Prof. M. Gerla & R. Dzhanidze. Class CS 117. Ch.2a 1

Chapter 2a

Introduction to Computer Communications

and

Networking

1. Communication Link

2. General Definition

3. Example of Computer Communication
Systems
4. Networking / a. Telephone Network
b. Computer Networks
c. Cable Television
d. Wireless Networks.
5. Communication Standards
6. OSI/RM
7. Layer Descriptions
8. The TCP/IP Reference Model / a. Protocol Hierarchies
b. Internet Layer
c. Transport Layer
d. Application Layer
e. Host-to-Network Layer
9. Packet Switching and Circuit Switching
10. Connection-oriented and Connectionless Services

Reference:

“Computer Networks”, Andrew S. Tanenbaum, pg. 1-84.

1.Communication Link

The goal of a communication is to transmit information from a source to a destination. The diagram in Figure 1 shows the steps in transmitting information. At each stage, the form of the signal may change, but the information should still be contained within the signal.

Figure 1. Block diagram of a general communication system.

For instance, a voice radio system receives voice information at stage 1. A microphone changes it to an electrical signal at stage 2, and the radio transmitter modulates this signal with a carrier signal at stage 3. The transmitted signal experiences attenuation and interference while in the transmission medium, so the received signal at stage 4 will not be identical to the transmitted signal. The receiver demodulates the signal at stage 5, and a speaker converts this electrical signal at stage 6. In a successful communication system, the output signal will carry the same information as the input signal.

2.General Definitions

Information is the meaning that a human being assigns to data by means of the conventions applied to those data.

Data is a representation of facts, concepts, or instructions in a formalized manner suitable for communication, interpretation, or processing by human beings or by automatic means. In other words, data conveys meaning.

Signals are the physical encoding of data, whether by material, electric, or electromagnetic means. Signals can be analog (continuous in time and amplitude), discrete (discrete in time, continuous in amplitude), or digital (discrete in time and amplitude).

An analog signal is a continuously varying signal. For example, voice is an analog signal, a continuously varying pattern of compression waves in frequencies from 20to20,000 Hz.

A discrete signal is a sequence of analog values which changes once every interval. For example, a sensor may report temperature once every second, and encode the temperature as a voltage. It will hold this voltage for one minute, then change the voltage on the next reading.

A digital signal is a sequence of digital values which changes once every interval. For example, a sensor may test the temperature of an engine once per second, and report whether the temperature is greater than 200 degrees. If the temperature is higher than 200 degrees, it will send a voltage of +5 Volts on the transmission line. If the temperature is less than 200 degrees, it will send a voltage of –5Volts. The digital signal is a voltage that changes once per second, with a 1 represented as +5Volts, and a 0 represented as –5Volts.

3.Example of a Computer Communications System

Figure 2 shows a point-to-point computer communications system. A digital signal from the source computer is encoded so that bit errors can be detected and corrected at the destination. (If transmission is sufficiently reliable, the encoder may use only an error detection code, and any messages with errors will be retransmitted.)

The encoded signal is modulated for transmission, and at the destination it is demodulated into an analog signal. The encoded digital signal is regenerated from this analog signal, the errors are corrected, and the original message reproduced.

Figure 2: Block diagram of a computer communicationsystem.

4. Networking

Communication networks enable many users to communicate to each other’s, and to transfer information in form of voice, video, electronic mail, and computer files. Principles of communication system are different for different approach.

a. In Telephone Network conversation is accomplished along one link. To set up a call, a set of circuits has to be connected, joining the two telephone sets. Circuit switching occurs at the beginning of new telephone call by switching of circuit, where "circuit" reefers to the capability of transmitting one telephone conversation. An electronic interface in the switch converts the analog signal traveling on the link from the telephone set to the switch into digital signal called a bit stream. The same interface converts the digital signal that travels between the switches into an analog signal before sending it from the switch to the telephone (Figure 3).

Figure 3. Phone connection to digital network.

The switches themselves are computers, which makes them very flexible. This flexibility allows such services as call waiting, call forwarding, and call back.Since the 1980s the transmission links of the telephone network have been changing to the SONET, or Synchronous Optical Network, standard. SONET rates are arranged in the Synchronous Transfer Signal (STS), hierarchy shown in

Table 1.


Table 1

STS-n rate is exactly n times the STS-1 rate. Because all clocks in SONET network are synchronized to the same master clock, it is possible to compose an STS-n signal by multiplexing exactly n numbers of STS-1 signals.

b. In Computer or Data Communication Networks organization of data exchange is in packets, and data communications are organized by packet switching.

Using the packet switching techniques and rules of operations (protocols) was developed packet switching networks, so called ARPANET. The ARPANET used single packet format and addressing scheme. Through the ARPANET protocols is easy to interconnect networks, and it was later evolved into the Internet. The Internet is used to interconnect a large number of computers and local area networks throughout the world. Development of the ARPANET architecture was formalized layered model of Open System Interconnection, or OSI model.

