Application Layer
The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services. The relationship of the application layer to the user and the presentation layer is shown in Figure 2.2-11. Of the many application services available, the figure shows only three, X.400 message handling services ,...
Network Layer 1
Once a connection has been established by the D channel, the B channel sends data using circuit switching, X.25, or other similar protocols. The network layer functions of the D channel, however, must be discussed here. These functions are defined by the ITU-T Q.93l standard. The network-layer packet is called a message. A message is encapsulated in the information field of an LAPD I-Frame for transport across a link see Figure 10.4-12 . Fig. 10.4-12 Network layer packet format Fig. 10.4-12...
Connectionoriented and Connectionless Services
At the network layer, the OSI model supports two types of protocols connection-oriented network services CONS and connectionless network services CLNS . A connection-oriented network service CONS establishes a virtual circuit for the transmission of data that is active for the entire transmission. All packets belonging to a single transmission are sent, in order, over the same route, thereby allowing a degree of control over the quality of service that would not otherwise be possible. In a CONS...
Packet Switching
Circuit switching was designed for voice communication. In a telephone conversation, for example, once a circuit is established it remains connected for the duration of the session. Circuit switching creates temporary dedicated links that are well suited to this type of communication. Circuit switching is less well suited to data and other nonconversational transmissions. Nonvoice transmissions tend to be bursty, meaning that data come in spurts with idle gaps between them. When...
Telephone Networks and Circuit Switching
In 1875, while working on the use of sinusoidal signals for multiplexing in telegraphy, Alexander Graham Bell recognized that direct transmission of a voice signal over wires was possible. In 1876 Bell developed a system that could transmit the entire voice signal and could form the basis for voice communication, which we now know as the telephone. The modern telephone network was developed to provide basic telephone service, which involve the two-way, real-time transmission of voice signals...
Complex Signals
So far, we have focused attention on simple periodic signals sine waves . But what about periodic signals that are not sine waves Many useful waveforms do not change in a single smooth curve between a minimum and maximum amplitude they jump, slide, wobble, spike, and dip. But as long as any irregularities are consistent, cycle after cycle, a signal is still periodic and logically must be describable in the same terms used for sine waves. In fact, it can be shown that any periodic signal, no...
ATM Cell Format
The cell has a length of 53 bytes and contains two principal fields see Figure 12.2-1 the header 5 bytes containing in particular a logical identifier used to route the cells in the network this identifier is comparable to the logic channel number of X.25 packets and the DLCI of frames and the information field 48 bytes corresponding to the payload. Cell relay is operated using two technologies which have common characteristics the asynchronous transfer mode ATM , chosen by the ITU-T cell used...
Data Link Layer
The data link layer is responsible for delivering data units groups of bits from one station to the next without errors. It accepts a data unit from the third layer and adds meaningful bits to the beginning header and end trailer that contain addresses and other control information. A data unit with this additional information is called a frame see Figure 2.2-2 . To get to its ultimate destination, a transmission may have to be passed along by a number of intermediate stations. Data link frame...
Frequency Shift Keying FSK
In frequency shift keying FSK , the frequency of the signal is varied to represent binary 1 or 0. The frequency of the signal during each bit is constant and its value depends on the bit 0 or 1 both peak amplitude and phase remain constant. Figure 4.3-6 gives the conceptual view of FSK. FSK avoids most of the noise problems of ASK. Because the receiving device is looking for specific frequency changes over a given number of periods, it can ignore voltage spikes. The limiting factors of FSK are...
Period and Frequency
Period refers to the amount of time, in seconds, a signal needs to complete one cycle. Frequency refers to the number of periods a signal makes over the course of one second. The frequency of a signal is its number of cycles per second. Mathematically, the relationship between frequency and period is that they are the inverse of each other if one is given, the other can be derived. Frequency l Period Period l Frequency Unit of Frequency Frequency is expressed in Hertz Hz , after the German...
