Network Topologies for Scalable Multi-User Virtual Environments Lingrui Liang

Syllabus Introduction Related work Network characteristics Network topologies Experimental result Conclusion

Introduction Network bandwidth and graphics performance are increased, distributed visual simulation systems are allowed multiple users to interact in a shared 3D virtual environment(VE) Workstations run 3D graphics interface program to simulate the virtual environment, each user can be a role to see other roles via wide-area network

Introduction Avatars controlled by users and kept the newest information in the workstation through messages Support: visual interactions bewteen multiple users in a shared 3D VE

Applications Distributed training simulationsCollaborative design Virtual meetingsMultiplayer games

Challenge Keep consistent state among a larger number of workstations E.g. One entity moves or modifies the shared environment, an update must be applied on each workstation N entities move, M times per second. Updates should be M*N per second to a shared database

Goal and Aim Goal: trade-offs of different network topologies and messaging protocols for multi-user virtual environments Aim: represent the design of system that can support very large numbers of users at the same time

Related Work A virtual environment is represented by these systems as a set of independent entities. Each entity has a geometric description and a behaviour Types of entity:  Static entity  Dynamic entity

Related Work Conditions for activating distributed simulation with multiple entities interact in a shared VE:  Sending message to one another to update the geomotry or behaviour of entities  Modifications to the shared environment  Impact on other entities

Related Work – Entity Management Every entity is managed by one of the workstation in the distributed system Workstation may map user input to control of entities and may include viewing capabilities To manage its own entities (local entities), each workstation maintains surrogate for some entities managed by other workstation (remote entities)

Related Work – Surrogate It contatins representations for the geometry and behaviour of entity If a workstation receives an update message from a remote entity, it will update the geometric and behavioural models for the entity’s local surrogate Surrogate behaviour is simulated by each workstation in the processing of update

Communication Mode for Network Nodes Four routing schemes:  Unicast  Multicast  Broadcast  Anycast

Unicast Unicast transmission is the sending of information packets to a single destination E.g. Reality Bulit for Two, VEOS and MR Toolkit are based on unicast peer-to-peer

Multicast Using multicast to send update message to a subset of participating workstation. The general idea is to map entity properties into multicast groups, and send update message only to relevant groups E.g. NPSNET and DIVE are multicast peer-to-peer systems

Broadcast Broadcast refers to transmitting a packet that will be received by every device on the network E.g. VERN and SIMNET are broadcast peer-to-peer systems

Anycast Anycast is a network addressing and routing scheme whereby data is routed to the "nearest" or "best" destination as viewed by the routing topology E.g. DNS and IPv6 are based on the anycast systems

Client-Server System Communication between client workstation is managed by message server with intelligent server message processing Key feature of client-server design: servers can process messages before propagating them to other cilents, selecting, augmenting, or modifying messages

Example for Client-Server Design A server may determine to send a particular update message only to a relevant small subset of clients and then broadcast the message only to those clients and their servers It can be applied to many users at the same time

Client-Server Design Motivations:  To support modem connections to clients  To simplify implementation Assumption: a client can send a message to any one or set of clients and/or servers at any time Result: with better processing and messaging properties, a variety of alternate system topologies are possible

Network Characteristics Communication between workstations participating can be implemented by a various possible network with different features Logical networks:  Transport is oriented connection or connectionless  Message delivery is unicast or multicast  Message latency  Data bandwidth

Wide-Area Network - Connections Connections:  data exchange between two workstations  two-way, oriented connection, unicast transport  low latency and low bandwidth(14.4Kb/s or 28.8Kb/s)  e.g. A network is a modem using a standard telephone cable

Wide-Area Network - Unicast Unicast:  random number of workstations are connected to a network logically  the network supports connectionless and unicast messages  e.g. Internet

Wide-Area Network - Multicast Multicast:  random number of workstations communicate with each other with connectionless and multicast messages  e.g. Mbone(Multicaste Backbone)

Network Characteristics Different networks can be constructed using combinations of different types of networks Each combination can affect the design of the multi-user virtual environments systems

Basic Concept - Network Topology Network topology is the study of the arrangement or mapping of the elements of a network, especially the physical (real) and logical (virtual) interconnections between nodes

Peer-to-Peer Topologies Arranged system with a set of workstations Communication method: peer-to-peer (P2P) directly Peers send a unicast message to other peers when an entity is updated (unicast message is available)

Peer-to-Peer Topologies - Unicast

Peer-to-Peer Topologies Filters must be applied What for? – Update messages are not sent to every peer for every update Peers maintain lists of entities in each cell Update messages are sent to all peers whenever an entity moves to a new cell, mapping must be changed among peers

Peer-to-Peer Topologies Peers can send a single multicast message to a subset of peers at one time (Multicast message is available) A multicast group can be assigned to each cell Peers do not maintain the lists of entities in each cell, but they join and leave multicast groups as their entities move between cells

Peer-to-Peer Topologies - Multicast

Hierarchical Topologies Hierarchical Topologies = Tree Topologies A hierarchical topology is created similar to an extended star topology. The primary difference is that it does not use a central node. Instead, it uses ad trunk node from which it branches to another nodes.

Hierarchical Topologies Multi-user VE can be designed by this topology Each entity’s update, a client sends update message to server, the it propogates to other servers and clients Types of network used for client-server and for server-server links

Hierarchical Topologies

Properties Advantages:  Message distribution is shifted out of the client and into the server  Server: litter processing, storage or messaging to keep entities consistent in a large VE  Client: precessing, storage and network bandwidth requirements scale unlimited Disadvantages  Extra latency may be introduced for each update message

Communications – Client/Server Oriented connection, unicast: each server manages message distribution for a subset of clients Every entity’s update, client server, server other servers and client with entities, but maintaining mappings and periodic update are required Not scale infinitely

Communications – Client/Server Connectionless, unicast: each server manages message distribution for separate regions of the VE Advantage: fewer server-to-sercer messages are generated Maintaining mappings and periodic update are also required Scales infinitely and server are limited to finite subsets of the VE

Communications – Server/Server Multicast: similar to the P2P multicast system design Region managed by each server is static, servers do not join and leave multicast group dynamically No periodic update required Scales infinitely

Experiment VE conditions: 800 rooms connected by hallways consisting of 23,168 polygons and 2,219 cells Runs 256 computer- controlled entites at the same time With 2,4, 8 or 16 servers

Experimental Scheme Two schemes to demonstrate the eect of system design on the message processing requirements of workstations in a multiuser virtual environment:  A) clients made static connections to one server while servers passed messages to each other using a connectionless unicast network  B) clients and servers both passed messages on a connectionless unicast network, each server managed message distribution for a separate region of the virtual environment

Experimental Results

Experimental Analysis A) the number of servers increased, the total number of server-server messages output by each server increased as separate unicast messages were sent to multiple servers. This is sublinear increase B) the number of servers increased, and the intervisibility between server regions decreased, server-server messaging was reduced

Experimental Result Using B) system design, the message processing burden of each client and server can be quite small, and these systems can scale many users at the same time

Conclusion Characteristics of the networks can greatly impact the message distribution performance of a particular system design for multi-user VE The results of experiments demonstrate that different network characteristics and different system designs can seriously affect the message processing rates required by workstations in a multi-user VE Perhaps, by identifying network characteristics and system designs that improve the message distribution properites or decrease the cost of multi-user VE systems, we can help software and network architects in the design of future systems