# Connecting two hosts¶

The first step when building a network, even a worldwide network such as the Internet, is to connect two hosts together. This is illustrated in the figure below.

To enable the two hosts to exchange information, they need to be linked together by some kind of physical media. Computer networks have used various types of physical media to exchange information, notably :

• electrical cable. Information can be transmitted over different types of electrical cables. The most common ones are the twisted pairs (that are used in the telephone network, but also in enterprise networks) and the coaxial cables (that are still used in cable TV networks, but are no longer used in enterprise networks). Some networking technologies operate over the classical electrical cable.

• optical fiber. Optical fibers are frequently used in public and enterprise networks when the distance between the communication devices is larger than one kilometer. There are two main types of optical fibers : multi-mode and single-mode. Multi-mode is much cheaper than single-mode fiber because a LED can be used to send a signal over a multi-mode fiber while a single-mode fiber must be driven by a laser. Due to the different modes of propagation of light, multi-mode fibers are limited to distances of a few kilometers while single-mode fibers can be used over distances greater than several tens of kilometers. In both cases, repeaters can be used to regenerate the optical signal at one endpoint of a fiber to send it over another fiber.

• wireless. In this case, a radio signal is used to encode the information exchanged between the communicating devices. Many types of modulation techniques are used to send information over a wireless channel and there is lot of innovation in this field with new techniques appearing every year. While most wireless networks rely on radio signals, some use a laser that sends light pulses to a remote detector. These optical techniques allow to create point-to-point links while radio-based techniques can be used to build networks containing devices spread over a small geographical area.

## The physical layer¶

These physical media can be used to exchange information once this information has been converted into a suitable electrical signal. Entire telecommunication courses and textbooks are devoted to the problem of converting analog or digital information into an electrical signal so that it can be transmitted over a given physical link. In this book, we only consider two very simple schemes that allow to transmit information over an electrical cable. This enables us to highlight the key problems when transmitting information over a physical link. We are only interested in techniques that allow transmitting digital information through the wire. Here, we will focus on the transmission of bits, i.e. either 0 or 1.

Note

Bit rate

In computer networks, the bit rate of the physical layer is always expressed in bits per second. One Mbps is one million bits per second and one Gbps is one billion bits per second. This is in contrast with memory specifications that are usually expressed in bytes (8 bits), KiloBytes (1024 bytes) or MegaBytes (1048576 bytes). Transferring one MByte through a 1 Mbps link lasts 8.39 seconds.

Bit rate

Bits per second

1 Kbps

$$10^3$$

1 Mbps

$$10^6$$

1 Gbps

$$10^9$$

1 Tbps

$$10^{12}$$

To understand some of the principles behind the physical transmission of information, let us consider the simple case of an electrical wire that is used to transmit bits. Assume that the two communicating hosts want to transmit one thousand bits per second. To transmit these bits, the two hosts can agree on the following rules :

• On the sender side :
• set the voltage on the electrical wire at +5V during one millisecond to transmit a bit set to 1

• set the voltage on the electrical wire at -5V during one millisecond to transmit a bit set to 0

• On the receiver side :
• every millisecond, record the voltage applied on the electrical wire. If the voltage is set to +5V, record the reception of bit 1. Otherwise, record the reception of bit 0

This transmission scheme has been used in some early networks. We use it as a basis to understand how hosts communicate. From a Computer Science viewpoint, dealing with voltages is unusual. Computer scientists frequently rely on models that enable them to reason about the issues that they face without having to consider all implementation details. The physical transmission scheme described above can be represented by using a time-sequence diagram.

A time-sequence diagram describes the interactions between communicating hosts. By convention, the communicating hosts are represented in the left and right parts of the diagram while the electrical link occupies the middle of the diagram. In such a time-sequence diagram, time flows from the top to the bottom of the diagram. The transmission of one bit of information is represented by three arrows. Starting from the left, the first horizontal arrow represents the request to transmit one bit of information. This request is represented by a primitive which can be considered as a kind of procedure call. This primitive has one parameter (the bit being transmitted) and a name (DATA.request in this example). By convention, all primitives that are named something.request correspond to a request to transmit some information. The dashed arrow indicates the transmission of the corresponding electrical signal on the wire. Electrical and optical signals do not travel instantaneously. The diagonal dashed arrow indicates that it takes some time for the electrical signal to be transmitted from Host A to Host B. Upon reception of the electrical signal, the electronics on Host B’s network interface detects the voltage and converts it into a bit. This bit is delivered as a DATA.indication primitive. All primitives that are named something.indication correspond to the reception of some information. The dashed lines also represents the relationship between two (or more) primitives. Such a time-sequence diagram provides information about the ordering of the different primitives, but the distance between two primitives does not represent a precise amount of time.

Time-sequence diagrams are useful when trying to understand the characteristics of a given communication scheme. When considering the above transmission scheme, it is useful to evaluate whether this scheme allows the two communicating hosts to reliably exchange information. A digital transmission is considered as reliable when a sequence of bits that is transmitted by a host is received correctly at the other end of the wire. In practice, achieving perfect reliability when transmitting information using the above scheme is difficult. Several problems can occur with such a transmission scheme.

