles yet in existence that use the protocol? The authors of this article develop test systems that allow the GSM GPRS developers to verify that our GPRS products are performing at a high level of quality and dependability.
A GPRS system consists of several network nodes including mobiles, base station system, serving GPRS service node (SGSN) and gateway GPRS service node (GGSN). These nodes are shown in Figure 2. When the mobile activates an Internet protocol (IP) packet data session, the base station notifies its SGSN. The SGSN activates the session, called a packet data protocol (PDP) context, with the GGSN. If necessary, the GGSN assigns a dynamic IP address to the mobile's context. When the GGSN acknowledges successful context creation, an IP tunnel using the GPRS tunneling protocol (GTP) is created between the SGSN and GGSN. The tunnel ID (TID) is tied to that mobile's context at both the GGSN and the SGSN.
Tracing mobile-originated IP packets through the GPRS nodes (overlooking the RF access details) illuminates some of the testing challenges. When the mobile sends IP data, the packets are compressed and segmented by the subnetwork dependent convergence protocol (SNDCP) and sent down through the logical link control (LLC) layer traversing the stack shown in Figure 3. The mobile's LLC layer ensures the packets arrive in sequence and resends any packets not acknowledged by the SGSN. The LLC layer sends the packets through the radio-specific portion of the RF stack, over the RF link to the base station and the SGSN. At the SGSN, the peer LLC layer acknowledges the received packets, and the SNDCP compression and segmentation are undone.
As shown in Figure 3, the resulting IP packet is enveloped in a GTP UDP/IP message with a header containing the mobile's context TID. The SGSN sends the GTP packet to the GGSN's GTP port. The GGSN extracts the IP packet and places it on the Gi interface. An example of a Gi interface for IP is the Internet. For mobile-bound packets, a similar procedure is repeated in the opposite direction.
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