Sierra Wireless CDPD Primer 2130006 Rev 1.
Sierra Wireless, Inc. CDPD Primer Copyright ©2001 Sierra Wireless, Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior permission of the publisher. The information in this manual is subject to change without notice and does not represent a commitment on the part of Sierra Wireless, Inc. Sierra Wireless, Inc.
Sierra Wireless, Inc. CDPD Primer Contents 1. About this Guide............................................................................1 1.1. Introduction ........................................................................................................... 1 1.2. Document Structure .............................................................................................. 1 1.2.1. Format.................................................................................................... 1 1.
Sierra Wireless, Inc. CDPD Primer 3.2. Features of CDPD................................................................................................ 12 4. 3.2.1. Packet-Switched .................................................................................. 12 3.2.2. Based on Internet Protocols ................................................................. 12 3.2.3. Full Duplex .......................................................................................... 12 3.2.4.
Sierra Wireless, Inc. 5. CDPD Primer Operations: Making a CDPD Connection ...................................23 5.1. The Registration Process .................................................................................... 23 5.1.1. Network Entity Identifier (NEI) and Home Subdomain ...................... 23 5.1.2. Temporary Equipment Identifier (TEI) ............................................... 23 5.1.3. Equipment Identifier (EID)..............................................................
Sierra Wireless, Inc. CDPD Primer List of Figures Figure 2-1: Forward and reverse channels ........................................................................................3 Figure 2-2: A cellular telephone system............................................................................................4 Figure 2-3: Cellular channel reuse using three sectors per cell .........................................................5 Figure 2-4: Cell handoff in three-sector cells...........................
1. About this Guide 1.1. Introduction The Sierra Wireless CDPD Primer is an overview of Cellular Digital Packet Data (CDPD), a standard for data transmission over wireless cellular telephone networks, such as those using the analog AMPS system, and widely available in North America. Many Sierra Wireless products support CDPD (see section 6).
Sierra Wireless, Inc. 2. CDPD Primer Telephones and Wireless Data Transmission 2.1. Telecommunications and the Telephone Today’s wireless data communications standards, including CDPD and more recent varieties, evolved from technologies in different industries, including radio and data communications. CDPD’s most direct and well known ancestor is the traditional telephone system, which is where we begin our history. 2.1.1.
Sierra Wireless, Inc. CDPD Primer called handsets—can both receive signals from, and send signals to, the base station. All the cell sites for one cellular service provider connect into an automated central management system, and from there into the wider PSTN; so AMPS users can seamlessly call both traditional wireline phones and other mobile phones (whether on their network or not), and vice versa. AMPS has been remarkably successful.
Sierra Wireless, Inc. CDPD Primer A-side carriers use Band A of the AMPS spectrum. Their cellular phones transmit in the frequency ranges 824-835 MHz and 845-846.5 MHz, and they receive in the 869-880 MHz and 890-891.5 MHz ranges. These correspond to cellular channels numbered 1-333, 667-716, and 991-1023. B-side carriers use Band B, which transmits at 835-845 MHz and 846.5-849 MHz, and receives at 880-890 MHz and 891.5-894 MHz, corresponding to cellular channels 334-666 and 717-799.
Sierra Wireless, Inc. • CDPD Primer the subscriber moves so that a different cell site provides better coverage, when the MTSO de-assigns the radio channel from the old cell site and assigns a new radio channel from the new cell site—one which provides better signal quality. This scenario is known as a handoff (see section 2.2.7). The user might notice a handoff as a very brief (about 1/10 second) gap in a voice call during the switch to a new channel.
Sierra Wireless, Inc. CDPD Primer 2.2.7. Cell Handoff Using a typical seven-cell, three-sector channel reuse pattern, call handoff is straightforward, allowing a cellular handset to move through and between cells while smoothly continuing a call. In this example, the cellular subscriber makes (or receives) a call in sector A3. They move to sector A1 and experience a handoff at location 1 where they are handed off to a new radio channel in sector A1.
Sierra Wireless, Inc. CDPD Primer switched—using up the entire bandwidth of a cellular channel for the audible tones used to modulate the data transmission. With the growing popularity of the Internet in the 1990s, circuit-switched became even less appropriate for most data transmissions. Like many computer data transmissions, Internet connections are “bursty”, with short flows of information interspersed with long idle periods.
Sierra Wireless, Inc. 3. CDPD Primer Background: Introduction to CDPD 3.1. Cellular Digital Packet Data (CDPD) In 1991 the U.S. cellular operators began a process to offer packet data technology for services such as e-mail and telemetry. The result was Cellular Digital Packet Data (CDPD), which the carriers began to deploy in 1993. Today, regions of CDPD coverage include most of North America’s population. CDPD is an open specification.
