The realm of LAN interconnection devices offers a number of options. This section discusses the nature of hubs, LAN switches, virtual LANs (VLANs), bridges, routers, and IP switches.
Hubs
Hubs interconnect the wiring that's connected to workstations. They are a building block of most networks. There are three major types of hubs:
• Active— Active hubs regenerate and retransmit signals, just as a repeater does. Because
hubs typically have 8 to 12 ports for network computers to connect to, they are sometimes called multiport repeaters. Active hubs require electrical power to run (that's why they're called active).
• Passive— Passive hubs serve as connection points and do not regenerate the signal; the
signal simply passes through the hub. They do not require electrical power to run. Wiring panels and punchdown blocks are examples of passive hubs.
• Hybrid— Hybrid hubs accommodate several different types of cables.
it easy to change or expand wiring systems, they use different ports to accommodate different cabling types, and they centralize the monitoring of network activity and traffic. Hubs, which are sometimes called concentrators or multistation access units (MSAUs), can also eliminate the need for NICs with onboard transceivers at each node or personal computer.
A group of transceivers can all be located in and managed by an intelligent hub. Intelligent hubs are modular and chassis based, with slots that accommodate the user's choice of interface modules— such at Ethernet, Token Ring, or FDDI—for connectivity to LANs, WANs, or other network devices. The number of ports on the NIC determines the number of users in the particular star. Intelligent hubs often provide integrated management and internetworking capabilities, as well as Simple Network Management Protocol (SNMP)-based network management. New generations also offer bridging, routing, and switching functions.
Figure 8.7 shows a network that uses a combination of interconnection devices. Intelligent hubs provide connectivity between workstations that comprise a given cluster. An internal backbone is used to internetwork the intelligent hubs to move between different clusters. Those intelligent hubs then connect into a backbone router for purposes of WAN, or campuswide, connectivity.
Figure 8.7. Using interconnection devices
LAN Switches
LAN switches are a very cost-effective solution to the need for increased bandwidth in workgroups. Each port on the switch delivers a dedicated channel to the device or devices attached to that port, thereby increasing the workgroup's total bandwidth and also increasing the bandwidth available to individual users.
Figure 8.8 shows a simple example of a switched Ethernet configuration. One workstation requires 10Mbps on its own, so it has the full services of a 10Mbps port on the switched Ethernet card. Five workstations, on the other hand, each need 2Mbps, so one 10Mbps port serves all five workstations. These five workstations connect into a hub, and that hub connects into the actual port. Servers have extra bandwidth requirements—the ones in Figure 8.8 require 25Mbps—so they are each served by a bonding of several 10Mbps ports.
The key applications for LAN switches are to interconnect the elements of a distributed computing system, to provide high-speed connections to campus backbones and servers, and to provide high bandwidth to individual users who need it. Instead of sharing a 10Mbps LAN among a number of terminals in a workgroup, a LAN switch can be used, and an individual workstation can get the entire 10Mbps. LAN switches provide great scalability because they enable the network to increase in bandwidth with the fairly simple addition of more switched ports. Thus, LAN switches have many benefits, including scalability in terms of bandwidth, flexibility, and high performance.
Figure 8.9 shows how an Ethernet switch can be used to connect devices that are on the same segment, some of which are served by one shelf of the Ethernet switch and others of which are served by connecting shelves together. On the backplane, you can provide internetworking between the Ethernet segments, so you can provide internetworking on a campuswide basis.
Figure 8.9. An Ethernet switch
more sophisticated, we have been increasing the bandwidth associated with LANs. Today, it is common to see 10Mbps being delivered to an individual desktop and 100Mbps serving as the cluster capacity. To facilitate internetworking between these high-capacity desktops and Fast Ethernet clusters, Gigabit Ethernet is increasingly being used in the backbone. As shown in Figure 8.3 earlier in the chapter, Gigabit Ethernet switches can connect underlying 100Mbps or 10Mbps LAN
segments, and the 10Mbps or 100Mbps LAN switches can deliver 10Mbps to the desktop and 100Mbps to the segment.
VLANs
Switched LANs enable us to create VLANs, which don't completely fit the earlier definition of a LAN as being limited in geographical scope. With a VLAN, geography has no meaning. You could have two people in a Singapore office, three in New York, one in London, and four in Cairo, and they could all be part of the same LAN, a VLAN, because the LAN is defined by software rather than by hardware and location. Figure 8.10 shows an example of a VLAN.
