Not your father's radio network

Jan 1, 2008 12:00 PM, By Jay M. Jacobsmeyer, P.E.

Modern land mobile radio systems are packet-switched networks that contrast sharply with legacy systems using circuit-switched connections. The circuit switch operates like the original telephone network, where each call requires a physical connection between two end users that is maintained until the call is complete. The circuit switch manages some number of narrowband channels, and each channel is dedicated to a single conversation for the duration of the call. A trunked radio network is an example of a circuit-switched network.

In contrast, the packet switch manages a single broadband channel by time-sharing the channel on a packet-by-packet basis. Access is controlled using a multiple access method such as carrier sense multiple access with collision avoidance (CSMA/CA). Computer professionals often call the broadband channel the medium, and methods for controlling the medium fall into a class of protocols called medium access control (MAC).

In packet networks, a single message or conversation is divided into many packets that typically are interleaved in time with packets from other users. Packets from the same conversation can arrive at different times and even out of order. The uncertain delivery time makes packet-switched networks less suitable for voice traffic, but special protocols used by voice-over-IP (VoIP) systems prioritize voice packets so they are assembled more quickly and cause no noticeable delays to the user. IEEE 802.11 (Wi-Fi) networks are packet-switched networks.

Circuit switching is inefficient for bursty traffic, but push-to-talk voice calls — and all data calls — are inherently bursty. Furthermore, practically all computer networks are packet-switched. Eventually, the convergence of computer and radio technology will draw all radio traffic into packet-switched networks. This article examines the protocols used on packet-switched radio networks and the methods for managing bandwidth on these networks.

The OSI model

Most computer networks follow the reference model for open systems interconnection (OSI) as published by the International Standards Organization (ISO). The OSI model has seven layers that communicate between end systems. The user generates a message at the top layer called the application layer (layer 7), and the message moves down the protocol stack to the physical layer (layer 1), where the message is physically transmitted over the communications medium (cable or radio). At the distant end, the message moves up the stack until it reaches the corresponding application layer.

Intermediate nodes in an OSI network require only the bottom layer functions of the OSI model. For example, if our network is a packet radio system, the network node or base station needs to implement only the physical, data link and network layers. Such a configuration is shown in Figure 1. Note that we are using the normal convention of arrows between equivalent layers. These arrows indicate a virtual connection except at the physical layer, where there is an actual connection. Messages must travel down the stack and across the channel at the physical layer.

Computer networks almost universally operate under a somewhat different model called the TCP/IP model. Unlike the OSI model, the TCP/IP model is not an international standard, and its definitions are somewhat different. The TCP/IP model has five layers, with the first four layers identical to the OSI model. The fifth layer, called application, combines the session, presentation and application layers of the OSI model.

When computer professionals speak of bandwidth management, they mean management of a medium characterized by some gross bit rate, whether it originates from a radio network or a landline network. When radio professionals speak of bandwidth management, they usually mean spectrum management, which is the efficient use of allocated radio spectrum shared by multiple users.