Repeaters

A repeater (or regenerator) is an electronic device that operates on only the physical layer of the OSI model (see Figure 13.1-1). Signals that carry information within a network can travel a fixed distance before attenuation (weakening of the signal due to friction) or interference from noise endangers the integrity of the data. A repeater installed on a link receives the signal before it becomes too weak or corrupted, regenerates the original bit pattern, and puts the refreshed copy back onto...

Physical Layer

The physical layer defines the transmission medium, bit transmission, encoding, and electrical to optical transformation. It provides convergence with physical transport protocols, such as SONET and T-3, as well as the mechanisms for transforming the flow of cells into a flow of bits. The ATM Forum has left most of the specifications for this level to the implementor. For example, the transport medium can be twisted-pair, coaxial, or fiber-optic cable (although the speed necessary to support...

Signaling

Signaling is indispensable to any communication system. All communications, no matter it is voice, data, telegraphs, fax messages, satellite communication, or picture communication, must depend on signaling to perform information transmission, control, supervision, management, etc. Signaling thus is the language for the communication between various parts of the telecom network. The set of all signaling parts used in the telecom network constitutes the signaling system. If the telecom network...

Telecommunications Networks

Telecommunications networks exist to convey communication signals from one point to another. The principal components of a network are nodes, or switching centers, transmission links, and terminal equipment. A modern telecommunication network is a highly complex arrangement of transmission, switching, terminal and management systems that provides a wide variety of services to its customers. In general, the complexity of a network mainly is a function of the volume of telecommunications traffic...

Performance

When evaluating the suitability of a particular medium to a specific application, five factors should be kept in mind cost, speed, attenuation, electromagnetic interference, and security. Cost. The cost of the materials, plus installation. Speed. The maximum number of bits per second that a medium can transmit reliably. Among other factors, speed varies with frequency (higher frequencies can transport more bits per second), with the physical size of the medium and or transmission equipment, and...

Telephone User Part

TUP is an obsolete circuit-related protocol. TUP does not support ISDN circuits prompting the development of a newer protocol which would support the requirements of ISDN. When we talk about the requirements of ISDN, we are speaking specifically of identifying the channels of an ISDN circuit. TUP was designed specifically to handle voice-type calls on voice circuits. When ISDN was developed, the signaling requirements changed since ISDN is capable of carrying both voice and data. The signaling...

The OSI Model

The Open Systems Interconnection model is a layered framework for the design of network systems that allows for communication across all types of computer systems. The OSI reference model divides the basic communication functions into the seven layers, i.e., it consists of seven separate but related layers, each of which defines a segment of the process of moving information across a network (see Figure 2.l-1). Understanding the fundamentals of the OSI model provides a solid basis for...

Wavelength Division Multiplexing

One variation of FDM is wavelength division multiplexing (WDM). WDM on optical fibers is very analogous to FDM in coaxial cable and microwave systems. Optical fiber is transparent in two windows centered on wavelengths of 1300 and 1550 nanometers or nm (10-9 m). The total bandwidth in these two windows exceeds 30,000 GHz. One bit s per hertz (Hz) would result in a bandwidth of over 30 trillion bit s per fiber The carrier frequency at the center of the 1300-nm window is 180 GHz and it is 125 GHz...

Reference Points

Used here, the term reference point refers to the label used to identify individual interfaces between two elements of an ISDN installation. Just as the functional grouping defines the function of each type of equipment used in the ISDN, a reference point defines the functions of the connections between them. Specifically, a reference point defines how two network elements must be connected and the format of the traffic between them. We mention here only those reference points that define the...

Mixed Network Traffic

As you can imagine, the variety of packet sizes makes traffic unpredictable. Switches, multiplexers, and routers must incorporate elaborate software systems to manage the various sizes of packets. A great deal of header information must be read, and each bit counted and evaluated to ensure the integrity of every packet. Internetworking among the different packet networks is slow and expensive at best, and impossible at worst. Another problem is that of providing consistent data-rate delivery...

Asynchronous Transfer Mode

We have described the two essential switching technologies circuit switching, which takes account of the isochronous character the telephone service and packet switching (and frame relay, which is variant of it), which is well suited to data transmission. When the CCITT started its standardization work for a broadband network it to choose a transfer mode for the access link, circuit switching was first considered. Starting with hypothetical service requirements, fixed-rate channels have been...