Let us consider the simplest model of company’s information system consisting one or more databases and some number of system users (employees). In this model, Figure 4, the data are stored on powerful computers called servers. Often these are centrally housed and maintained by a system administrator. In contrast, the employees have simpler machines, called clients, on their desks.

The packet switching technique in some of networks is based on so called multiple access method of computer interconnections. It dramatically reduces the cost of interconnection nearby computers in a Local Area Network (LAN).

Figure 4. Model of the information system.

Broadly spiking, there are two types of transmission technology:

  1. Broadcast links.
  2. Point-to-point links.

Broadcast networks have a single communication channel that is shared by all the machines on the network. Short messages, called packets, sent by any machine are received by all the others. An address field within the packet specifies the intended recipient. Broadcast systems generally also allow the possibility of addressing a packet to all destinations by using a special code in the address field. When a packet with this code is transmitted, it is received and processed by every machine on the network, this mode of operation is called broadcasting.

Point-to-point networks consist of connections between two machines. To go from the source to the destination, a packet on this type of network may have to first visit one or more intermediate machines, often multiple routers. As a general rule, smaller networks, geographically localized networks tend to use broadcasting, whereas larger networks usually are presented as a point-to-point interconnection of the small networks. Point-to-point transmission with one sender and one receiver is called uncasting.

ALocal Area networks (LAN) are privately-owned networks within a single building or campus of up to few kilometers in size. LANs are distinguished from other kinds of networks by three characteristics: (1) their size, (2) their transmission technology, (3) their topology.

A popular implementation of multiple accesses method in LAN is an Ethernet Network, IEEE 802.3 standard. Ethernet network is a buss-based broadcast networks with decentralized control. In Ethernet computers are attached to a common cable via an interface that today consists of small chip set mounted on the main board. When computer, attached to the network, Figure 5 a, wants to send a packet to another computer, it puts the own address and destination address into the packet header and transmits the packet on the cable. All the computers attached to the network read the packet, but only the computer with the destination address indicated on the packet copies it. Ethernet transmission rate 10 Mbps, 100 Mbps even 1Gbps are available.

Another popular multiple access method is so called Token Ring Network, IEEE 802.5 standard. Token Ring networks are ring-based broadcast LAN networks operating at 4 and 16 Mbps rate. In token rings computers are attached by point-to-point links in a unidirectional ring configuration, Figure 5 b, using token ring interface boards. When the computers have no information to transmit, the interfaces pass a token around the ring. The interface boards between the computers and the network are configured so that they put back on the ring whatever information they receive with a delay equal to a few bit transmission times. This enables the token to circulate very fast around the ring.

Suppose computer “A” wants to send a packet to computer “B”. Computer “A” puts the source address “A” and the destination address “B” into the packet header and gives the packet to its interface, which then waits for the token. As soon as it

Figure 5. Topology of the Ethernet (a) and Token Ring networks (b).

gets the token, the interface of computer “A” transmits this packet, instead of forwarding the token. The other computers keep forwarding the packet they receive while making a copy for themselves. In particular, the interface of computer “B” copies the packet destined to it. The other interfaces discard their copy of the packet when they find out that it is not for them. When “A” receives the last bit of its own packet, after the packet has traveled around the ring, it puts the token back on the ring. What is important is that the computers get to transmit in turn, when they get the token. Hardware is available for token ring networks at 4 Mbps and at 16 Mbps.

The maximum time a computer waits before it gets to transmit in a token ring or Ethernet network is small enough for many applications but too large for interactive audio or video applications. Also, the transmission rate of token ring (4 or 16 Mbps) or 10-Mbps Ethernet networks are too slow for some multimedia applications.

These limitations of Token Ring network led to develop a new network called: Fiber Distributed Data Interface (FDDI). FDDI network use optical fibers to transmit at 100 Mbps; access to the channel is regulated by a timed-token mechanism. This mechanism is similar to the access control of a token ring, but with FDDI, the arrivals of token are timed to assure that they are retransmitted within a fixed time. The high speed of FDDI makes it suitable for networking workstations with instruction rates of a few hundred Mbps. Because it can guarantee a token rotation time, FDDI can offerintegrated services for applications that combine audio and video with data.

AMetropolitan Area Networks (MAN) covers a city. A MAN represents as an interconnected LANs by point-to-point communication links. The interconnection is controlled by switches that are finding the better way to connect with desired LAN.