Virtual Circuit Approach
In the virtual circuit approach to packet switching, the relationship between all packets belonging to a message or session is preserved. A single route is chosen between sender and receiver at the beginning of the session. When the data are sent, all packets of the transmission travel one after another along that route. So what is the difference between circuit switching and virtual circuits Although circuit switching can use multiplexing at the end-user level, no multiplexing is done at the...
Network Topology and Network Functions
There are various kinds of telecom networks. According to services, they can be divided into telephone network, telegraph network, fax network, data network, CATV network, ISDN and B-ISDN according to signal forms, analog network, data network and hybrid network according to network purposes, bearer network, switching network and supporting network according to network topology, meshed network, star network, compound network, tree network, chain network, ring network and bus network according...
Datagram Approach
In the datagram approach to packet switching, each packet is treated independently from all others. Even when one packet represents just a piece of a multipacket transmission, the network and network layer functions treats it as though it existed alone. Packets in this technology are referred Figure 9.2-2 shows how the datagram approach can be used to deliver four packets from station A to station X. In this example, all four packets or datagrams belong to the same message but may go by...
Bipolar
Bipolar encoding, like RZ, uses three voltage levels positive, negative, and zero. Unlike RZ however, the zero level in bipolar encoding is used to, represent binary 0. Positive and negative voltages represent alternating 1s. If the first 1 bit is represented by the positive amplitude, the second will be represented by the negative amplitude, the third by the positive amplitude, and so on. This alternation occurs even when the 1s bits are not consecutive. Three types of bipolar encoding are in...
Simple Analog Signals
The sine wave is the most fundamental form of a periodic analog signal. Visualized as a simple oscillating curve, its change over the course of a cycle is smooth and consistent, a continuous, rolling flow. Figure 3.3-1 shows a sine wave. Each cycle consists of a single arc above the time axis followed by a single arc below it. Sine waves can be fully described by three characteristics amplitude, period or frequency, and phase.
Integrated Services Digital Network ISDN
The ISDN integrates customer services with the IDN. As we saw in the discussion of packet-switched networks in chapter 9, fully digital services are much more efficient and flexible than analog services. To receive the maximum benefit from the integrated digital networks the next step is to replace the analog local loops with digital subscriber loops. Voice transmissions can be digitized at the source, thereby removing the final need for analog carriers. It then becomes possible to send data,...
Network Layer
The network layer is responsible for the source-to-destination delivery of a packet across multiple network links. Whereas the data link layer oversees station-to-station node-to-node delivery, the network layer ensures that each packet gets from its point of origin to its final destination successfully and efficiently. To make such end-to-end delivery possible, the network layer provides two related services switching and routing. Switching refers to temporary connections between physical...
Medium Bandwidth and Significant Bandwidth
A transmission medium has a limited bandwidth, which means that it can transfer only some range of frequencies. In other words, a transmission medium with a particular bandwidth is capable of transmitting only digital signals whose significant bandwidth is less than the bandwidth of the medium. If a signal is sent on a transmission medium whose bandwidth is less than the required significant bandwidth, the signal may be so distorted that it is not recognizable at the receiver see Figure 3.4-7 ....
Optical Fiber
Up until this point, we have discussed conductive metal cables that transmit signals in the form of current. Optical fiber, on the other hand, is made of glass or plastic and transmits signals in the form of light. To understand optical fiber, we first need to explore several aspects of the nature of light. The Nature of Light Light is a form of electromagnetic energy. It travels at its fastest speed in a vacuum 300,000 kilometers second approximately l86,000 miles second . The speed of light...
Functional Grouping
In the ISDN standard, the devices that enable users to access the services of the BRI or PRI are described by their functional duties and collected in functional groupings. Subscribers choose the specific devices best suited to their needs from these groupings. Remember that the ISDN defines only the functional behavior of each group. The standard does not say anything about implementation. Each functional grouping is a model that can be implemented using devices, or equipment chosen by the...