Figure 8.10. A VLAN
A switched VLAN is a high-speed, low-latency broadcast group that unites an arbitrary collection of endstations on multiple LAN segments. Switched virtual networking eliminates the bottlenecks that are normally associated with a physical LAN topology by creating high-speed switched connections between endstations on different LAN segments. Users who want to belong to a particular broadcast domain do not have to be physically located on that LAN segment.
VLANs provide a software-based, value-added function by enabling the creation of a virtual broadcast domain, a shared LAN segment within a switched environment. Switching latencies on VLANs are typically one-tenth those of fast routers. However, routers are still required for inter- VLAN communications.
Bridges
Bridges entered the networking scene before routers. Applications for bridges include connecting network segments (for example, by taking 5 to 10 individual clusters and creating the appearance of a single logical VLAN). A bridge can also be used to increase the number of computers on a network or to extend the distance of a segment beyond what the specifications allow. Similarly, a bridge can
network traffic. Bridges can connect similar as well as dissimilar networks. Bridges have several important functions:
• Learning— When the bridge is first connected to the network, it sends an announcement that
says, "Hello. I'm your new bridge. What's your address?" All the other devices respond with, "Hello. Welcome to the neighborhood," along with their addresses. The bridge builds a table of local addresses, called the Media Access Control sublayer addresses. The MAC sublayer (which is equivalent to OSI Layer 2) controls access to the shared transmission medium. It is responsible for making the data frames and putting bits in fields that make sense, and it works with the physical layer, Layer 1. MAC standards, including IEEE 802.3, 802.4, and 802.5, define unique frame formats. Every NIC ever made has a globally unique burned-in MAC address.
• Performing packet routing— Bridges either filter, ignore, or forward packets.
• Using the Spanning Tree Algorithm— Bridges use the Spanning Tree Algorithm to select the
most efficient network path and to disable all the other potential routes.
Figure 8.11 illustrates a local bridge installed between two LAN segments that are located at the same local premise. When the bridge is plugged in, it sends out a hello message to its community; the devices answer, and the bridge builds an addressing table. Say that PC A wants to send a
document to Printer 1. The bridge realizes that that printer resides within its community. It knows the address and it therefore does not do anything except filter the packet. On the other hand, if PC A is attempting to communicate with Server Z, the bridge says, "Well, I don't know where that server is. It's not part of my local community, so it must be somewhere else on the other side of this bridge." The bridge then broadcasts that information out to as many LAN segments as are connected to the other side of the bridge. In essence, the bridge creates broadcast storms.
Figure 8.11. An example of a local bridge
Bridges are not networkable devices; they can't target a destination network. All they can determine is whether a destination is or is not on its segment, and if the destination is somewhere else, the bridge sends a message to every somewhere else that it knows about. This can be an especially big problem if you use a bridge in a remote mode, as shown in Figure 8.12, because, in essence, you are trying to connect together remote locations by using a WAN link, which is expensive in terms of bandwidth. You pay for every bit sent, so sending messages to LAN segments that don't need to see
them across a WAN link that doesn't need to be congested is inefficient.
Figure 8.12. An example of remote bridges
Although bridges can operate in local and remote areas, today they are mostly used in the local environment. They operate at OSI Layer 2 and they are point-to-point—they do not understand networking or routing and relaying through a series of nodes. Bridges are protocol independent (Layer 3 and up), which keeps the software simple and inexpensive. Bridges cannot translate between different Layer 2 protocols (for example, between Ethernet and Token Ring). Bridges are primarily used to isolate traffic loads in the local environment because they offer fast throughput; a bridge doesn't have to do intelligent routing, which makes it faster and less expensive than a traditional router. Over time we've merged together the best features of bridges and routers so that some of the problems with each have begun to disappear.
Routers
The most popular internetworking device today is the router (see Figure 8.13). The applications for routers are quite similar to those for bridges. You use them for network segmentation and
connection; that is, you use them either to segment larger networks into smaller ones or to connect smaller networks into a larger virtual whole. You can use a router to switch and route packets across multiple communications paths and disparate Layer 2 network types, and because it is a Layer 3 device, a router is networkable—it understands how to read network addresses and how to select the destination or target network, so it prohibits broadcast storms. This capability allows routers to act as firewalls between LAN segments. Routers can be associated with traffic filtering and isolation, and because they can read information about the network and transport protocols used, they can make