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Static Configuration

 

The scope of this section is limited to initial configuration of the wireless system in a static environment, i.e. no handoffs take place. This is the primary difference between this section and Section 9. This includes the initial configuration at each level including: radio topology configuration, ATM switch configuration, and the standard protocol configuration, such as the HDLC-like and IP layers. The beamform process determines all possible topologies. Other parameters such as distance, load distribution, and signal power can be used to determine the best configuration.

There are two main levels of configuration which are required. At the lowest level, the RN radios must configure themselves to communicate with EN radios. Also, the ATM switches must learn the network topology in order to properly switch cells. The interaction between these two configuration tasks in a mobile environment is under study.

Each layer of the high speed radio connection will have a corresponding layer in the network configuration system, as shown in Table 1.

The following is a description and ordering of events for the establishment of the wireless connections.

  table207
Table 1:   Network Configuration System Layers.

  table215
Table 2:   MYCALL Packet Contents.

  table222
Table 3:   NEWSWITCH Packet Contents.

  table229
Table 4:   SWITCHPOS Packet Contents.

  table236
Table 5:   TOPOLOGY Packet Contents.

  table243
Table 6:   USER_POS Packet Contents.

  table250
Table 7:   HANDOFF Packet Contents.

  table257
Table 8:   VC_SETUP Packet Contents.

  table264
Table 9:   ARPSERVER Packet Contents.

  table271
Table 10:   MOBAGENT Packet Contents.

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

The network will have one master switch (EN), which will run the topology configuration algorithm [13] and distribute the resulting topology information to all the connected ENs over point-to-point packet radio links. The point-to-point link layer is AX.25 [14]. It provides automatic retries, and returns link errors to the orderwire.

The master EN could initially be the first active EN, and any EN would have the capability of playing the role of the master.

The first EN to become active would initially broadcast its callsign(where callsign = radio address) and start-up-time in a MYCALL packet (Table 2), and listen for responses from any other ENs. Since it is the first active EN, there would be no responses in a given time period, say T. At the end of T time, the EN could rebroadcast its MYCALL packet and wait another T seconds. At the end of 2T seconds, if there are still no responses from other ENs, the EN assumes that it is the first EN active and takes on the role of the master. If the first two or more ENs start up within T seconds of each other, at the end of the interval T, the EN could compare the start-up times in all the received MYCALL packets and the EN with the oldest start-up time would become the master.

Each successive EN that becomes active would initially broadcast its callsign in a MYCALL packet. The master on receipt of a MYCALL packet would extract the callsign of the source of the packet, establish a point-to-point link to the new EN and send it a NEWSWITCH packet (Table 3). The new EN on receipt of the NEWSWITCH packet over a point-to-point link, would obtain its position from its GPS receiver and send its position to the master as a SWITCHPOS packet (Table 4) over the point-to-point link. On receipt of a SWITCHPOS packet, the master would record the position of the new EN in its "switch position" table (table of EN positions), and run the topology configuration algorithm [13], to determine the best possible interconnection of all the ENs. The master would then distribute the resulting information to all the ENs in the form of a TOPOLOGY packet (Table 5) over the point-to-point links. The EN can then use this information to setup the high-speed links as specified by the topology algorithm. The master would also distribute a copy of its ``switch position'' table to all the ENs (over the point-to-point links), which they can use in configuring RNs as discussed below. This sequence of operations is illustrated in Figure 14 and Figure 15. Figure 16 and Figure 17 show the steps in configuring ENs and RNs respectively. Also, the EN can then use the callsign information in the ``switch position'' table to setup any additional point-to-point packet radio links (corresponding to the high-speed links) required to exchange any link quality information. Thus this scheme would result in point-to-point packet radio links from the master to every EN (a point-to-point star network with the master as the center of the star) and also between those ENs that have a corresponding high-speed link, as shown in Figure 13.

 

  figure300


Figure 13: Example Orderwire Topology.

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 ENs exchange MYCALL packets, elect a new master node, and the network of ENs is reconfigured using the topology configuration algorithm [13]. The efficiency of this method of handling failure of the master node versus maintaining a hot backup for the master node is to be studied.

   figure307
Figure 14: State Diagram for Master.

   figure312
Figure 15: State Diagram for EN not serving as Master.

   figure317
Figure 16: Flow Diagram for processes during Reconfiguration Mode State.

   figure322
Figure 17: Flow Diagram for processes during Handle RN State.

Each RN that becomes active would obtain its position from its GPS receiver and broadcast its position as a USER_POS packet (Table 6). This packet would be received by all the ``nearby'' ENs. Each candidate EN would then compute the distance between the RN and all the candidate ENs (which is possible since each EN has the positions of all the other ENs from the ``switch position'' table). An initial guess at the best EN to handle the RN would be the closest EN. This EN would then feed the new RN's position information along with the positions of all its other connected RNs to a beamsteering algorithm that returns the steering angles for each of the beams on the EN so that all the RNs could be configured. If a time slot and/or beam is available to fit in the new RN (this information will be returned by the beamforming algorithm), the EN would steer its beams so that all its connected RNs and the new RN are configured, record the new RN's position in its ``user position'' table (table of positions of connected users), establish a point-to-point link to the new RN and send it a HANDOFF packet (Table 7) with link setup information indicating that the RN is connected to it. If the new RN cannot be accommodated, the EN would send it a HANDOFF packet with the callsign of the next closest EN, to which the RN could send another USER_POS packet over a point-to-point link. This EN could then use the beamform algorithm to determine if it could handle the RN, and so on. Figure 18 shows the states of operation and transitions between the states for a RN.

This scheme thus 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 [14] ensures error free transmissions over point-to-point links. Also the point-to-point 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 can use a retry mechanism to ensure successful broadcast of its USER_POS packet.

   figure337
Figure 18: State Diagram for RN.




next up previous
Next: Establishing an End-to-End Connection Up: Rapidly Deployable Radio Networks Previous: Open Issues

Steve Bush
Sun Aug 25 17:29:09 CDT 1996