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Bandwidth Service. The VNS bandwidth service is used during the call setup when a connection request is made. Video teleconferencing users may require a CIR of 10M bps whereas the terminal emulation users may only require 1M bps. This is where ATM end stations and ATM switches negotiate the amount of bandwidth dedicated to a particular virtual circuit using UNI signaling. Ethernet, Token Ring, and FDDI nodes do not recognize The UNI signaling, but the switches they attach to could proxy signal for the end station, thus allowing a single bandwidth manager for the entire network, not just the ATM portion.
Broadcast Service. The VNS broadcast service uses as its base the concept of the broadcast unknown server BUS that is part of the ATM Forums LAN emulation draft standard. This is how broadcasts are flooded through the network to remain compatible with the operation of many of todays protocols and NOSs. A degree of intelligence can be assigned to the VNS that would allow for broadcasts or multicasts based on protocol type or even policy group.
Virtual Routing Service. The VNS virtual routing service is one of the most critical components of a virtual network. Just as traditional networks required traditional routers for interconnection, virtual LANs will require virtual routers for internetworking between virtual LANs. In other words, routing is required, but routers may not be. Some protocols such as TCP/IP actually require a router for users on two different subnetworks to speak with each other. In addition, most networks today are logically divided based on network layer protocol addresses with routers acting as the building block between segments.
The difference in operation between a virtual router and a traditional router goes back to the connection-oriented vs. connectionless distinction. Routing allows for address resolution between the layer 3 protocol address and the layer 2 MAC address just as it happens through the ARP process in TCP/IP networks. The VNS virtual routing service performs the address resolution function, but once the end station addresses are resolved, establishes a virtual connection between the two users. Two users separated by a traditional router would always have the router intervening on every single packet because the router would have resolved the protocol addresses to its own MAC address rather than the actual end stations MAC address. This VNS routing service allows the network to route once for connection setup and switch all successive packets.
Accounting Service. The VNS accounting service is beneficial because it allows the creation of the network bill. Similar to the way a telephone bill is broken down, the accounting service details connection duration with date and time stamp along with bandwidth consumption details. This is most directly applicable in the WAN. For many network managers, WAN usage is never really accounted for on an individual user basis, yet it can consume up to 40% of the operations budget.
As usage-based WAN service options such as ISDN gain popularity, accounting becomes that much more critical. IXCs, competitive access providers, and the RBOCs continue to deliver higher-bandwidth links with usage-based tariff. In the future, they could install a 155M-bps SONET OC3 links and only charge for the actual bandwidth used. Unless network managers have tools to control access to and account for usage of WAN links, WAN costs will continue to rise. This service lets network managers know who is using the WAN.
Throughout this discussion, words have been carefully chosen to describe the operation of switched virtual networks. Many of the current vendor offerings on the market have as their goal the construction of a switched Virtual LAN. These Virtual LANs are interconnected using a traditional router device. However, the router has been viewed as the performance bottleneck. Routers should be deployed when segmentation or separation is the need; switches should be used to deliver more bandwidth. The Virtual LAN concept is merely an interim step along the way to realizing the fully virtual network.
The ATM Forums draft LAN emulation standard allows ATM devices to internetwork with traditional LAN networks such as Ethernet and Token Ring. However, it seems ironic that it essentially tries to make ATM networks operate like a traditional shared-access LAN segment. Although it is required for near-term deployment of ATM solutions into existing LAN architectures, its position as an end-all solution is questionable. A more logical approach uses ATM as the model that LANs must emulate.
Each vendors approach to virtual networking features will vary slightly in implementation. Most vendors have agreed, however, that the router is moving to the periphery of the network and the core will be based on switching technologies with virtual network capabilities. The three critical success factors that a virtual network vendor must display to effectively deliver on all the promise of virtual networks are connectivity, internetworking, and network management.
Connectivity expertise through a demonstrated leadership in the intelligent hub industry ensures the user a broad product line with numerous options in regards to topology and media types. The product should fit the network, rather than the network design being dictated by the capability of the product. This indicates a vendors willingness to embrace standards-based connectivity solutions as well as SNMP management and RMON analyzer capabilities.
Internetworking expertise ensures that the vendor is fully equipped to deal with layer 2 as well as layer 3 switching issues through an understanding of protocols and their operation. This is not something that can be learned overnight. The integration of these technologies is still unattainable.
Network management software is crucial virtual networks do not exist or operate without it. The virtual network services provide all the value to the switch fabric. Users should look for a vendor that has delivered distributed management capabilities. Just as the telephone network relies on distributed software intelligence for its operations, so too must the switched virtual network provide the same degree of redundancy and fault tolerance. Users should also consider whether the vendor embraces all of the popular network management platforms (e.g., SunNet Manager, HPOV, Cabletron SPECTRUM, and IBM NetView for AIE) or only one. Finally, users should make sure the vendor has experience managing multiple types of devices from vendors other than itself. It would be naive to think that all of the components that make up a network are of one type from one vendor.
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