Say what? QoS in English

By Eric Hindin, Network World |  Networking

Most likely, a combination of overengineering and QoS features will emerge as the solution of choice. Several vendors favor this approach, saying it's better to overengineer the network by throwing smart bandwidth, rather than raw bandwidth, at the problem. Of course, vendors are hardly objective: They use software capabilities such as QoS to differentiate their products.

In the WAN, overengineering is less practical. Declining WAN bandwidth costs will make higher speeds more affordable, somewhat mitigating the need for QoS in the WAN. However, WAN bandwidth costs will still be a significant expense for most corporations, so overengineered WANs will never be as prevalent as overengineered LANs.

Setting priorities

In lieu of overengineering, data prioritization and queuing systems provide the most mainstream QoS tools available today. Routers have supported data prioritization and queuing for many years. Some new Gigabit Ethernet switches are designed to support data prioritization and queuing, but policy-based management software to fully harness the technology isn't yet available. These new switches include 3Com's CoreBuilder 3500 and 9000 and SuperStack II, Bay Networks' Accelar models, Cabletron Systems' SmartSwitch Router, and Cisco's Catalyst 5000 and 8000.

Data prioritization systems can be characterized as implicit or explicit. With implicit QoS, a router or switch automatically allocates service levels based on administrator-specified criteria, such as the type of application, protocol or source address. Every incoming packet is examined or filtered to see if it meets the specified criteria.

Just about all routers support implicit QoS. Several switches also are designed to provide implicit QoS, but only offer limited prioritization capabilities. For example, the switches can prioritize based on type of virtual LAN and source or destination address rather than higher level information such as application or protocol type. Emerging policy-based network systems will bring more robust prioritization capabilities to these switches.

Explicit QoS, in contrast, lets the user or application request a particular level of service, and switches and routers attempt to meet the request. IP Precedence, also called IP Type Of Service (TOS), is likely to become the most widely used explicit QoS technique.

Part of the IP Version 4 protocol, IP TOS reserves a field in the IP packet where delay, throughput and reliability service attributes can be specified. The latest version of Winsock in Windows 98 and NT lets administrators use applications to set the field. With the exception of multimedia software, few popular applications support IP TOS.

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