Switching

Switches in the frame relay network have only two functions. When a frame is received, the switch checks it for errors using the FCS field, which is a CRC. If the frame is intact, the switch then compares the DLCI to an entry in a switching table. The table tells the switch which outgoing port corresponds to that DLCI and therefore to the correct PVC (see Figure 11.2-3). The switch then sends the frame out through that port. If an error is found in the frame the switch discards it. What happens...

Frame Relay Operation

Frame relay transmission is based on permanent virtual circuit (PVC) connections. Virtual circuits in other standards are implemented by the network layer. Frame relay uses data link connection identifiers (DLCIs). The DLCI identifies a permanent virtual circuit that is set up when the system is put in place. All traffic between two given stations takes the same path (hence the appellation permanent virtual circuit). Figure 11.2-1 shows a network where stations are connected directly to a frame...

Data Link Layer

At the data link layer, frame relay employs a simplified version of HDLC. The simpler version is used because HDLC provides extensive error and flow control fields that are not needed in frame relay. Figure 11.1-2 shows the format of a frame relay frame. The frame is similar to that of HDLC. In fact, the flag, FCS, and information fields are the same. The address and control fields, however, have been combined into a single field. This field, though still labeled as an address field, is...

Frame Relay

X.25 provides extensive error checking and flow control. Packets are checked for accuracy at each station (node) to which they are routed. Each station keeps the copy of the original frame until it receives confirmation from the next station that the frame has arrived intact. Such station-to-station checking is implemented at the data link layer of the OSI model. But X.25 does not stop there. It also checks for error from source to receiver at the network layer. The source keeps a copy of the...

Services

Broadband ISDN provides two types of services interactive and distributive (see Figure 10.5-2). Interactive services are those that require two-way exchanges between either two subscribers or between a subscriber and a service provider. These services are of three types conversational, messaging, and retrieval. Conversational Conversational services are those, such as telephone calls, that support real-time exchanges (as opposed to store and forward). These real-time services can be used for...

Broadband ISDN

When the ISDN was originally designed, data rates of 64 kbit s to l.544 Mbit s were sufficient to handle all existing transmission needs. As applications using the telecommunications networks advanced, however, these rates proved inadequate to support many applications. In addition, the original bandwidths proved too narrow to carry the large numbers of concurrent signals produced by a growing industry of digital service providers. Figure 10.5-1 shows the bit rates required and by a variety of...

User Interfaces

Digital subscriber loops can currently be of two types basic rate interface (BRI) and primary rate interface (PRI). Each type is suited to a different level of customer needs. Both include one D channel and some number of either B or H channels. The basic rate interface (BRI) specifies a digital pipe consisting of two B channels and one 16 kbit s D channel (2B+D) see Figure 10.3-1. The total rate of two B channels of 64 kbit s each plus one D channel of l6 kbit s equals l44 kbit s. In addition,...

Channels

A data channel (D channel) can be either l6 or 64 kbit s, depending on the needs of the user. Although the name says data, the primary function of a D channel is to carry control signaling for the B channels. Up to this point, the transmission protocols we have examined all use in-channel signaling. Control information (such as call establishment, ringing, call interrupt, or synchronization) is carried by the same channel that carries the message data. The ISDN separates control signals onto a...

Transmission Media

As discussed in Chapter 3, computers and other telecommunication devices use signals to represent data. These signals are transmitted from one device to another in the form of electromagnetic energy. Electromagnetic signals can travel through a vacuum, through air, or through other transmission media. Electromagnetic energy, a combination of electrical and magnetic fields vibrating in relation to each other, includes power, voice, radio waves, infrared light, visible light, ultraviolet light,...

Analog and Digital

Both data and the signals that represent them can take either analog or digital form. Analog refers to something that is continuous--- a set of specific points of data and all possible points between. Digital refers to something that is discrete. Time is an analog quantity. It is a continuous stream that can be divided up into quarters, hundredths, thousandths, and so on ad infinitum. The measurement of time, however, can be either analog or digital. The hands of a traditional, or analog, clock...