The first problem is that electrical transmission can be affected by electromagnetic interference. Interference can have various sources including natural phenomenons (like thunderstorms, variations of the magnetic field,…) but also other electrical signals (such as interference from neighboring cables, interference from neighboring antennas,…). Due to these various types of interference, there is unfortunately no guarantee that when a host transmit one bit on a wire, the same bit is received at the other end. This is illustrated in the figure below where a DATA.request(0) on the left host leads to a Data.indication(1) on the right host.

With the above transmission scheme, a bit is transmitted by setting the voltage on the electrical cable to a specific value during some period of time. We have seen that due to electromagnetic interference, the voltage measured by the receiver can differ from the voltage set by the transmitter. This is the main cause of transmission errors. However, this is not the only type of problem that can occur. Besides defining the voltages for bits 0 and 1, the above transmission scheme also specifies the duration of each bit. If one million bits are sent every second, then each bit lasts 1 microsecond. On each host, the transmission (resp. the reception) of each bit is triggered by a local clock having a 1 MHz frequency. These clocks are the second source of problems when transmitting bits over a wire. Although the two clocks have the same specification, they run on different hosts, possibly at a different temperature and with a different source of energy. In practice, it is possible that the two clocks do not operate at exactly the same frequency. Assume that the clock of the transmitting host operates at exactly 1000000 Hz while the receiving clock operates at 999999 Hz. This is a very small difference between the two clocks. However, when using the clock to transmit bits, this difference is important. With its 1000000 Hz clock, the transmitting host will generate one million bits during a period of one second. During the same period, the receiving host will sense the wire 999999 times and thus will receive one bit less than the bits originally transmitted. This small difference in clock frequencies implies that bits can “disappear” during their transmission on an electrical cable. This is illustrated in the figure below.

A similar reasoning applies when the clock of the sending host is slower than the clock of the receiving host. In this case, the receiver will sense more bits than the bits that have been transmitted by the sender. This is illustrated in the figure below where the second bit received on the right was not transmitted by the left host.

From a Computer Science viewpoint, the physical transmission of information through a wire is often considered as a black box that allows transmitting bits. This black box is commonly referred to as the physical layer service and is represented by using the DATA.request and DATA.indication primitives introduced earlier. This physical layer service facilitates the sending and receiving of bits, by abstracting the technological details that are involved in the actual transmission of the bits as an electromagnetic signal. However, it is important to remember that the physical layer service is imperfect and has the following characteristics :

• the Physical layer service may change, e.g. due to electromagnetic interference, the value of a bit being transmitted

• the Physical layer service may deliver more bits to the receiver than the bits sent by the sender

• the Physical layer service may deliver fewer bits to the receiver than the bits sent by the sender

Many other types of encodings have been defined to transmit information over an electrical cable. All physical layers are able to send and receive physical symbols that represent values 0 and 1. However, for various reasons that are outside the scope of this chapter, several physical layers exchange other physical symbols as well. For example, the Manchester encoding used in several physical layers can send four different symbols. The Manchester encoding is a differential encoding scheme in which time is divided into fixed-length periods. Each period is divided in two halves and two different voltage levels can be applied. To send a symbol, the sender must set one of these two voltage levels during each half period. To send a 1 (resp. 0), the sender must set a high (resp. low) voltage during the first half of the period and a low (resp. high) voltage during the second half. This encoding ensures that there will be a transition at the middle of each period and allows the receiver to synchronize its clock to the sender’s clock. Apart from the encodings for 0 and 1, the Manchester encoding also supports two additional symbols : InvH and InvB where the same voltage level is used for the two half periods. By definition, these two symbols cannot appear inside a frame which is only composed of 0 and 1. Some technologies use these special symbols as markers for the beginning or end of frames.

All the functions related to the physical transmission or information through a wire (or a wireless link) are usually known as the physical layer. The physical layer allows thus two or more entities that are directly attached to the same transmission medium to exchange bits. Being able to exchange bits is important as virtually any information can be encoded as a sequence of bits. Electrical engineers are used to processing streams of bits, but computer scientists usually prefer to deal with higher level concepts. A similar issue arises with file storage. Storage devices such as hard-disks also store streams of bits. There are hardware devices that process the bit stream produced by a hard-disk, but computer scientists have designed filesystems to allow applications to easily access such storage devices. These filesystems are typically divided into several layers as well. Hard-disks store sectors of 512 bytes or more. Unix filesystems group sectors in larger blocks that can contain data or inodes representing the structure of the filesystem. Finally, applications manipulate files and directories that are translated in blocks, sectors and eventually bits by the operating system.

Computer networks use a similar approach. Each layer provides a service that is built above the underlying layer and is closer to the needs of the applications. The datalink layer builds upon the service provided by the physical layer. We will see that it also contains several functions.