Sierra Wireless, Inc. CDPD Primer Channel Usage AMPS Voice Unused Channel 1 Channel 2 Channel 3 Unused capacity 3 2 1 0 Time Figure 3-1: Cellular radio channel usage within a single cell sector In this example, the unused channel capacity ranges between 0 and 3 radio channels. The CDPD concept is based on sending packet-switched data on radio channels within a sector when they are not used for cellular voice communications.
Sierra Wireless, Inc. CDPD Primer Planned channel hops occur at a time specified by the CDPD network. For example, the CDPD network management function may configure the CDPD base station to use only a specific channel for a fixed period of time and then hop in a round-robin fashion to another of the radio channels used within a specific sector.
Sierra Wireless, Inc. CDPD Primer 3.1.4. Base Station Broadcast Parameters CDPD base stations regularly broadcast information (known as cell configuration messages) to all CDPD subscriber devices so that they know all channels available both within the cell and in neighboring cells. This information is then used by the CDPD subscriber device to find new CDPD channels in the event of a planned handoff or forced channel hop.
Sierra Wireless, Inc. CDPD Primer The CDPD infrastructure provides a buffer so that, to the wider Internet, packets destined for a CDPD device can be routed as normal, through a fixed series of addresses. The CDPD system captures these packets and then routes them to the CDPD device according to information the CDPD network maintains about the modem’s current location. From the user’s point of view, only the destination address is known, since the CDPD network manages the packet routing (see section 5.2.
Sierra Wireless, Inc. CDPD Primer 3.2.4. Transmission Rate and Peak Throughput CDPD offers raw transmission rates of 19.2 kbps. Error control overhead means that actual throughput of useful data can be up to 12 kbps on a clean, lightly-loaded channel. Data transfer rates may be lower on congested networks with many voice or CDPD transmissions underway. Data compression can, of course, increase the effective throughput.
Sierra Wireless, Inc. CDPD Primer The signal could be weaker than desired in locations such as: • Underground parking garages • Tunnels • Buildings with all metal construction • Old concrete buildings with many steel reinforcing bars In these cases, the CDPD subscriber may be able to get a stronger signal by locating the antenna near an opening or window.
Sierra Wireless, Inc. CDPD Primer 3.2.9. CDPD Data Transmission Format CDPD data is sent in Reed-Solomon blocks of 378 bits, each consisting of 63 six-bit symbols. Of the 63 symbols, 47 carry user data or protocol, while the remaining 16 symbols provide parity. Since the airlink is inherently vulnerable to interference and noise, the high number of parity bits enables the protocol to recover blocks in which up to 7 of the 63 symbols have been damaged.
Sierra Wireless, Inc. 4. CDPD Primer Infrastructure: CDPD Network Architecture 4.1. Physical: Components of the CDPD Network As mentioned in section 3.2.6, a cellular carrier can construct a CDPD network with a few new components added to the existing AMPS cellular phone infrastructure. Such a process costs less than creating a completely separate wireless data infrastructure.
Sierra Wireless, Inc. CDPD Primer 4.1.2. Mobile Data Base Station (MDBS) The Mobile Data Base Station (MDBS), together with the Mobile Data Intermediate System, connects the traditionally non-mobile protocols of the Internet to mobile CDPD subscriber M-ESs. The MDBS provides a relay function between the MD-IS and the M-ESs.
Sierra Wireless, Inc. CDPD Primer 4.2. Services: CDPD Network Services When a cellular carrier constructs a CDPD network, it provides a number of services to provide security, enable easy connection to the Internet, permit roaming, track usage, maintain accounting information, and prevent unauthorized access. 4.2.1. Domain Name Server As on the Internet as a whole, the Domain Name Server (DNS) system translates human-readable host names into numerical IP addresses in the network.
Sierra Wireless, Inc. CDPD Primer 4.2.4. Network Management Services Network Management Services are administrative services for the network provider itself, and do not involve the subscriber. They include: • Configuration Management of the various components of the CDPD network, which includes collecting data from and sending data to them, as well as controlling them remotely. • Fault Management to detect, isolate, and correct, abnormal operations in any portion of the CDPD network.
Sierra Wireless, Inc. CDPD Primer Figure 4-1: ISO layered communications architecture 4.3.1. The Application Layer (Layer 7) The top layer of the stack includes interaction with the end user. It allows for protocols and services required by a particular user-designed or commercially written application. Particular user requirements and application services that can be used by more than one application are contained in this layer. 4.3.2.
Sierra Wireless, Inc. CDPD Primer 4.3.6. The Data Link Layer (Layer 2) The data link layer provides communication between two or more connected systems. It performs frame formatting, error checking, addressing, and other functions to provide accurate data transmission between systems. It also governs access of users to the communication medium, such as the radio channel. 4.3.7.
Sierra Wireless, Inc. CDPD Primer Figure 4-2: Example of a CDPD virtual terminal subprofile 2130006 Rev 1.
Sierra Wireless, Inc. 5. CDPD Primer Operations: Making a CDPD Connection Understanding a CDPD network requires more than knowing how it is physically and logically configured. The actual process by which a mobile end system (M-ES), such as a CDPD modem plugged into a notebook, connects to the network and moves data through it is particularly important, and requires the M-ES to have a verified address on the system. 5.1.