Error Detection

In this section we discuss the idea of error detection in general terms, using the single parity check code as an example throughout the discussion. The basic idea in performing error detection is very simple. As illustrated in Figure 6.2-1, the information produced by an application is encoded so that the stream that is input into the communication channel satisfies a specific pattern or condition. The receiver checks the stream coming out of the communication channel to see whether the...

Multiple Bit Error

The term multiple-bit error means that two or more nonconsecutive bits in a data unit have changed from 1 to 0 or from 0 to 1. Figure 6.1-3 shows the effect of a multiple-bit error on a byte of data. In this example, if we read the bit pattern as an ASCII character, 010000010 (ASCII B) was sent, but 00001010 (ASCII LF) meaning line feed, was received.

Single Bit Error

The term single-bit error means that only one bit of a given data unit (such as a byte, character, data unit, or packet) is changed from 1 to 0 or from 0 to 1. Figure 6.1-2 shows the effect of a single-bit error on a data unit. To understand the impact of the change, imagine that each group of eight bits is an ASCII character with a 0 bit appended to the end. In the example, 00000010 (ASCII STX) was sent, meaning start of text, but 00001010 (ASCII LF) was received, meaning line feed.

Cell Networks

Many of the problems associated with packet internetworking are solved by adopting a concept called cell networking. A cell is small data unit of fixed size. In a cell network, which uses the cell as the basic unit of data exchange, all data are loaded into identical cells that can be transmitted with complete predictability and uniformity. As packets of different sizes and formats reach the cell network from a tributary network, they are split into multiple small data units of equal length and...

Telegraph Networks and Message Switching

Morse demonstrated a practical telegraph that provided the basis for telegram service, the transmission of text messages over long distances. The text was encoded using the Morse code into sequences of dots and dashes. Each dot or dash was communicated by transmitting short and long pulses of electrical current over a copper wire. By relying on two signals, telegraphy made use of a digital transmission system. The time required to transmit a message is minimized by having more...

The composition of A Telecommunication Network

The telecommunication network logically consists of the nodes, the links, ends, and signaling protocols. But physically, the perfect telecom network consists of the switching network, the transmission bearer network, terminal equipment, and the supporting system. The switching network and the transmission bearer network are the basic networks of the telecommunication network, and the supporting system is the auxiliary network of the telecommunication network. The essential function of a network...

Types of Bridges

To select between segments, a bridge must have a look-up table that contains the physical addresses of every station connected to it. The table indicates to which segment each station belongs. How this table is generated and how many segments are connected by a single bridge determine the type and cost of the bridge. There are three types of bridges simple, learning, and multiport. Simple bridges are the most primitive and least expensive type of bridge. A simple bridge links two segments and...

Subscriber Access to the ISDN

To allow flexibility, digital pipes between customers and the ISDN office (the subscriber loops) are organized into multiple channels of different sizes. The ISDN standard defines three channel types, each with a different transmission rate bearer channels, data channels, and hybrid channels (see Table 10.3-1).

Data Link Protocols

In general, the word protocol refers to a set of rules or conventions for executing a particular task. In data communications, protocol is used in a narrower sense to mean the set of rule or specifications used to implement one or more layers of the OSI model. Data link protocols are sets of specifications used to implement the data link layer. To this end, they contain rules for link discipline, flow control, and error handling, among others. Data link protocols can be divided two subgroups...

Overview

Whenever the transmission capacity of a medium linking two devices is greater than the transmission needs of the devices, the link can be shared, much as a large water pipe can carry water to several separate houses at once. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. A concentrator, like a multiplexer, is a device for economy of line use, but unlike the multiplexer it usually involves queuing. Both multiplexers...

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

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...

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...

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

Sinusoidal Curve Designs

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,...

Medium Bandwidth and Significant Bandwidth

Bandwidth Signal

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

Fiber Optical Cable Propgation Mode

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

Functional Grouping Isdn

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

Aperiodic Signal Example

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

Telephone 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...

N n n n

Bit Interval

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

Fsk Bandwidth

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

And Modulation

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

How Packet Move The Network Router

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

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

Manchester Encoding

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

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

Example Redundancy Bit Calculation

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

Stop And Wait

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

Time Division Multiplexing

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

Phase Shift Keying Waveform

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...

