InformationWeek
Defining The Business Value Of Technology
October 19, 1998
Strengthen Your Backbone
continued...page 3 of 3
Virtual LANs
In the evolution of networks, what often occurs is that each department has a local switch or hub, and those are connected to switches that consolidate buildings, which are connected to switches that consolidate campuses, and so on. If a user is moved from one cubicle to another or from one department or location to another, his or her PCs must be re-configured with a new network address for the new location.
Many Ethernet switches, both 10/100 switches and Gigabit Ethernet, support Virtual LANs, which are intended to address this configuration problem. VLANs allow an administrator to create one LAN on some ports of a switch, and a different LAN on other ports, bypassing the need to have a hub or switch for each separate LAN. This means a relocated user can keep his or her existing network address--the administrator simply assigns the port for the new office to the same LAN as the port in the old office.
VLANs can be configured very simply, through a graphical user interface, often by clicking on the port in a graphical representation of the switch, then selecting a LAN to assign to the port from a list of available LANs.
Network Protocols On ATM
LAN Emulation (LANE) is used to route existing network protocols over ATM. ATM is a separate protocol with very small packets--LANE allows TCP/IP and other network protocols such as IPX/SPX or AppleTalk to be routed over ATM. If only TCP/IP is used in a network, IP over ATM can be used instead, while multiprotocol encapsulation (ME) is generally used if traffic needs to go over a WAN link.
All these methods of routing network traffic require a server, which may be incorporated in a router or switch, or may be a separate device. Configuration of the servers is often quite cumbersome and complex, requiring manual editing of a text table or the use of a text-based interface to enter port numbers, TCP/IP, or other network information, and so forth. LANE can be used to create separate networks on the same switch. However, configuration is much more complex and the user interfaces are much less usable.
Additionally, since the network protocols are being encapsulated in ATM packets, a protocol analyzer must understand this to be able to decode the actual network traffic the packets contain. This means that the analyzer must understand ATM as well as the other network protocols, which adds considerably to the cost of the analyzer. With Gigabit Ethernet, the analyzer needs only to have a Gigabit NIC installed.
Performance
Performance in the network is generally analyzed in two areas, especially at high speeds. The first area is switch or router performance; the second is the performance of the NICs used to connect servers to the backbone.
Switches and routers can usually communicate with one another at the full speed of the medium--either 622 Mbps or 1 Gbps. In fact, many devices support port aggregation, which combines two or more ports to link to another device, providing higher speeds and failover if one of the links fails. Newer switches have backplanes that support anything from 12 Gbps to more than 1,000 Gbps, which is more than most labs can test, to say nothing of use.
However, many backbones don't just aggregate traffic between LANs; they also contain servers that are accessed by many users from different LANs in an enterprise--mail servers or in-house Web servers, for instance. Administrators who connect these servers directly to an ATM or Gigabit Ethernet switch and expect to see 622 Mbps or 1 Gbps transmission will be disappointed--recent tests by InformationWeek's sister publication, Internet Week, showed that even with optimization, Windows NT servers were hard-pressed to get half of the rated transmission speed. Other servers had less difficulty--Sun Solaris and NetWare servers came much closer to using the full speed of the medium.
In addition, throughput on a network is not going to be the same as the network's advertised speed--overhead of the protocol will play a role, as will congestion on the network. In general, ATM uses more overhead than does Gigabit Ethernet, which means that slightly less of the total bandwidth is available for actual traffic. In practice, this amount is not very significant.
In all, ATM is a good fit for large enterprises linked over a WAN, for installations with a lot of token-ring, or when quality of service is a major consideration, especially if different divisions or companies are using different network hardware. Gigabit Ethernet is simpler and cheaper, and coming up fast with quality-of-service features, although current solutions are limited to a single vendor's equipment. Once Gigabit vendors standardize on 802.1q, that should change. Gigabit also offers substantial advantages for administrators already familiar with Ethernet.
return to page 1, 2
Back to Labs
Send Us Your Feedback
Top of the Page
Featured Microsite