Sierra Wireless, Inc. CDPD Primer 5.1.3. Equipment Identifier (EID) Each M-ES device has an Equipment Identifier (EID), which, unlike the changeable NEI, is a fixed number completely unique to that M-ES. The EID is a 48-bit number based on the IEEE Organizationally Unique Identifier (OUI). The EID represents a universal address that is unique to a subscriber unit such as a modem or cellular handset.
Sierra Wireless, Inc. CDPD Primer update the Subscriber Directory Profile (see section 5.1.3) so that the old NEI can be re-used with the new device. 5.1.5. Service Provider Network Identifier (SPNI) Just as each M-ES has its unique numbers, including the NEI and EID, each CDPD carrier has a number identifying it to M-ESs connecting to its network equipment. That 16-bit number is known as the Service Provider Network Identifier (SPNI).
Sierra Wireless, Inc. CDPD Primer In Figure 5-1, Notebook B sends a transmission to Notebook A. The packet takes these steps: 1. From ME-S (B) to the local MDBS. 2. MDBS to the controlling MD-IS (A) where the destination user is determined to be at home. 3. MD-IS (A) back to the local MDBS. 4. MDBS to ME-S (A); the destination.
Sierra Wireless, Inc. CDPD Primer 1 2 3 7 8 M-ES (A) IS (Router) MDBS MD-IS (A) 4 6 Internet M-ES (B) MD-IS (B) 5 Figure 5-2: CDPD mobility management scenario 2 The difference between scenarios 1 and 2 is transparent to the notebook users, except for the delays introduced by the longer transmission in scenario 2. If either notebook were communicating with a fixed system on the Internet, the paths would be similar.
Sierra Wireless, Inc. CDPD Primer When receiving data on the forward (receive) channel from the MDBS, it must: • Receive packet data from the network • Request retransmission of packets depending on error correction performance • Decrypt the packets • Disassemble the packets to extract the application data • Pass the serial data to the end-user’s application equipment 5.3.
Sierra Wireless, Inc. CDPD Primer MAC also measures and maintains statistics about: • The number of frames, blocks, and bursts transmitted, and how many succeeded • How many frames had to be rescheduled and why 5.5.1. Details of MAC Transmission Access Management The MAC protocol enables the CDPD carrier to set a limit on how many times an M-ES can attempt to transmit the same data block, to prevent an overflow of stale data that a modem is unable to send.
Sierra Wireless, Inc. CDPD Primer 5.6.1. The Radio Resource Management Entity (RRME) The forward and reverse channels of a CDPD data conversation, like AMPS voice channels, are different radio frequencies, widely separated in the assigned spectrum. The M-ES selects these channels according to a list made regularly available by the local MDBS, but the overall selection process is managed by an entity in the CDPD network known as the Radio Resource Management Entity (RRME).
Sierra Wireless, Inc. CDPD Primer In most urban areas, the maximum power level allowed is 2 (600 mW) because of the high density of cells. However, the carrier may set a lower value (a higher level number), believing that CDPD traffic can cause noise problems on the voice channels. There is little evidence that this actually happens, but voice-conscious carriers may arbitrarily set the maximum to a level from 3 to 5. This may cause serious problems for CDPD operation.
Sierra Wireless, Inc. 6. CDPD Primer Sierra Wireless Products and CDPD Sierra Wireless produces a number of devices and applications that work with CDPD, as well as other wireless data technologies. For more information about any of the following items, visit the Sierra Wireless Web site at www.sierrawireless.com. 6.1.
Sierra Wireless, Inc. CDPD Primer 6.5. Original Equipment Manufacturer (OEM) CDPD Devices The SB300 Type III-size CDPD modem is a cost-effective wireless data modem that can be integrated into original equipment manufacturer (OEM) devices. The more fully-featured SB320 also offers circuit-switched cellular (CSC) support and wireline connections for data and voice. 6.6.
Sierra Wireless, Inc. 7. CDPD Primer Additional Resources 7.1. Books Cellular Digital Packet Data System Specification Release 1.1, January 19, 1995 CDPD Forum Inc./Wireless Data Forum Computer Networks Andrew S. Tannenbaum Prentice Hall Inc., 1981 ISBN 0-13-165183-8 Data and Computer Communications William Stallings MacMillan Publishing Inc.
Sierra Wireless, Inc. CDPD Primer 7.2.1. CDPD Coverage and Carriers Wireless Data Forum’s CDPD Coverage Maps. www.wirelessdata.org/maps CDPD details for major U.S. cellular carriers: • AT&T Wireless Services – Wireless IP www.attws.com/general/explore/wireless_ip/downloads/wip_mail_whitepaper.pdf • Cingular Wireless www.cingular.com/cingular/products_services/wireless_data • Verizon Wireless – AirBridge. http://www.bam.
