Group world transitions

that their activities will be independent and uncorrelated. Rather, at least when partic- ipants are involved in a common task, there is a small but statistically highly signifi- cant element of correlation between their activities. This argues for additional caution when considering, for example, the scope for exploiting statistical multiplexing of movement-related traffic for larger numbers of users (i.e. being able to require or reserve less bandwidth on the basis that while some users are moving - and generating network updates - many other users will not be).

A.2.2. Group world transitions

Having considered one key aspect of movement within a world this analysis now con- siders two aspects of moving between worlds. First it considers the likelihood and form of coordinated inter-world transitions by groups of participants and then whether and in what circumstances participants return to previously visited worlds.

It might be expected from some of the activities organised in the meetings being ana- lysed that group world transitions would occur. For example, participants would typi- cally gather initially in the gateway world and wait for others to arrive. Then at some point the meeting organiser would invite everyone to go through to the meeting world for the formal start of the meeting and all of the participants would move - in a vaguely coordinated fashion - through to the meeting world. The purpose of this aspect of the analysis is therefore not simply to discover whether such transitions occur. Rather, the purpose is to assess the significance and character of such transi- tions. Once the data has been presented its significance will be considered.

For the purpose of the automated analysis a group world transition is defined as an event in which two or more participants who are in a world at the same time move via a single portal jump to another world so that they are together again. Figure 44 on page 181 shows the incidence of singleton and group world transitions in the meet- ings analysed. The solid line shows the number of incidents, while the dashed line shows the total number of participants involved in those incidents, e.g. each group transition incident for group size four must have involved four participant transitions. Summarising the underlying data:

• participants jumped to new worlds on a total of 584 occasions; • of these, 337 (58%) were in groups of two or more;

• individuals or groups made world transitions on 350 occasions; • of these 103 (29%) were group transitions;

• the average size of those groups was 3.27 participants.

Figure 45 on page 181 shows the distribution of world entry delay for participants involved in group transitions. For each member of a group (excepting the leader) it shows how much time elapsed between the group leader and the group member reach- ing the destination world. The range of delay shown on the graph, up to 30 seconds, accounts for 203 (87%) of the 234 non-leading participants to make group world tran- sitions. 104 (44%) of these occur within 5 seconds, while 159 (67%) occur within 10 seconds.

Group world transitions are important when considering the design of and require- ments for CVEs because moving to a new world is a significant event which will almost always involve an exchange of data between the participant’s process(es) and

A.2.2. Group world transitions

the rest of the system. In MASSIVE-1 for example, on entering a new world the user’s client applications will:

• terminate connections to objects in the old world;

• be informed of the existence and identity of nearby objects in the new world;

Figure 44: distribution of group size for group world transitions.

0 50 100 150 200 250 0 1 2 3 4 5 6 7 8 occurance group size incidents participants

Figure 45: distribution of group member arrival delay for group world transitions. 0 50 100 150 200 250 0 5 10 15 20 25 30 occurances delay, seconds distribution (1% buckets) cumulative

A.2.2. Group world transitions

• establish network associations with each; and

• exchange general and medium-specific information such as location, awareness, name, graphical appearance, etc.

This can result in a significant but transient burst of network traffic and a correspond- ing load on other processes. Different systems will organise this information in differ- ent ways and obtain it from different sources, but there will still be some requirement for the participant’s application(s) to learn about the new world. This makes the occurrence of group world transitions important in two respects:

• a coordinated movement by a large group could generate a much greater transient load than might be expected if inter-world movement were assumed to be inde- pendent and uncorrelated; and

• specialised system support for group world transitions (for example, based on the use of network-supported multicasting of new world information to all group members) could both alleviate this problem and also reduce the total network load relating to world transitions when compared to a model of independent and uncor- related movement.

For example, for the meetings analysed, imposing a world transition delay of 10 sec- onds would both ensure that state transfers for the same world did not need to be per- formed more than once in any 10 second, and would require approximately 325 unicast and 100 multicast state transfers rather than 584 unicast state transfers. Impos- ing longer delays on world transitions would increase the effectiveness of group trans- fers by allowing more transitions to be grouped whereas shorter delays would include fewer transitions.

Before moving on to consider participants returning to worlds a little must be said about the general applicability (or otherwise) of this result. As was noted at the begin- ning of this section group world transitions were an organised aspect of the activities being analysed; will they occur in other applications and situations? Such an question cannot be answered definitively without gathering a great deal more data about a wide range of different applications and scenarios. However some more subjective and ten- tative observations can be made:

• MASSIVE-1 is not alone in adopting a multiple world model with portals between worlds (see for example DIVE [Carlsson and Hagsand, 1993] or in a more limited sense dVS [Grimsdale, 1991]);

• the same effects and results would apply for systems structured using regions (as in MASSIVE-2 and Spline [Barrus et al., 1996]) as for worlds (but at the granularity of regions);

• the world designer for the ITW meetings (Adrian Bullock) chose independently to structure the meeting space as a number of different worlds with tailored content and form; and

• the world and portal model was widely accepted and effectively employed by par- ticipants.

It may be argued from these observations that a multi-world structure (or an equiva- lent regionalised structure) is a generally useful and appropriate virtual design style. So it may be anticipated that group world transitions will occur in many applications involving formal or informal cooperation and interaction, for example, as common