Twisted Pair Cable

Effect Noise Parallel Line

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

How The Packet Transfer And The Layers

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...

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

Return Zero Encoding

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...

Session Layer

Synchronization Points 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

Ami Encoding

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...

The ISDN Layers

Isdn Layer Diagram

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...

Terrestrial Microwave

Waves Hitting Satellite Dish

Microwaves do not follow the curvature of the earth and therefore require line-of-sight transmission and reception equipment. The distance coverable by a line-of-sight signal depends to a large extent on the height of the antenna the taller the antennas, the longer the sight distance. Height allows the signal to travel farther without being stopped by the curvature of the planet and raises the signal above many surface obstacles, such as low hills and tall buildings that would otherwise block...

Circuit Switching

Space Division Switch

Circuit switching creates a direct physical connection between two devices such as phones or computers. For example, in Figure 9.1-1, instead of point-to-point connections between the three phones on the left A, B, and C to the four phones on the right D, E, F, and G , requiring 12 links, we can use four switches to reduce the number and the total length of the links. In Figure 9.1-1, phone A is connected through switches I, II, and III to phone D. By moving the levers of the switches, any...

Layered Architecture

Layered Division Multiplexing

The OSI model is built of seven ordered layers physical layer 1 , data link layer 2 , network layer 3 , transport layer 4 , session layer 5 , presentation layer 6 , and application layer 7 . Figure 2.1-2 shows the layers involved when a message is sent from device A to device B. As the message travels from A to B, it may pass through many intermediate nodes. These intermediate nodes usually involve only the first three layers of the OSI model. In developing the model, the designers distilled...

Pulse Amplitude Modulation PAM

Modulateur Pam

The first step in analog-to-digital encoding is called pulse amplitude modulation PAM . This technique takes analog information, samples it, and generates a series of pulses based on the results of the sampling. The term sampling means measuring the amplitude of the signal at equal intervals. The method of sampling used in PAM is more useful to other areas of engineering than it is to data communication. However, PAM is the foundation of an important analog-to-digital encoding method called...

Amplitude

On a graph, the amplitude of a signal is the value of the signal at any point on the wave. It is equal to the vertical distance from a given point on the waveform to the horizontal axis. The maximum amplitude of a sine wave is equal to the highest value it reaches on the vertical axis. Amplitude is measured in volt, ampere, or watt, depending on the type of signal. Volt refers to voltage ampere refers to current and watt refers to power. Amplitude is the height of the signal. The unit for...

Amplitude Shift Keying ASK

Amplitude Shift Keying

In amplitude shift keying ASK , the strength of the signal is varied to represent binary 1 or 0. Both frequency and phase remain constant while the amplitude changes. Which voltage represents 1 and which represents 0 is left to the system designers. A bit duration is the period of time that defines one bit. The peak amplitude of the signal during each bit is constant and its value depends on the bit 0 or 1 . The speed of transmission using ASK is limited by the physical characteristics of the...

Bandwidth for ASK

Ask Bandwidth

As you will recall from Chapter 3, the bandwidth of a signal is the total range of frequencies occupied by that signal. When we decompose an ASK signal, we get a spectrum of simple frequencies. The signal at the center of this spectrum is the carrier fc. At either side are signals with frequencies fc-Nbaud 2, fc Nbaud 2, fc-3 Nbaud 2, fc 3 Nbaud 2, and so on. For practical purposes, however, only the carrier frequency and the two closest side frequencies are needed see Figure 4.3-4 . Fig. 4.3-4...

Integrated Digital Network IDN

Idn Integrated Digital Network

Next, customers began to require access to a variety of networks, such as packet-switched networks and circuit-switched networks. To meet these needs, the telephone companies created integrated digital networks IDNs . An IDN is a combination of networks available for different purposes see Figure 10.2-4 . Access to these networks is by digital pipes, which are time-multiplexed channels sharing very high-speed paths. Customers can use their local loops to transmit both voice and data to their...