A Wide Area Network (WAN) spans a large geographical area, often a country or continent. It contains a collection of machines intended for running user programs, Figure 6. Traditionally these machines called hosts. The hosts are connected by a communication subnet, or just subnet. The hosts are owned by the costumers, whereas the subnet is typically owned and operated by a telephone company or Internet service provider. The job of the subnet is to carry messages from host to host. Separation of the pure communication aspects of the network (task of a subnet) from the application aspects (task of host), simplifies the complete network design.

The subnet consists of two distinct components:transmission lines and switching elements. Transmission lines move bits between machines. They can be made of copper wire, optical fiber, or radio links. Switching elements are specialized computers that connect three or more transmission lines. When data arrive on an incoming line, the switching element must choose an outgoing line on which to forward them. These switching computers call router. The collections of communication lines and routers (but not the hosts) form the subnet.

Faster LAN and WAN networks are today being deployed, using Asynchronous Transfer Mode (ATM) technology. With ATM, a computer transmits information at rates between 25 Mbps and 2.5 Gbps in packets of 53 bytes. These fixed-size packets, called cells, can be switched rapidly by ATM switches. Part of the header of ATM packet (cell) is the connection identifier and contains a virtual circuit address, instead of a source and destination address. With the appropriate control software, network can be connected many ATM switches together to build large networks. Moreover, the links between ATM switches can be long optical fibers. Using this technology companies can build a worldwide network. In the ATM network, data is transmitted from source to destination over a fixed route, just like in a telephone connection. Unlike telephone networks, however, an ATM connection is not allocated at a fixed bandwidth. The ATM network determines how much bandwidth needed to allocate signal, so that information is transmitted with very low loss rate or delay, as required by the application. Since ATM is connection-oriented, sending data requires first sending a packet to set up the connection. As the setup packet wends its way through the subnet, all the routers on the path make an entry in their internal tables nothing the existence of the connection and reserving whatever resources are needed for it. ATM communication is similar to leased lines in the telephone world. Each connection, temporarily or permanent has a unique connection identifier. Once a connection has been established, either side can being transmitting data.

Thus, this technology is suited for building large integrated services networks. Figure 7 summarizes the increase in speed of data networks. Over the 30 years since 1970, speed has increased by six orders of magnitude, from 10 Kbps to 10 Gbps.

Figure 6. Wide Area Network (WAN)


Figure 7. The speed of data networks increased by six orders of magnitude

c. Cable Television, originally known as Community Antenna Television or CATV. CATV was created when the signal from one master antenna was distributed over a large area using coaxial cable and amplifiers. The key innovations in cable TV are optical feeder links, digital compression techniques, and service integration. Today cable TV uses frequency-division multiplexing to transmit up to 69 analog TV channels, each 4.5 MHz wide. Transmission is over coaxial cables arranged as a unidirectional tree, with wideband amplifiers used to compensate for the attenuation of the cable signal. The number of TV a channels is limited by the bandwidth of coaxial cables. The span of a CATV network is limited by the noise power, which increases as more amplifiers are added to compensate for the signal power loss during propagation.

Optical fibers are used to transmit the TV signals over longer distance. Fibers have a much lower attenuation than coaxial cables, so they can transmit signals over longer distances before it becomes necessary to use an amplifier. In this implementation, the transmission over the fiber is still analog. The signal is fed into the coaxial cable network at various points, where the optical signal is converted into electrical signals. The cost of each optical transmission line is spread over a few hundred users. Moreover, existing coaxial cables can be reused. This hybrid fiber/coaxial cable distribution system has a longer span and better signal quality than a coaxial cable network. This network called fiber-to-the-curb (FTTC) network, where "curb" designates a location in some neighborhood where the fiber is connected to the local coaxial distribution network.

To increase the number of TV channels, the CATV industry is now migrating to a digital transmission technology. Before transmitting the TV signals, the CATV company uses a TV codec (coder-decoder) that converts each signal into a bit stream that represents the video frames. Using algorithms that have been standardized by Motion Pictures Expert Group (MPEG), the codec compresses the bit stream to reduce its rate. The bit streams are transmitted over fibers to the curb and then distributed by the neighborhood coaxial network. The compression gain now allows the network to transmit about 500 TV channels. Using the first version of the MPEG standard, MPEG1, a moderate-quality TV signal is encoded as a 1.5 Mbps bit stream, which can be modulated in a signal that has a bandwidth of about 600 kHz. Set-up boxes at the user residence perform the decompression. This CATV network is still unidirectional. To provide new services, such as video on demand, Internet access, and telephony, the CATV industry is organizing bidirectional networks. Such a network connects video servers to users by means of control messages. The user choices these messages to select the video program, and the video program is sent over the network to the user. A cable modem can give users access to a shared 3 Mbps (or 10 Mbps) upstream channel to the Internet.

d. Wireless Networks. The first packet-switched wireless network was developed in 1971. Alohanet, as the network was called, interconnected computers on four islands in a star topology: two computers could exchange packets through a central computer hub.