Aperiodic Signals
An aperiodic, or nonperiodic, signal changes constantly without exhibiting a pattern or cycle that repeats over time. Figure 3.2-2 shows examples of aperiodic signals. An aperiodic, or nonperiodic, signal has no repetitive pattern. Figure 3.2-2 Examples of aperiodic signals It has been proved, however, by a technique called a Fourier transform, that any aperiodic signal can be decomposed into an infinite number of periodic signals. Understanding the characteristics of periodic signals,...
Message Switching
Message switching is best known by the descriptive term store and forward. In this mechanism, a node usually a special computer with a number of disks receives a message, stores it until the appropriate route is free, and then sends it along. Store and forward is considered a switching technique because them is no direct link between the sender and receiver of transmission. A message is delivered to the node along one path then rerouted along another to its destination. Note that in message...
Satellite Communication
Satellite transmission is much like line-of-sight microwave transmission in which one of the stations is a satellite orbiting the earth. The principle is the same as terrestrial microwave, with a satellite acting as a supertall antenna and repeater see Figure 5.2-14 . Although in satellite transmission signals must still travel in straight lines, the limitations imposed on distance by the curvature of the earth are reduced. In this way, satellite relay allow microwave signals to span continents...
Flow Control
The second aspect of data link control, following line discipline, is now control. In most protocols flow control is a set of procedures that tell the sender how much data it can transmit before it must wait for an acknowledgment from the receiver. Two issues are at stake The flow of data must not be allowed to overwhelm the receiver. Any receiving device has a limited speed at which it can process incoming data, and a limited amount of memory, in which to store incoming data. The receiving...
Frequency Spectrum and Bandwidth
Two terms need mentioning here spectrum and bandwidth. The frequency spectrum of a signal is the collection of all the component frequencies it contains and is shown using a frequency domain graph. The bandwidth of a signal is the width of the frequency spectrum see Figure 3.3-10 . In other words, bandwidth refers to the range of component frequencies, and frequency spectrum refers to the elements within that range. To calculate the bandwidth, subtract the lowest frequency from the highest...
Amplitude Modulation AM
In AM transmission, the carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal. The frequency and phase of the carrier remain the same only the amplitude changes to follow variations in the information. Figure 4.4-3 shows how this concept works. The modulating signal becomes an envelope to the carrier. The total bandwidth required for AM can be determined from the bandwidth of the audio signal BWt 2 BWm. The bandwidth of an AM signal is...
n n n n
b. ISO degree phase shift Fig. 3.4-2 Amplitude, period, and phase for a periodic digital signal Most digital signals are aperiodic and thus period or frequency is not appropriate. Two new terms, bit interval instead of period and bit rate instead of frequency are used to describe digital signals. The bit interval is the time required to send one single bit. The bit rate is the number of bit intervals per second. This means that the bit rate is the number of bits sent in one second, usually...
Amp
Fig. 4.3-7 Relationship between baud rate and bandwidth in FSK Fig. 4.3-7 Relationship between baud rate and bandwidth in FSK Find the bandwidth for an FSK signal transmitting at 2000 bit s. Transmission is in half-duplex mode and the carriers must be separated by 3000 Hz. For FSK, if fc1 and fc0 are the carrier frequencies, then BW Baud rate fc1 - fc0 However, the baud rate here is the same as the bit rate. Therefore BW V Bit rate fc1 - 2000 3000 5000 Hz Find the maximum bit rates for an FSK...
Voice Communication over Analog Networks
Initially, telecommunications networks were entirely analog and were reserved for the transmission of analog information in the form of voice. The local loops connecting the subscriber's handset to the telephone company's central office were also analog see Figure 10.2-1 . Fig. 10.2-1 Voice communication over an analog telephone network Fig. 10.2-1 Voice communication over an analog telephone network
Frequency Modulation FM
In FM transmission, the frequency of the carrier signal is modulated to follow the changing voltage level amplitude of the modulating signal. The peak amplitude and phase of the carrier signal remain constant, but as the amplitude of the information signal changes, the frequency of the carrier changes proportionately. Figure 4.4-6 shows the relationships of the modulating signal, the carrier signal, and the resultant FM signal. The bandwidth of an FM signal is equal to 10 times the bandwidth of...
