The physical layer must generate the electrical, optical, or wireless signals that represent the “1” and “0” on the media. The way that bits are represented is called the signaling method. The physical layer standards must define what type of signal represents a “1” and what type of signal represents a “0”. This can be as simple as a change in the level of an electrical signal or optical pulse. For example, a long pulse might represent a 1 whereas a short pulse might represent a 0.
This is similar to the signaling method used in Morse code, which may use a series of on-off tones, lights, or clicks to send text over telephone wires or between ships at sea.
Figures 30-7 through 30-9 show illustrations of signaling for copper cable, fiber-optic cable, and wireless media.
Figure 30-7 Electrical Signals Over Copper Cable
Figure 30-8 Light Pulses Over Fiber-Optic Cable
Figure 30-9 Microwave Signals Over Wireless
Video – Bandwidth (30.2.5)
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Different physical media support the transfer of bits at different rates. Data transfer is usually discussed in terms of bandwidth. Bandwidth is the capacity at which a medium can carry data. Digital bandwidth measures the amount of data that can flow from one place to another in a given amount of time. Bandwidth is typically measured in kilobits per second (kbps), megabits per second (Mbps), or gigabits per second (Gbps). Bandwidth is sometimes thought of as the speed that bits travel, however this is not accurate. For example, in both 10Mbps and 100Mbps Ethernet, the bits are sent at the speed of electricity. The difference is the number of bits that are transmitted per second.
A combination of factors determines the practical bandwidth of a network:
• The properties of the physical media
• The technologies chosen for signaling and detecting network signals
Physical media properties, current technologies, and the laws of physics all play a role in determining the available bandwidth.
Table 30-1 shows the commonly used units of measure for bandwidth.
Bandwidth Terminology (30.2.7)
Terms used to measure the quality of bandwidth include:
• Latency
• Throughput
• Goodput
Latency refers to the amount of time, including delays, for data to travel from one given point to another.
In an internetwork, or a network with multiple segments, throughput cannot be faster than the slowest link in the path from source to destination. Even if all, or most, of the segments have high bandwidth, it will only take one segment in the path with low throughput to create a bottleneck in the throughput of the entire network.
Throughput is the measure of the transfer of bits across the media over a given period of time.
Due to a number of factors, throughput usually does not match the specified bandwidth in physical layer implementations. Throughput is usually lower than the bandwidth. There are many factors that influence throughput:
• The amount of traffic
• The type of traffic
• The latency created by the number of network devices encountered between source and destination
There are many online speed tests that can reveal the throughput of an internet connection. The figure provides sample results from a speed test.
There is a third measurement to assess the transfer of usable data; it is known as goodput. Goodput is the measure of usable data transferred over a given period of time. Goodput is throughput minus traffic overhead for establishing sessions, acknowledgments, encapsulation, and retransmitted bits. Goodput is always lower than throughput, which is generally lower than the bandwidth.
Check Your Understanding – Physical Layer Characteristics (30.2.8)
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