Redundancy

Cyclical Redundancy Check

One mechanism that would satisfy these requirements would be to send every data unit twice. The receiving device would then be able to do a bit for bit comparison between the two versions of the data. Any discrepancy would indicate an error, and an appropriate correction mechanism could be set in place. This system would be completely accurate the odds of errors being introduced onto exactly the same bits in both sets of data are infinitesimally small , but it would also be insupportably slow....

Periodic Signals

A signal is periodic if it completes a pattern within a measurable time frame, called a period, and repeats that pattern over identical subsequent periods. The completion of one full pattern is called a cycle. A period is defined as the amount of time expressed in seconds required to complete one full cycle. The duration of a period, represented by T, may be different for each signal, but is constant for any given periodic signal. Figure 3.2-1 illustrates hypothetical periodic signals. A...

Plesiochronous Digital Hierarchy

Synchronous Time Division Multiplexing

As above mentioned, thirty-two digital circuits are multiplexed to form the first level of the digital hierarchy, that is 2048 kbit s or 32 bytes every 125 microseconds. Four groups of 32 circuits form the second level, that is 8192 kbit s, and so on to the fourth level, which corresponds to a digital rate of 139264 kbit s. These four time division multiplexing levels are illustrated in Figure 7.5-1 and figure 7.5-2 respectively. This technique of multiplexing does not make any assumption...

Bit Rate Baud Comparison

Baud Rate Bit Rate

Assuming that an FSK signal over voice-grade phone lines can send 1200 bits per second, it has a bit rate of l200. Each frequency shift represents a single bit so it requires 1200 signal elements to send 1200 bits. Its baud rate, therefore, is also 1200. Each signal variation in an 8-QAM system, however, represents three bits. So a bit rate of 1200, using 8-QAM, has a baud rate of only 400. As Figure 4.3-17 shows, a dibit system has a baud rate of one-half the bit rate. A tribit system has a...

Frequency Division Multiplexing

Fdm Frequency Domain Multiplexing

Frequency-division multiplexing FDM is an analog technique that can be applied when the bandwidth of a link is greater than the combined bandwidths of the signals to be transmitted. In FDM, signals generated by each sending device modulate different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the link. Carrier frequencies are separated by enough bandwidth to accommodate the modulated signal. These bandwidth ranges are...

Constellation diagram

Qam Signal

Instead of 90 degrees, we now vary the signal by shifts of 45 degrees. With 8 different phases, each shift can represent three bits one tribit at a time. As you can see, the relationship of number of bits per shift to number of phases is a power of two. When we have four possible phases, we can send two bits at a time---22 equals 4. When we have eight possible phases, we can send three bits at a time 23 equals 8 . Figure 4.3-12 shows the relationships between...

Physical Specifications

Symmetrical Fiber Link

The B-ISDN model is divided into layers that are different from those of N-ISDN. These layers are closely tied to the design of ATM. For this reason, we will postpone our discussion of them to our discussion of ATM see Chapter 12 . Physical aspects of B-ISDN not related to ATM include access methods, functional equipment groupings, and reference points, described below. B-ISDN defines three access methods designed to provide for three levels of user needs. They are symmetrical 155.520 Mbit s,...

Time division multiplexing

Tdm Multiplexing Device

Time-division multiplexing TDM is a digital process that can be applied when the data rate capacity of the transmission medium is greater than the data rate required by the sending and receiving devices. In such a case, multiple transmissions can occupy a single link by subdividing them and interleaving the portions. Figure 7.3-1 gives a conceptual view of TDM. Note that the same link is used as in the FDM here, however, the link is shown sectioned by time rather than frequency. In the TDM...

Networking And Internetworking Devices

Two or more devices connected for the purpose of sharing data or resources are stations on a network. Putting together a network is often more complicated than simply plugging cable into a hub. A local area network LAN may need to cover more distance than its media can handle effectively. Or the number of stations may be too great for efficient frame delivery or management of the network, and the network may need to be subdivided. In the first case, a device called a repeater or regenerator is...

Phase Division Multiplexing

Time Domain Plot Phase 270 Degree

The term phase describes the position of the waveform relative to time zero. If we think of the wave as something that can be shifted backward or forward along the time axis, phase describes the amount of that shift. It indicates the status of the first cycle. Phase is measured in degrees or radians 360 degrees is 2n radians . A phase shift of 360 degrees corresponds to a shift of a complete period a phase shift of 180 degrees corresponds to a shift of half a period and a phase shift of 90...

