The successful operation of any communications network requires cooperating system components. These network components or entities perform predefined and a priori agreed upon functions. The components must also communicate with each other in a predefined manner.
The component entities defined by the CDPD architecture (see Figure 3.6) include several that are unique to CDPD and others which are standard "off the shelf" components Since the CDPD network is an overlay on the existing cellular network, it only made sense to leverage off the existing infrastructure. The CDPD network model defines component elements reflecting the cellular network, and is illustrated in Figure 3.6.
In a typical cellular telephone network, cell sites are deployed throughout the coverage area. At each cell site, a base station transmits and receives RF signals from the cellular telephones through an antenna tower. The demodulated signals are then digitized and placed on voice communications channels of a multiplexing channel. These are typically 1.544 Mbps T1 connections which can carry up to 24 voice calls, each one occupying a 56 kbps digital channel. The voice circuits terminate at a mobile switching center (MSC), which interfaces with the land-line telephone network.
The design of the CDPD system is aimed at minimizing the changes in network operation and maximizing the reuse of the existing cellular network infrastructure. Given this constraint, the next specification team objective was using the system's resources effectively. Without a doubt, the most precious resource is the radio spectrum. Beyond the radio resource, the precious system resources to consider are the physical plant and the communications links.
Many people may not immediately realize that the collection of cell sites constitute a significant investment by cellular service providers.3.15 Many cell sites have to be constructed while others involve ongoing leasehold arrangements. In addition, the antenna tower space is also tied with space, height, zoning and licensing issues. Furthermore, each base station typically has a T-1 circuit dedicated to carrying its traffic.
The CDPD system architecture reuses these components by specifying a piece of additional infrastructure equipment at the existing cell sites. This new piece of equipment, named the Mobile Data Base Station (MDBS), can use the existing antenna feeds and towers. Furthermore, the functionality of the box has been limited to allow small compact designs that can fit within most existing cell sites. Recent designs can be deployed in microcell environments.
The MDBSs handle the RF communications to and from the mobile devices and relay the data to and from more centralized CDPD network infrastructure equipment. This communication path can reuse channels on the existing cellular communications links. In most cell sites, there are more than enough channels to justify the deployment of a T1 connection. However, there are typically a few unused channels in these T1 links. The MDBS can use the idle channels for CDPD data. Once all the CDPD data channels are brought back to the MSC site, they can be "groomed" off to the CDPD central infrastructure equipment, the Mobile Data Intermediate System (MD-IS).
With this design approach, the CDPD specification team defined the conceptual reference model depicted in Figure 3.7 . In the following sections, each of the identified components are described.
In OSI terminology, an End System or ES is a network node which is the ultimate source and destination of NPDUs. This is the same as a host in Internet terminology. The Mobile End System or M-ES is any network host which happens to be mobile (i.e., CDPD radio and software-equipped). Example M-ESs could include telemetry devices, personal communicators and personal computers. Even communicators in vending machines (which don't actually move) could be considered to be M-ESs.
The M-ES is a network device with protocols specified up to and including Layer 3. The M-ES has a Layer 3 address which is globally unique in both the CDPD and conventional networking environments. M-ESs are true mobile hosts and CDPD networks are extensions of IP-based (and CLNP-based) networks, such as the Internet. There is no need for gateways as in other wireless packet data services.
Applications on the M-ES access the network via conventional means-sockets, TLI, NDIS and ODI are a few example APIs. Standard APIs and protocols are emphasized, especially at the M-ES; this allows immediate portation of applications from existing PCs to CDPD.
Both full and half duplex M-ESs are supported by CDPD. This allows low-cost devices with only a single radio to provide mobile data services to low traffic generating applications, such as the previously-mentioned vending machines.
The M-ES architecture consists of three distinct functional blocks, as illustrated in Figure 3.8 : the subscriber unit, the subscriber identity module and the mobile application subsystem.
The Subscriber Unit (SU) constitutes the portion of the device that establishes and maintains data communications with the CDPD network infrastructure. It achieves this through proper execution of the CDPD airlink protocols, which extend from the physical layer to the network layer. Also included are administrative layers above the network layer and the layer management entities necessary to ensure proper cooperation between the M-ES and the network.
The Subscriber Identity Module (SIM) is the repository of identity and authentication credentials for the network address in use at the M-ES. Every subscriber device must have the proper authentication credentials and identity. This function was separated from the rest of the M-ES functions to enable implementation of removable SIM cards as in GSM. This is in consideration for users that may find it easier to carry a small smart card with all the necessary identity information than to carry a complete Mobile End System3.16 . The specification used to define the SIM is based on the appropriate GSM standards.
