CN Unit-2

Telephony: Multiplexing: Many to one, one to many

Multiplexing is sending multiple signals from a single link. It means n number of inputs and

single output. Multiplexing is sending multiple signals or streams of information on a carrier at

the same time in the form of a single, complex signal and then recovering the separate signals at

the receiving end.

There are basically three types of multiplexing is used.

1. WDM(wave division multiplexing)

2. TDM (Time division multiplexing)

3. FDM (Frequency division multiplexing)

                    Wavelength Division Multiplexing (WDM):

·In optical transmissions, FDM is known as Wavelength Division Multiplexing (WDM).

·With light different frequencies correspond to different colors.

·Several transmissions can be sending over the same fiber by using different light colors, and

combining into a single light stream. 

·Prisms are used as multiplexors and demultiplexors.

Time Division Multiplexing (TDM):

·It means dividing the available transmission time into time slots, and allocating a different slot to each

transmitter. 

·One method for transmitters to take turns is to transmit in round-robin order.

Frequency Division Multiplexing (FDM):

·It is the basis for broadcast radio.

·Several stations can transmit simultaneously without interfering with each other provided they use separate carrier frequencies (separate channels).

·In data communications FDM is implemented by sending multiple carrier waves over the same copper wire.

·At the receiver’s end, demultiplexing is performed by filtering out the frequencies other than the one carrying the expected transmission. 

·Any of the modulation methods discussed before can be used to carry bits within a channel.

Error control:

Error detection and correction:

Error:

· Frame = m data bits + r bits for error control. – n = m + r.

· Given the original frame f and the received frame f’, how many corresponding bits differ?

– Hamming distance (Hamming, 1950).

Parity Bit:

· Simple error detecting code.

· Even- or odd parity.

· Example:

– Transmit 1011010.

– Add parity bit 1011010 0 (even parity) or 1011010 1 (odd parity).

Hamming Code:

· Check bits in power-of-two positions.

· Each check bit verifies a set of data bits.

· A data bit is checked by multiple check bits.

· Bits in positions that are power of 2 are check bits. The rest are data bits.

· Each check bit used in parity (even or odd) computation of collection of bits.

– Example: check bit in position 11, checks for bits in positions, 11 = 1+2+8. Similarly, bit 11 is checked by bits 1, 2, and 8.

· Parity computations:

– 11: 1, 2, 8 - 6: 2, 4

– 10: 2, 8 - 5: 1, 4

– 9: 1, 8 - 3: 1, 2

– 7: 1, 2, 4

Hamming Code: Example 1

11 10 9 8 7 6 5 4 3 2 1

1: 1, 3, 5, 7, 9, 11

Data: 1001101 using even parity (counting from right to left).

11 10 9 8 7 6 5 4 3 2 1

2: 3, 6, 7, 10, 11

10011011

10011100101

Hamming Code: Example 2

What if instead of 1 0 0 1 1 1 0 0 10 1, receiver gets 1 0 0 1 0 1 0 0 1 0 1?

Receiver takes frame received and re-computes check bits.

1: 3, 5, 7, 9, 11: 1, 1, 0, 1, 0, 1 => 1

2: 3, 6, 7, 10, 11: 0, 1, 1, 0, 0, 1 => 1

4: 5, 6, 7 : 0, 0, 1, 0 => 1

8: 9, 10, 11: 1, 0, 0, 1 => 0

11 10 9 8 7 6 5 4 3 2 1

0111

Result: Bit in position 0 1 1 1 is wrong!

How much code redundancy?

· How many check bits needed, i.e., given m data bits, how many more bits (r) are needed

to allow all single-bit errors to be corrected?

– Resulting frame is m + r.

– (m+r+1) <= 2r.

– Given m, then find r.

– Example: If m = 7 (ASCII 7 code), minimum r is 4.

Hamming Code: Example 7-bit

. Hamming codes can only correct single errors.

. But, to correct bursts of errors, send column by column.

Error Detecting Codes

· Typically used in reliable media.

· Examples: parity bit, polynomial codes (CRC, or Cyclic redundancy Check).

Polynomial Codes

· Treat bit strings as representations of polynomials with coefficients 1’s and 0’s.

· K-bit frame is coefficient list of polynomial with k terms (and degree k-1), from xk-1to x0.

– Highest-order bit is coefficient of xk-1, etc.

– Example: 110001 represent x5 + x4 +x0.

· Generator polynomial G(x).

– Agreed upon by sender and receiver.

CRC:

· Checksum appended to frame being transmitted.

– Resulting polynomial divisible by G(x).

· When receiver gets checksum frame, it divides it by G(x). 

