A channel is a communication medium, the path that data takes from source to destination. A channel can be comprised of so many different things: wires, free space, and entire networks. Signals can be routed from one type of network to another network with completely different characteristics. In the Internet, a packet may be sent over a wireless WiFi network to an ethernet lan, to a DSL modem, to a fiber-optic backbone, et cetera. The many unique physical characteristics of different channels determine the three characteristics of interest in communication: the latency, the data rate, and the reliability of the channel.
Bandwidth and Bitrate
Bandwidth is the difference between the upper and lower cutoff frequencies of, for example, a filter, a communication channel, or a signal spectrum. Bandwidth, like frequency, is measured in hertz (Hz). The bandwidth can be physically measured using a spectrum analyzer.
Bandwidth, given by the variables Bw or W is closely related to the amount of digital bits that can be reliably sent over a given channel:
where rb is the bitrate. If we have an M-ary signaling scheme with m levels, we can expand the previous equation to find the maximum bit rate for the given bandwidth.
Example: Bandwidth and Bitrate
Let's say that we have a channel with 1KHz bandwidth, and we would like to transmit data at 5000 bits/second. We would like to know how many levels of transmission we would need to attain this data rate. Plugging into the second equation, we get the following result:
However, we know that in M-ary transmission schemes, m must be an integer. Rounding up to the nearest integer, we find that m = 3.
The "capacity" of a channel is the theoretical upper-limit to the bit rate over a given channel that will result in negligible errors. Channel capacity is measured in bits/s.
Shannon's channel capacity is an equation that determines the information capacity of a channel from a few physical characteristics of the channel. A communication systems can attempt to exceed the Shannon's capacity of a given channel, but there will be many errors in transmission, and the expense is generally not worth the effort. Shannon's capacity, therefore, is the theoretical maximum bit rate below which information can be transmitted with negligible errors.
The Shannon channel capacity, C, is measured in units of bits/sec and is given by the equation:
C is the maximum capacity of the channel, W is the available bandwidth in the channel, and SNR is the signal to noise ratio, not in DB.
Because channel capacity is proportional to analog bandwidth, some people call it "digital bandwidth".
Channel Capacity Example
The telephone network has an effective bandwidth less then 3000Hz (but we will round up), and transmitted signals have an average SNR less than 40dB (10,000 times larger). Plugging those numbers into Shannon's equation, we get the following result:
we can see that the theoretical maximum channel capacity of the telephone network (if we generously round up all our numbers) is approximately 40Kb/sec!. How then can some modems transmit at a rate of 56kb/sec? it turns out that 56k modems use a trick, that we will talk about in a later chapter.
Digital information packets have a number of overhead bits known as a header. This is because most digital systems use statistical TDM (as discussed in the Time-Division Multiplexing chapter). The total amount of bits sent in a transmission must be at least the sum of the data bits and the header bits. The total number of bits transmitted per second (the "throughput") is always less than the theoretical capacity. Because some of this throughput is used for these header bits, the number of data bits transmitted per second (the "goodput") is always less than the throughput.
In addition, since we all want our information to be transmitted reliably, it makes good sense for an intelligent transmitter and an intelligent receiver to check the message for errors.
An essential part of reliable communication is error detection, a subject that we will talk about more in depth later. Error detection is the process of embedding some sort of checksum (called a CRC sum in IP communications) into the packet header. The receiver uses this checksum to detect most errors in the transmission.
forward error correction
Some systems use forward error correction (FEC), a subject that we will talk about more in depth later. In such a system, the transmitter builds a packet and adds error correction codes to the packet. Under normal conditions -- with very few bit errors -- that gives the receiver enough information to not only determine that there was some sort of error, but also pinpoint exactly which bits are in error, and fix those errors.
ARQ: ACK and NAK
In addition, since we all want our information to be transmitted reliably, it makes good sense for an intelligent transmitter and an intelligent receiver to communicate directly to each other, to ensure reliable transmission. This is called acknowledgement, and the process is called hand-shaking.
In an acknowledgement request (ARQ) scheme, the transmitter sends out data packets, and the receiver will then send back an acknowledgement. A positive acknowledgement (called "ACK") means that the packet was received without any detectable errors. A negative acknowledgement (called "NAK") means that the packet was received in error. Generally, when a NAK is received by the transmitter, the transmitter will send the packet again.
If the transmitter fails to receive a ACK in a reasonable amount of time, the transmitter will send the packet again.
In some streaming protocols, such as RTP, the transmitter is sending time-sensitive data, and it can therefore not afford to wait for acknowledgement packets. In these types of systems, the receiver will attempt to detect errors in the received packets, and if an error is found, and it cannot be immediately corrected with FEC, the bad packet is simply deleted.