The Mobile Application Subsystem (MAS) is the portion of the M-ES that contain all protocols at the network layer and above. This is what gives the M-ES something interesting to communicate with. The application is whatever needs mobile network connectivity-Email, remote database access, vending machines, etc.
M-ESs span a wide range of feature sets and form factors. Some of the units currently available support the CDPD protocols only, while others also provide AMPS cellular modem capability. Still others support voice communications with the addition of a handset.
Along with varying form factors, M-ESs also come with different power source options. Some require large external power sources such as would be available in a vehicle. Others supply their own power through internal batteries. Still others draw on the power source of a laptop or notebook computer.
The Mobile Data Base Station or MDBS is the system end of the MAC sublayer over the airlink. The MDBS arbitrates activities on the channels it hosts at the MAC sublayer much like an Ethernet hub. It relays frames at the LLC sublayer. This device is physically located at cell sites.
The MDBS is the network infrastructure device that bridges the different media between the Mobile End System and the CDPD network. The MDBS communicates with the M-ESs through the airlink physical interface. It performs all the necessary modulation of the data bits onto the RF channel. It also demodulates the RF signal into digital bits of data. These operations are carried out within the specifications and rules required to operate on the cellular frequency bands.
In a CDPD system, multiple mobile devices share the use of a single radio channel. To ensure proper sharing of the RF channel, a medium access control mechanism is used to arbitrate access to that channel. An MDBS is an active participant in the CDPD medium access control scheme, Digital Sense Multiple Access (DSMA). Once the data stream is successfully received and decoded by the MDBS, it relays the Link Layer frames between the M-ES and the MD-IS.
The MDBS is Layer 3-addressable for network management and radio resource management purposes. In terms of user data, the MDBS is little more than a Layer 2 relay between the RF and the conventional networking worlds. For user traffic at Layer 3, the MDBS is a "phantom" element.3.17
The Mobile Data Intermediate System or MD-IS is the focal point of CDPD mobility management. It has two functions in its role as a mobility-enabling router-the mobile serving function and the mobile home function.
The mobile serving function or MSF of the MD-IS provides the system end-point of the LLC sublayer MDLP link, opposite the mobile. This connection-oriented link serves as the foundation for the registration of mobiles to the system. When a mobile announces itself to the system, it is the mobile serving function which receives this registration request.
The mobile home function or MHF provides the anchor for the mobility of the M-ES. Whenever some network entity sends packets destined to the M-ES, the packets are routed in the conventional manner to the mobile home function, which then forwards them to the mobile serving function (which could be located in another MD-IS), based on previously exchanged messages between the mobile serving and mobile home functions.
The MD-IS is the most important data networking entity in the CDPD system. These devices are responsible for most of the mobility management functions of the network. The MD-ISs perform the functions necessary to track the local access point of the mobile devices. In other words, the MD-IS deals with the determination of and tracking of the exact radio coverage cell each M-ES is operating from.
The MD-IS is responsible for presenting an interface to the external networks on behalf of all the M-ESs in the CDPD network. This interface is necessary to ensure that hosts wishing to communicate to the any M-ES can traverse the external networks and enter the CDPD network at the proper point of presence.
The MD-IS is also responsible for routing all network traffic to the appropriate M-ES destination. The MD-ISs within a CDPD network must cooperate to ensure this task is achieved irrespective of whether the M-ES is in a local area, or if it is at the far end of the continent.
Finally, since the CDPD network is a commercial public data network, the routing nodes-the MD-ISs-must also perform the necessary administrative functions such as usage accounting.
The MDBS/MD-IS Interface is an internal interface between the MDBS and the MD-IS. Since this interface is internal to the CDPD "cloud," it is recommended rather than specified. Proprietary solutions have no need to adhere to this interface definition. However (CDPD service providers beware!), allowing a proprietary solution here is tantamount to losing control of one's system architecture.
This interface is specified for the sole purpose of an open system definition. This allows a multiplicity of vendors for each side of the interface in a more or less plug-'n-play manner. There was much discussion amongst members of the CDPD specification team regarding the need for any specification of this interface. In the end, this interface definition met the needs of the CDPD service providers wishing to order equipment from their vendors.