Example 222
Now imagine that in Fig.2.2-5 we want to send data from a node with network address A and physical address 10, located on one local network, to a node with a network address P and physical address 95, located on another local area network. Because the two devices are located on different networks, we cannot use link addresses only the link addresses have only local jurisdiction. What we need here are universal addresses that can pass through the boundaries of local area networks. The network...
Pulse Code Modulation PCM
PCM modifies the pulses created by PAM to create a completely digital signal. To do so, PCM first quantizes the PAM pulses. Quantization is a method of assigning integral values in a specific range to sampled instances. The result of quantization is presented in Figure 4.2-3. Figure 4.2-4 shows a simple method of assigning sign and magnitude values to quantized samples. Each value is translated into its seven-bit binary equivalent. The eighth bit indicates the sign. The binary digits are then...
ATM Topology
Although much of ATM is still under study, the basic topology has been mapped out. At this point, ATM is intended to be a hierarchy of networks connected by associated interfaces. The two most important of these interfaces are the user network interface UNI and the network-to-network interface NNI . A UNI is the interface between a user and the wide area ATM network. An NNI is the interface between two wide area ATM networks between two B-ISDN nodes , see Figure 12.4-1. In addition, we can have...
Biphase
Probably the best existing solution to the problem of synchronization is biphase encoding. In this method, the signal changes at the middle of the bit interval but does not return to zero. Instead, it continues to the opposite pole. As in RZ, these midinterval transitions allow for synchronization. As mentioned earlier, there are two types of biphase encoding in use on networks today Manchester and Differential Manchester. Manchester Manchester encoding uses the inversion at the middle of each...
Presentation Layer
The presentation layer ensures interoperability among communicating devices. Functions at this layer make it possible for two computers to communicate even if their internal representations of data differ e.g., when one device uses one type of code and the other uses another . It provides the necessary translation of different control codes, character sets, graphics characters, and so on to allow both devices to understand the same transmission the same way. The presentation layer is also...
Coaxial Cable
Coaxial cable or coax carries signals of higher frequency ranges than twisted-pair cable see Figure 5.1-9 , in part because the two media are constructed quite differently. Instead of having two wires, coax has a central core conductor of solid or stranded wire usually copper enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two also usually copper . The outer metallic wrapping serves both as a shield against noise...
Hamming Code
So far, we have examined the number of bits required to cover all of the possible single-bit error states in a transmission. But how do we manipulate those bits to discover which state has occurred A technique developed by R.W. Hamming provides a practical solution. The Hamming code can be applied to data units of any length and uses the relationship between data and redundancy bits discussed above. For example, a seven-bit ASCII code requires four redundancy bits that can be added to the end...
Error Control
In the data link layer, the term error control refers primarily to methods of error detection and retransmission. Error correction in the data link layer is implemented simply anytime an error is detected in an exchange, a negative acknowledgment NAK is returned and the specified frames are retransmitted. This process is called automatic repeat request ARQ . It sometimes happens that a frame is so damaged by noise during transmission that the receiver does not recognize it as a frame at all. In...
Tsi
9.1.3 Space- and Time-division Switching Combinations When we compare space-division and time-division switching, some interesting facts merge. The advantage of space-division switching is that it is instantaneous. Its disadvantage is the number of crosspoints required to make space-division switching acceptable in terms of blocking The advantage of time-division switching is that it needs no crosspoints. Its disadvantage is that processing each connection creates delays. Each time slot must be...
Phase Shift Keying PSK
In phase shift keying PSK , the phase is varied to represent binary 1 or 0. Both peak amplitude and frequency remain constant as the phase changes. For example, if we start with a phase of 0 degrees to represent binary 0, then we can change the phase to 180 degrees to send binary l. The phase of the signal during each bit is constant and its value depends on the bit 0 or 1 . Figure 4.30 gives a conceptual view of PSK. The above method is often called 2-PSK, or binary PSK, because two different...