– If remainder, then error!

Cyclic Redundancy Check

At Transmitter, with M = 1 1 1 0 1 1, compute 2rM= 1 1 1 0 1 1 0 0 0 with G = 1 1 0 1

T = 2rM + R [note G starts and ends with “1” R = 1 1 1 Transmit T= 1 1 1 0 1 1 1 1 1

Cyclic Redundancy Check

At the Receiver, compute:

Note remainder = 0 no errors detected

CRC Performance

· Errors go through undetected only if divisible by G(x)

· With “suitably chosen” G(x) CRC code detects all single-bit errors.

Flow and error control: Different techniques to control the overflow of data and different errors in

transmission are called as Flow and error control techniques. Some techniques are as Follows:

Simplex Stop-and-Wait Protocol:

Simplex: Data transmission in one direction. The receiver may not be always ready to receive the next frame (finite buffer storage). Receiver sends a positive acknowledgment frame to sender to transmit the next data frame. Error-free communication channel assumed. No retransmissions used.

A Simplex Positive Acknowledgment with Retransmission (PAR) Protocol. The receiver may not be always ready to receive the next frame (finite buffer storage). Noisy communication channel; frames may be damaged or lost. Frame not received correctly with probability P Receiver sends a positive acknowledgment frame to sender to transmit the next data frame. Any frame has a sequence number,

either 0 or 1 Maximum utilization and throughput similar to protocol 2 when the effects of errors are ignored.

A Simplex PAR Protocol (continued) Effect of Errors The sender starts a timer when transmitting a data frame. If data frame is lost or damaged (probability = p): Receiver does not send an acknowledgment Sender times out and retransmits the data frame

Flow Control Sliding Window Protocols:

These protocols allow both link nodes (A, B) to send and receive data and acknowledgments simultaneously. Acknowledgments are piggybacked into an acknowledgment field in the data frame header not as separate frames. If no new data frames are ready for transmission in a specified time, a separate acknowledgment frame is generated to avoid time-out. Each outbound frame contains a sequence

number ranging from 0 to 2 n-1 (n-bit field). N = 1 for stop-and-wait sliding window protocols.

Sending window: A set of sequence numbers maintained by the sender and correspond to frame sequence numbers of frames sent out but not acknowledged. The maximum allowed size of the sending window w correspond to the maximum number of frames the sender can transmit before receiving any acknowledgment without blocking (pipelining). All frames in the sending window may be lost or damaged and thus must be kept in memory or buffers until they are acknowledged. Sliding Window Data

Link Protocols

Receiving window: A set of sequence numbers maintained by the receiver and indicate the frames sequence numbers it is allowed to receive and acknowledge. The size of the receiving window is fixed at a specified initial size. Any frame received with a sequence number outside the receiving window is discarded. The sending window and receiving window may not have the same upper or lower limits or

have the same size. When pipelining is used, an error in a frame is dealt with in one of two ways:

Go back n:

The receiver discards all subsequent frames and sends no acknowledgments. The sender times out and resends all the discarded frames starting with faulty frame.

Selective repeat:

The receiving data link stores all good frames received after a bad frame. Only the bad frame is retransmitted upon time-out by the sender.

Circuit switching:

Circuit switching is the most familiar technique used to build a communications network. It is used for ordinary telephone calls. It allows communications equipment and circuits, to be shared among users. Each user has sole access to a circuit (functionally equivalent to a pair of copper wires) during network use. Consider communication between two points A and D in a network.

The connection between A and D is provided using (shared) links between two other pieces of equipment, B and C.

                           A connection between two systems A & D formed from 3 links

Network use is initiated by a connection phase, during which a circuit is set up between source and destination, and terminated by a disconnect phase. These phases, with associated timings, are illustrated in the figure below.

                                      A circuit switched connection between A and D

(Information flows in two directions. Information sent from the calling end is shown in pink and information returned from the remote end is shown in blue)

After a user requests a circuit, the desired destination address must be communicated to the local switching node (B). In a telephony network, this is achieved by dialing the number.

Node B receives the connection request and identifies a path to the destination (D) via an intermediate node (C). This is followed by a circuit connection phase handled by the switching nodes and initiated by allocating a free circuit to C (link BC), followed by transmission of a call request signal from node B to node C. In turn, node C allocates a link (CD) and the request is then passed to node D after a similar delay.

The circuit is then established and may be used. While it is available for use, resources (i.e. in the intermediate equipment at B and C) and capacity on the links between the equipment are dedicated to the use of the circuit.