The Intermediate System or IS is a fancy name for a network router3.18 . It is standard OSI terminology and function. In CDPD, ISs handle packet forwarding for both IP and CLNP connectionless Layer 3 protocols, just as in conventional data networks. Typically, a packet traversing between networks will be handled by several ISs on its journey. MD-ISs must also support IS functionality.
In addition to the packet forwarding protocols-IP and CLNP-supported by the ISs, they must also support routing information exchange protocols (in order to function as routers, unless static routing only is used-a bad choice for dynamically changing networks!). The internal routing information exchange protocols defined by the CDPD specification include OSPF for IP and IS-IS for CLNP. External gateway routing information exchange protocols include BGP-43.19 for IP and IDRP for CLNP. It is wise, whenever possible, to have ISs supporting integrated IS-IS to prevent the "ships in the night" phenomenon, as described in [PERL92].
The CDPD network is a multi-protocol network. This means that the CDPD network is intended to support routing of multiple network layer protocols, including IP and CLNP. The architecture of CDPD allows future extensions for additional connectionless network protocols, such as IPv6. All ISs used in the CDPD network must also support multiple network layer protocols.
For the purpose of providing interconnection between a CDPD network and external networks, and between two CDPD networks, some level of security protection is valuable. For this requirement, the ISs at the borders of the CDPD network are further required to provide security filtering and access control functions, whose definition is beyond the scope of the CDPD System Specification.
The Fixed End System 3.20 or F-ES is a conventional network node, which could be either external to the CDPD service provider network, such as a transport layer peer of an M-ES, or internal, such as the servers which provide network support or application services. The only purpose in explicitly defining an F-ES is to distinguish between a conventional network host and a mobile host.
External F-ESs are the hosts that most CDPD subscribers should be familiar with. They are typically the systems hosting the applications the mobile user wishes to access. Some examples of these F-ESs might include: an inventory system at a home office, an electronic mail server for a road warrior and a user's favorite World Wide Web site. The most significant thread through these F-ES descriptions is that they are simply common hosts that support standard network layer protocols.
Internal F-ESs are hosts much like external F-ESs. The primary difference is that internal F-ESs operate within the boundaries of the CDPD network. As such, they conceivably are under the control of the service provider and can thus be presented with additional internal network data. Such data may include usage accounting information, mobile location information, subscriber authentication information, etc.
Internal F-ESs allow CDPD service providers to operate administrative servers and value added servers without the need for non-standard communications protocols. This capability is representative of the CDPD network architecture's flexibility. This flexibility results from the use of standard network protocols and adherence to the ISO Open System Interconnect reference model.
The Accounting Server or AS is an internal F-ES which serves two basic functions-collection and distribution of usage accounting data. Subscriber activity is recorded at the serving MD-IS over a configurable time period, then flushed to the AS. The AS then collates the detailed accounting records and distributes them to the appropriate peer accounting servers.
The distribution of detailed accounting records to home accounting servers is called the serving accounting distributor or SAD function. The reception and redistribution of these records at the home accounting server is called the home accounting distributor or HAD function. Other AS functions are defined in CDPD to enable near real-time accounting, separate accounting for large customers and separation of accounting (business) from network (operation) customer relationships.
The usage accounting capability is described in Chapter 7 and defined in Part 630 of the CDPD System Specification and Part 1023 of the CDPD Implementor Guidelines.
The authentication server is an internal F-ES which is intended to support the authentication function in CDPD. Since this function can be implemented in many different ways, it is not required to be a separately addressable entity; it could in fact be contained within the MD-IS. Authentication is discussed in Chapter 6 and defined in Parts 406 and 640 of the CDPD System Specification.
The directory server is an internal F-ES which provides support for directory services within the CDPD network. Directory services are used primarily for network management and other support services. Like the authentication server, this can be implemented in many different ways. Depending on the needs of a CDPD service provider and its customers, the directory server could support either DNS or X.500 capabilities, or both. Directory services are described in Chapter 7 and defined in Part 610 of the CDPD System Specification.
The network management system is the means by which a CDPD service provider operates the CDPD network. Network management includes configuration management, fault management, performance management and other functions. Like other servers, it can be implemented in many different ways. Typically network managers run in stand-alone processors because of the resource-intensive nature of their activities. Network management is discussed in Chapter 7 and defined in Parts 700 through 750 of the CDPD System Specification.
The message transfer system is the means by which CDPD accounting and other messaging functions are supported. CDPD accounting is supported by an X.400 messaging model, which is embodied in the message transfer system entity. It can be implemented in many ways and is discussed in Chapter 7.