Connectivity Devices
The previous hour dealt extensively with the important topic of routers on TCP/IP networks. Although routers are an extremely important and fundamental concept, they are just one of many connectivity devices you’ll find on a TCP/IP network.
Many types of connectivity devices exist, and they all play a role in managing traffic on TCP/IP networks. The following sections discuss bridges, hubs, and switches.
Bridges
A bridge is a connectivity device that filters and forwards packets by physical address. Bridges operate at the OSI Data Link layer (which, as described in Hour 3, falls within the TCP/IP Network Access layer). In recent years, bridges have become much less common as networks move to more versatile devices, such as switches. However, the simplicity of the bridges makes it a good starting point for this discussion of connectivity devices.
Although a bridge is not a router, a bridge still uses a routing table as a source for delivery information. This physical address–based routing table is considerably different from and less sophisticated than the routing tables described later in this hour.
A bridge listens to each segment of the network it is connected to and builds a table showing which physical address is on which segment. When data is transmitted on one of the network segments, the bridge checks the destination address of the data and consults the routing table. If the destination address is on the segment from which the data was received, the bridge ignores the data. If the destination address is on a different segment, the bridge forwards the data to the appropriate segment. If the destination address isn’t in the routing table, the bridge forwards the data to all segments except the segment from which it received the transmission.
By the Way
It is important to remember that the hardware-based physical addresses used by a bridge are different from the logical IP addresses. See Hours 1–4 for more on the difference between physical and logical addresses.
Bridges were once common on LANs as an inexpensive means of filtering traffic, and therefore increasing the number of computers that can participate in the network. As you learned earlier in this hour, the bridge concept is now embodied in certain network access devices such as cable modems and some DSL devices. Because bridges use only Network Access layer physical addresses and do not examine logical addressing information available in the IP datagram header, bridges are not useful for connecting dissimilar networks. Bridges also cannot assist with the IP routing and delivery schemes used to forward data on large networks such as the Internet.
Hubs
In the early years of ethernet, most networks used a scheme that connected the computers with a single, continuous coaxial cable. In recent years, 10BASE-T–style hub-based ethernet has become the dominant form. Almost all ethernet networks today use a central hub or switch to which the computers on the network connect (see Figure 9.19).
Figure 9.19. A hub-based ethernet network.
As you’ll recall from Hour 3, the classic ethernet concept calls for all computers to share the transmission medium. Each transmission is heard by all network adapters. An ethernet hub receives a transmission from one of its ports and echoes that transmission to all of its other ports (refer to Figure 9.19). In other words, the network behaves as if all computers were connected using a single continuous line. The hub does not filter or route any data. Instead, the hub just receives and retransmits signals.
One of the principal reasons for the rise of hub-based ethernet is that in most cases a hub simplifies the task of wiring the network. Each computer is connected to the hub through a single line. A computer can easily be detached and reconnected. In an office setting where computers are commonly grouped together in a small area, a single hub can serve a close group of computers and can be connected to other hubs in other parts of the network. With all cables connected to a single device, vendors soon began to realize the opportunities for innovation. More sophisticated hubs, called intelligent hubs, began to appear. Intelligent hubs provided additional features, such as the capability to detect a line problem and block off a port. The hub has now largely been replaced by the switch, which you learn about in the next section.
Switches
A hub-based ethernet network still faces the principal liability of the ethernet: Performance degrades as traffic increases. No computer can transmit unless the line is free. Furthermore, each network adapter must receive and process every frame placed on the ethernet. A smarter version of a hub, called a switch, was developed to address these problems with ethernet. In its most fundamental form, a switch looks similar to the hub shown in Figure 9.19. Each computer is attached to the switch through a single line. However, the switch is smarter about where it sends the data received through one of its ports. Most switches associate each port with the physical address of the adapter connected to that port (see Figure 9.20). When one of the computers attached to the port transmits a frame, the switch checks the destination address of the frame and sends the frame to the port associated with that destination address. In other words, the switch sends the frame only to the adapter that is supposed to receive it. Every adapter does not have to examine every frame transmitted on the network. The switch reduces superfluous transmissions and therefore improves the performance of the network.
Figure 9.20. A switch associates each port with a physical address.
Note that the type of switch I just described operates with physical addresses (see Hour 3) and not IP addresses. The switch is not a router. Actually, a switch is more like a bridge—or, more accurately, like several bridges in one. The switch isolates each of its network connections so that only data coming from or going to the computer on the end of the connection enters the line (see Figure 9.21).
Several types of switches are now available. Two of the most common switching methods are
Cut-through— The switch starts forwarding the frame as soon as it obtains the destination address.
Store and forward— The switch receives the entire frame before retransmitting. This method slows down the retransmission process, but it can sometimes improve overall performance because the switch filters out fragments and other invalid frames.
Switches have become increasingly popular in recent years. Corporate LANs often use a collection of layered and interconnected switches for optimum performance.
By the Way
Some vendors now view the fundamental switch concept described earlier in this section as a special case of a larger category of switching devices. More sophisticated switches operate at higher protocol layers and can, therefore, base forwarding decisions on a greater variety of parameters. In this more general approach to switching, devices are classified according to the highest OSI protocol layer at which they operate. Thus, the basic switch described earlier in this section, which operates at OSI’s Data Link layer, is known as a Layer 2 switch. Switches that forward based on IP address information at the OSI Network layer are called Layer 3 switches. (As you might guess, a Layer 3 switch is essentially a type of router.) If no such layer designation is applied to the switch, assume it operates at Layer 2 and filters by physical (MAC) address, as described in this section.