Layer 2 switches

By J.B. MilesSpecial to GCNIf your legacy Ethernet LANs are stretched to the breaking point under the weight of new users, servers and high-end applications, try replacing old Ethernet hubs with one or more 10/100-Mbps Ethernet switches.All the switches in this guide are reasonably priced, flexible and suitable for running small networks or for connecting many small workgroups at larger sites. Unmanaged eight- or 16-port switches sell for as little as $20 per port, and many managed 16- or 24-port switches sell for as low as $100 per port.Unlike hubs, which share available bandwidth among attached devices, nonblocking switches provide dedicated 10- or 100-Mbps bandwidth to networked PCs, workstations and servers. And if you can't yet part with your existing hubs, you can use 10/100 switches to connect them for better network segmentation.At the desktop, 10/100 switches are the best possible devices for connecting multiple users to the network at top speeds.The accompanying chart lists 66 10/100 dual Ethernet workgroup switches from 26 manufacturers. They are equipped with eight, 16 or 24 auto-sensing 10/100 ports to determine the speed of attached devices and automatically configure the ports for 10-Mbps or 100-Mbps throughput. All can automatically negotiate between half- and full-duplex modes'the latter effectively creating dedicated 20-Mbps or 200-Mbps connections.Most switches listed also provide shared or dedicated 10/100 uplinks to other switches or network backbones, and some even provide one or two dedicated Gigabit Ethernet links for especially heavy bandwidth requirements.The products in this guide are Layer 2 switches, meaning they operate at the second, or data link, layer of the Open Systems Interconnection model. Their function is to check incoming network traffic, keep track of the physical addresses of all stations attached to their ports and forward traffic to specific station addresses according to the forwarding tables they have built. Layer 2 switches can deliver data at wire speeds because they don't have to examine individual data packets.Layer 3 switches, also called routing or IP switches, perform both switching and routing tasks. Because they make heavy use of integrated circuits, specialized crossbar switching fabric, fast CPUs and other high-end components, they are expensive but well-suited to network backbone service.But less expensive Layer 2 switches also provide an extensive range of features and options, making them cost-effective for managing traffic at the workgroup and small network levels.Among their features and characteristics are: A switch's auto-sensing capability means it can determine the speed of attached devices and automatically configure the ports for 10-Mbps or 100-Mbps throughput. It does this by reading the speed of the network adapter or network interface card attached to the networked PC or server. A switch's ability to automatically negotiate between half- and full-duplex modes is extremely useful for particularly bandwidth-hungry client applications, such as a remote computer-aided design and manufacturing workstation.Full-duplex Fast Ethernet mode temporarily shuts down the Ethernet carrier-sense multiple-access with collision detection and loopback functions, allowing the switch to provide a theoretical bandwidth of 200 Mbps; that is, 100 Mbps in each direction.But this capability comes at the expense of some of the collision detection features of half-duplex mode and should be only be used for isolated high-speed workstations where collision detection isn't required. Aggregate bandwidth is the top theoretical speed limit of each switch and is related to the number of ports employed. Thus, the aggregate bandwidth of an eight-port switch running at 100 Mbps per port is 800 Mbps, while it's 1.6 Gbps for a 16-port switch. The simplest switches operate as cut-through devices, meaning they only examine a packet's destination before forwarding it. More advanced store-and-forward switches analyze the entire packet before forwarding it. The tradeoff? Store-and-forwarding eliminates many traffic errors but takes longer than the cut-through technique. Workgroup switches may be managed or unmanaged. Unmanaged switches are plug-and-play devices and are a good match for small, flat networks with a mixture of 10-Mbps and 100-Mbps legacy devices. They can automatically learn network addresses and can handle both auto-sensing and auto-negotiation.The best thing about them is their price. 3Com Corp.'s OfficeConnect Dual Speed Switch 8 is priced at $210, and Accton Technology Corp.'s popular CheetahSwitch Workgroup 3008A has been reduced to $199. D-Link Systems Inc.'s DSS-8+ can be bought for as low as $87 from third-party vendors, and ARK Technologies' $75 CT2208D unmanaged switch is the cheapest I've seen'less than $10 per port. Managed switches, in the $1,000 and up price range, are more expensive because they perform more services. Network management features are important when remote switch management is required, or when several switches are linked as part of a more complex network than a LAN.Products such as Allied Telesyn International Corp.'s $1,369, 16-port AT-8118 and all of Cisco Systems Inc.'s Catalyst 2900XL models'priced at $964 and up'come with proprietary management software. The software most often combines Simple Network Management Protocol and Remote Monitoring network management standards, with their own network management flavors and a little Web-based management tossed in. Simply adding users to a switch or connecting your switch to a network backbone switch is one way to scale your network upward. Rackmountable switches are more easily scalable than standalone desktop switches. Managed Layer 2 switches such as Accton's $785 CheetahSwitch Workgroup 3514F and all of IBM's 8275 series can support virtual LANs, either as a bundled or optional feature.Users can be linked by media access control addresses, IP addresses, network protocols or even the port numbers of a LAN switch into a virtual grouping that can share network resources and bandwidth.VLANs are tied to Layer 2 technology, however, and as Layer 3 switches providing full Gigabit Ethernet wire-speed routing enter the market, they may disappear. LED indicators at each switch port generally indicate the activity of the link, such as 10-Mbps or 100-Mbps bandwidth utilization or collision/full-duplex. An on/off switch LED light is included with virtually all switches. A big advantage to 100Base-T Fast Ethernet is that it runs over the same 10Base-T media that standard Ethernet uses. 100Base-TX'two pairs of data-grade Category 54 unshielded and shielded twisted-pair wiring'and 100Base-FX'two strands of multimode fiber'have emerged as the most popular cabling schemes in most wiring closets, so be sure that your switch supports them with appropriate connectors. The most popular media for Gigabit Ethernet uplinks is 1000Base-SX, which uses multimode fiber, but the more versatile, copper-based 1000Base-CX technology is coming on strong.XXXSPLITXXX-XXXSPLITXXX-