destination is on a different segment, the frame can be transmitted only to that segment. This is called transparent bridging.
When a switch interface receives a frame with a destination hardware address that isn’t found in the device’s filter table, it will forward the frame to all connected segments. If the unknown device that was sent the “mystery frame” replies to this forwarding action, the switch updates its filter table regarding that device’s location. But in the event the destination address of the transmitting frame is a broadcast address, the switch will forward all broadcasts to every connected segment by default.
All devices that the broadcast is forwarded to are considered to be in the same broadcast domain. This can be a problem because layer 2 devices propagate layer 2 broadcast storms that can seriously choke performance, and the only way to stop a broadcast storm from propagating through an internetwork is with a layer 3 device – a router!
The biggest benefit of using switches instead of hubs in your internetwork is that each switch port is actually its own collision domain. Remember that a hub creates one large collision domain, which is not a good thing! But even armed with a switch, you still don’t get to just break up broadcast domains by default because neither switches nor bridges will do that. They’ll simply forward all broadcasts instead.
Another benefit of LAN switching over hub-centered implementations is that each device on every segment plugged into a switch can transmit simultaneously. Well, at least they can as long as there’s only one host on each port and there isn’t a hub plugged into a switch port! As you might have guessed, this is because hubs allow only one device per network segment to communicate at a time.
The Physical Layer
Finally arriving at the bottom, we find that the Physical layer does two things: it sends bits and receives bits. Bits come only in values of 1 or 0 – a Morse code with numerical values. The Physical layer communicates directly with the various types of actual communication media. Different kinds of media represent these bit values in different ways. Some use audio tones, while others employ state transitions– changes in voltage from high to low and low to high. Specific protocols are needed for each type of media to describe the proper bit patterns to be used, how data is encoded into media signals, and the various qualities of the physical media’s attachment interface.
The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. This layer is also where you identify the interface between the data terminal equipment (DTE) and the data communication equipment (DCE). (Some old phone-company employees still call DCE “data circuit-terminating equipment.”) The DCE is usually located at the service provider, while the DTE is the attached device. The services available to the DTE are most often accessed via a modem or channel service unit/data service unit (CSU/DSU).
The Physical layer’s connectors and different physical topologies are defined by the OSI as standards, allowing disparate systems to communicate. The Cisco exam objectives are interested only in the IEEE Ethernet standards.
Hubs at the Physical Layer
A hub is really a multiple-port repeater. A repeater receives a digital signal, reamplifies or regenerates that signal, then forwards the signal out the other port without looking at any data. A hub does the same thing across all active ports: any digital signal received from a segment on a hub port is regenerated or reamplified and transmitted out all other ports on the hub. This means all devices plugged into a hub are in the same collision domain as well as in the same broadcast domain. Figure 1.18 shows a hub in a network and how when one host transmits, all other hosts must stop and listen.
FIGURE 1.18 A hub in a network
Hubs, like repeaters, don’t examine any of the traffic as it enters or before it’s transmitted out to the other parts of the physical media. And every device connected to the hub, or hubs, must listen if a device transmits. A physical star network, where the hub is a central device and cables extend in all directions out from it, is the type of topology a hub creates. Visually, the design really does resemble a star, whereas Ethernet networks run a logical bus topology, meaning that the signal has to run through the network from end to end.
Topologies at the Physical layer
One last thing I want to discuss at the Physical layer is topologies, both physical and logical. Understand that every type of network has both a physical and a logical topology.
■ The physical topology of a network refers to the physical layout of the devices, but mostly the cabling and cabling layout.
■ The logical topology defines the logical path on which the signal will travel on the physical topology.
Figure 1.19 shows the four types of topologies.
FIGURE 1.19 Physical vs. Logical Topolgies
Here are the topology types, although the most common, and pretty much the only network we use today is a physical star, logical bus technology, which is considered a hybrid topology (think Ethernet):
■ Bus: In a bus topology, every workstation is connected to a single cable, meaning every host is directly connected to every other workstation in the network.
■ Ring: In a ring topology, computers and other network devices are cabled together in a way that the last device is connected to the first to form a circle or ring.
■ Star: The most common physical topology is a star topology, which is your Ethernet switching physical layout. A central cabling device (switch) connects the computers and other network devices together. This category includes star and extended star topologies. Physical connection is commonly made using twisted-pair wiring.
■ Mesh: In a mesh topology, every network device is cabled together with connection to each other. Redundant links increase reliability and self-healing. The physical connection is commonly made using fiber or twisted-pair wiring.
■ Hybrid: Ethernet uses a physical star layout (cables come from all directions), and the signal travels end-to-end, like a bus route.
Summary
Whew! I know this seemed like the chapter that wouldn’t end, but it did – and you made it through! You’re now armed with a ton of fundamental information; you’re ready to build upon it and are well on your way to certification.
I started by discussing simple, basic networking and the differences between collision and broadcast domains.
I then discussed the OSI model – the seven-layer model used to help application developers design applications that can run on any type of system or network. Each layer has its special jobs and select responsibilities within the model to ensure that solid, effective communications do, in fact, occur. I provided you with complete
