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Physical Layer of the Network Control Protocol


At the physical level we will be using the orderwire to exchange position, time and link quality information and to setup the wireless connections. The process of setting up the wireless connections involves setting up links between ESs and between ESs and RNs.

The network will have one master ES, which will run the topology configuration algorithm [15] and distribute the resulting topology information to all the connected ESs over point-to-point orderwire packet radio links. In the current prototype the point-to-point link layer for the orderwire uses AX.25 [16]. The master ES is initially the first active ES, and any ES has the capability of playing the role of the master.

The first ES to become active initially broadcasts its callsign and start-up-time in a MYCALL packet, and listens for responses from any other ESs. In this prototype system, the packet radio callsign is assigned by the FCC and identifies the radio operator. Since it is the first active ES, there would be no responses in a given time period, say T. At the end of T seconds, the ES rebroadcasts its MYCALL packet and waits another T seconds. At the end of 2T seconds, if there are still no responses from other ESs, the ES assumes that it is the first ES active and takes on the role of the master. If the first two or more ESs start up within T seconds of each other, at the end of the interval T, the ESs compare the start-up times in all the received MYCALL packets and the ES with the oldest start-up time becomes the master. In this system, accurate time stamps are provided by the GPS.

Each successive ES that becomes active initially broadcasts its callsign in a MYCALL packet. The master on receipt of a MYCALL packet extracts the callsign of the source, establishes a point-to-point link to the new ES and sends it a NEWSWITCH packet. The new ES on receipt of the NEWSWITCH packet over a point-to-point orderwire link, obtains its position from its GPS receiver and sends its position to the master as a SWITCHPOS packet over the point-to-point orderwire link. On receipt of a SWITCHPOS packet, the master records the position of the new ES in its switch position table, which is a table of ES positions, and runs the topology configuration algorithm [15] to determine the best possible interconnection of all the ESs. The master then distributes the resulting information to all the ESs in the form of a TOPOLOGY packet over the point-to-point orderwire links. Each ES then uses this information to setup the inter-ES links as specified by the topology algorithm. The master also distributes a copy of its switch position table to all the ESs over the point-to-point orderwire links, which they can use in configuring RNs as discussed below. This sequence of operations is illustrated in Figure 5 and Figure 6. Also, the ES then uses the callsign information in the switch position table to setup any additional point-to-point orderwire packet radio links corresponding to the inter ES links required to exchange any link quality information. Thus this scheme results in a point-to-point star network of orderwire links with the master at the center of the star and also point-to-point orderwire links between those ESs that have a corresponding inter ES link, as shown in Figure 3.

In the event of failure of the master node which can be detected by listening for the AX-25 messages generated on node failure, the remaining ESs exchange MYCALL packets, elect a new master node, and the network of ESs is reconfigured using the topology configuration algorithm [15].

Figure 5: State Diagram for Master EN.

Figure 6: State Diagram for EN not serving as Master.

Each RN that becomes active obtains its position from its GPS receiver and broadcasts its position as a USER_POS packet over the orderwire network. This packet is received by all the nearby ESs. Each candidate ES then computes the distance between the RN and all the candidate ESs which is possible since each ES has the positions of all the other ESs from the switch position table. An initial guess at the best ES to handle the RN is the closest ES. This ES then feeds the new RN's position information along with the positions of all its other connected RNs to a beamforming algorithm that returns the steering angles for each of the beams on the ES so that all the RNs can be configured. If the beamforming algorithm determines that a beam and TDMA time slot are available to support the new RN, the ES steers its beams so that all its connected RNs and the new RN are configured. It also records the new RN's position in its user position table which contains positions of connected RNs, establishes a point-to-point orderwire link to the new RN and sends it a HANDOFF packet with link setup information indicating that the RN is connected to it. If the new RN cannot be accommodated, the ES sends it a HANDOFF packet with the callsign of the next closest ES, to which the RN sends another USER_POS packet over a point-to-point orderwire link. This ES then uses the beamform algorithm to determine if it can handle the RN. Figure 7 shows the states of operation and transitions between the states for a RN.

This scheme uses feedback from the beamforming algorithm together with the distance information to configure the RN. It should be noted that the underlying AX.25 protocol [16] provides error free transmissions over point-to-point orderwire links. Also the point-to-point orderwire link can be established from either end and the handshake mechanism for setting up such a link is handled by AX.25. If the RN does not receive a HANDOFF packet within a given time it uses a retry mechanism to ensure successful broadcast of its USER_POS packet.

Figure 7: State Diagram for RN.

A point-to-point orderwire link is retained as long as a RN is connected to a particular ES and a corresponding high-speed link exists between them to enable exchange of link quality information. The link can be torn down when the mobile RN migrates to another ES in case of a hand-off. Thus at the end of this network configuration process, three overlaid networks are setup, namely, an orderwire network, an RN to ES network and an inter-ES network. The orderwire network has links between the master ES and every other active ES in a star configuration, links between ESs connected by inter-ES links as well as links between RNs and the ESs to which they are connected, as shown in Figure 3. Raw pipes for the user data links between RNs and appropriate ESs as well as for the user data links between ESs are also set up.

next up previous
Next: ATM Network Configuration Layer Up: Network Control Protocol Overview Previous: Network Control Protocol Overview

Steve Bush
Tue Nov 12 11:48:31 CST 1996