PCM 30Channel System

Channel Hierarchy

The digital baseband signal adopted in Europe, China and North America is at a 64kbit s rate. Chapter 4 describes how a 4 kHz speech channel is converted into digitized samples each sample is allocated an 8-bit code corresponding to one of 256 28 quantizing levels, and the sampling rate is 8 kHz. The same 8-bit word or octet may also be obtained from the IA5 alphabet 7 bits plus a parity bit. A signaling bit is sometimes added to each octet increasing the rate to 72 kbit s , but this is...

Elements of A Digital Communication System

Elements The Communication Process

Figure 1.1-1 illustrates the functional diagram and the basic elements of a digital communication system. The source output may be either an analog signal, such as audio or video signal, or a digital signal, such as the output of a teletype machine, that is discrete in time and has a finite number of output characters. In a digital communication system, the messages produced by the source are converted into a sequence of binary digits. Ideally, we should like to represent the source output...

Types of Errors

Three Types Multiplexing

Whenever an electromagnetic signal flows from one point to another, it is subject to unpredictable interference from heat, magnetism, and other forms of electricity. This interference can change the shape or timing of the signal. If the signal is carrying encoded binary data, such changes can alter the meaning of the data, changing 0 to 1 or 1 to 0. Bits can be changed singly or in clumps. For example, a 0.0l second burst of impulse noise on a transmission with a data rate of 1200 bps might...

Analog and Digital Services to Subscribers

Modem Digital Analog

To reduce cost and improve performance, the telephone companies gradually began to add digital technologies while continuing their analog services to their customers see Figure 10.2-3 . Three types of customers were identified at this time traditional customers using their local loops only for analog purposes customers using analog facilities to transmit digital information via modem and customers using digital services to transmit digital information. Of these, the first group was still the...

Asynchronous TDM

Dte Data Terminal Equipment

As we saw in the previous section, synchronous TDM does not guarantee that the full capacity of a link is used. In fact, it is more likely that only a portion of the time slots is in use at a given instant. Because the time slots are preassigned and fixed, whenever a connected device is not transmitting the corresponding slot is empty and that much of the path is wasted. For example, imagine that we have multiplexed the output of 20 identical computers onto a single line. Using synchronous TDM,...

Vertical Redundancy Check VRC

Bit Even Parity Generator Diagram

The most common and least expensive mechanism for error detection is the vertical redundancy check VRC often called a parity check. In this technique, a redundant bit, called a parity bit, is appended to every data unit so that the total number of 1s in the unit including the parity bit becomes either even if the system is checking for even parity or odd if the system is checking for odd parity . Both the sending and receiving systems must use the same type of parity so that if an even-parity...

Cyclic Redundancy Check CRC

Binary Subtraction With Complement

The third and most powerful of the redundancy checking techniques is the cyclic redundancy check CRC . Unlike VRC and LRC which are based on addition, CRC is based on binary division. In CRC, instead of adding bits together to achieve a desired parity, a sequence of redundant bits, called the CRC or the CRC remainder, is appended to the end of a data unit so that the resulting data unit becomes exactly divisible by a second, predetermined binary number. At its destination, the incoming data...

Bipolar 8Zero Substitution B8ZS

B8ZS is the convention adopted in North America to provide synchronization of long strings of 0s. In most situations, B8ZS functions identically to bipolar AMI. Bipolar AMI changes poles with every 1 it encounters. These changes provide the synchronization needed by the receiver. But the signal does not change during a string of 0s, so synchronization is often lost. The difference between B8ZS and bipolar AMI occurs whenever eight or more consecutive 0s are encountered in the data stream. The...

Propagation of Radio Waves

Radio transmission utilizes five different types of propagation surface, tropospheric, ionospheric, line-of-sight, and space see Figure 5.2-2 . Radio technology considers the earth as surrounded by two layers of atmosphere the troposphere and the ionosphere. The troposphere is the portion of the atmosphere extending outward approximately 30 miles from the earth's surface in radio terminology, the troposphere includes the high-altitude layer called the stratosphere and contains what we generally...