TwistedPair Cable
Twisted-pair cable comes in two forms unshielded and shielded. Unshielded Twisted-Pair UTP Cable Unshielded twisted-pair UTP cable is the most common type of telecommunication medium in use today. Although most familiar from its use in telephone systems, its frequency range is suitable for transmitting both data and voice see Figure 5.1-2 . A twisted pair consists of two conductors usually copper , each with its own colored plastic insulation. The plastic insulation is color-banded for...
Transport Layer
The transport layer is responsible for source-to-destination end-to-end delivery of the entire message. Whereas the network layer oversees end-to-end delivery of individual packets, it does not recognize any relationship between those packets. It treats each one independently, as though each piece belonged to a separate message, whether or not it does. The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow...
Chapter 6 Error Detection And Correction
In most communication channels a certain level of noise and interference is unavoidable. Even after the design of the digital transmission system has been optimized, bit errors in transmission will occur with some small but nonzero probability. For example, typical bit error rates for systems that use copper wires are in the order of 10-6, that is, one in a million. Modern optical fiber systems have bit error rates of 10-9 or less. In contrast, wireless transmission systems can experience error...
Return to zero RZ
As you can see, anytime the original data contain strings of consecutive 1s or 0s, the receiver can lose its place. As we mentioned in our discussion of unipolar encoding, one way to assure synchronization is to send a separate timing signal on a separate channel. However, this solution is both expensive and prone to errors of its own. A better solution is to somehow include synchronization in the encoded signal, something like the solution provided by NRZ-I, but one capable of handling strings...
HighDensity Bipolar 3 HDB3
The problem of synchronizing strings of consecutive 0s is solved differently in Europe and China than in the United States. This convention, called HDB3, introduces changes into the bipolar AMI pattern every time four consecutive 0s are encountered instead of waiting for the eight expected by B8ZS in North America. Although the name is HDB3, the pattern changes whenever there are four 0s in succession see Figure 4.1-11 . In HDB3 if four 0s come one after another, we change the pattern in one of...
Session Layer
The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction between communicating devices. It also ensures that each session closes appropriately rather than shutting down abruptly and leaving the user hanging. For example, imagine that a user wants to transfer a file of 200 pages. What happens if the transfer is interrupted after only 52 pages When the problem is removed and the connection can be made again, should the session be canceled and...
Bipolar Alternate Mark Inversion AMI
Bipolar AMI is the simplest type of bipolar encoding. In the name alternate mark inversion the word mark comes from telegraphy and means 1. So AMI means alternate 1 inversion. A neutral, zero voltage represents binary 0. Binary 1s are represented by alternating positive and negative voltages. Figure 4.1-9 gives an example. In bipolar encoding we use three levels positive, zero, and negative. Fig. 4.1-8 Types of bipolar encoding By inverting on each occurrence of a 1, bipolar AMI accomplishes...
Application Adaptation Layer AAL
The application adaptation layer AAL allows existing networks such as packet networks to connect to ATM facilities. AAL protocols accept transmissions from upper-layer services e.g., packet data and map them into fixed-sized ATM cells. These transmissions can be of any type voice, data, audio, video and can be of variable or fixed rates. At the receiver, this process is reversed-segments are reassembled into their original formats and passed to the receiving service. The AAL layer reformats...
The ISDN Layers
It is difficult to apply the simple seven-layer architecture specified by the OSI to the ISDN. One reason is that the ISDN specifies two different channels B and D with different functionalities. As we saw earlier in this chapter, B channels are for user-to-user communication information exchange . D channels are predominantly for user-to-network signaling. The subscriber uses the D channel to connect to the network then the B channel to send information to another user. These two functions...
Longitudinal Redundancy Check LRC
Longitudinal redundancy check LRC is VRC in two dimensions. To increase the likelihood of detecting multiple-bit and burst errors, we need to increase the complexity of the checks we run on the data by looking at each bit twice. LRC error detection groups a predetermined number of data units, each already containing a VRC panty bit, together into a block. The corresponding bits of each data unit all the first bits, all the second bits, etc. are passed through a generator to find the parity of...