After completion of the connection, a signal confirming circuit establishment (a connect signal in the diagram) is returned; this flows directly back to node A with no search delays since the circuit has been established. Transfer of the data in the message then begins. After data transfer, the circuit is disconnected; a simple disconnect phase is included after the end of the data transmission.

Delays for setting up a circuit connection can be high, especially if ordinary telephone equipment is used. Call setup time with conventional equipment is typically on the order of 5 to 25 seconds after completion of dialing. New fast circuit switching techniques can reduce delays. Trade-offs between circuit switching and other types of switching depend strongly on switching times.

Packet switching:

Packet switching is similar to message switching using short messages. Any message exceeding a network-defined maximum length is broken up into shorter units, known as packets, for transmission; the packets, each with an associated header, are then transmitted individually through the network. The fundamental difference in packet communication is that the data is formed into packets with a pre-defined header format (i.e. PCI), and well-known "idle" patterns which are used to occupy the link when there is no data to be communicated.

Packet network equipment discards the "idle" patterns between packets and processes the entire packet as one piece of data. The equipment examines the packet header information (PCI) and then either removes the header (in an end system) or forwards the packet to another system. If the out-going link is not available, then the packet is placed in a queue until the link becomes free. A packet network is formed by links which connect packet network equipment.

              Communication between A and D using circuits which are shared using packet switching.

                                Packet-switched communication between systems A and D

                        (The message in this case has been broken into three parts labeled 1-3)

There are two important benefits from packet switching.

1. The first and most important benefit is that since packets are short, the communication links between the nodes are only allocated to transferring a single message for a short period of time while transmitting each packet. Longer messages require a series of packets to be sent, but do not require the link to be dedicated between the transmission of each packet. The implication is that packets belonging to other messages may be sent between the packets of the message being sent from A to D. This provides a much fairer sharing of the resources of each of the links.

2. Another benefit of packet switching is known as "pipelining". Pipelining is visible in the figure above. At the time packet 1 is sent from B to C, packet 2 is sent from A to B; packet 1 is sent from C to D while packet 2 is sent from B to C, and packet 3 is sent from A to B, and so forth. This simultaneous use of communications links represents a gain in efficiency; the total delay for transmission across a packet network may be considerably less than for message switching, despite the inclusion of a header in each packet rather

than in each message.

Message switching:

Sometimes there is no need for a circuit to be established all the way from the source to the destination. Consider a connection between the users (A and D) in the figure below (i.e. A and D) is represented by a series of links (AB, BC, and CD).

                         A connection between two systems A & D formed from 3 links

For instance, when a telex (or email) message is sent from A to D, it first passes over a local connection (AB). It is then passed at some later time to C (via link BC), and from there to the destination (via link CD). At each message switch, the received message is stored, and a connection is subsequently made to deliver the message to the neighboring message switch.

Message switching is also known as store-and-forward switching since the messages are stored at intermediate nodes en route to their destinations.

                          The use of message switching to communicate between A and D

The figure illustrates message switching; transmission of only one message is illustrated for simplicity. As the figure indicates, a complete message is sent from node A to node B when the link interconnecting them becomes available. Since the message may be competing with other messages for access to facilities, a queuing delay may be incurred while waiting for the link to become available. The message is stored at B until the next link becomes available, with another queuing delay before it can be forwarded. It repeats this process until it reaches its destination.

Circuit setup delays are replaced by queuing delays. Considerable extra delay may result from storage at individual nodes. A delay for putting the message on the communications link (message length in bits divided by link speed in bps) is also incurred at each node en route.

Message lengths are slightly longer than they are in circuit switching, after establishment of the circuit, since header information must be included with each message; the header includes information identifying the destination as well as other types of information.

Most message switched networks do not use dedicated point-to-point links and therefore a call must be set-up using a circuit switched network. The figure below illustrates the use of message switching over a circuit switched network, in this case using one intermediate message switch.

               Message switching using circuit switched connections between message switches.

Although message switching is still used for electronic mail and telex transmission, it has largely been replaced by packet switching (in fact, most electronic mail is carried using message switching with the links between message switches provided by packet or circuit-switched networks).

Data Link control protocols:

Line discipline

Various synchronous protocols manage communications on computer motherboards.

The terms "synchronous" and "asynchronous" refer to the two different styles of exchanging information in a digital system between two ports or devices. In both styles, messages need to be organized in order to ensure that they are properly handled. Synchronous messages typically use some sort of external clock to

match data exchange, while asynchronous messages simply move at their own individual rates of speed, relying on established systems of rules to ensure proper routing. All computer systems employ both methods of communication and there are a number of different protocols for each.

synchronous and asynchronous protocols overview:

File Transfer Protocols

File transfer protocols are examples of asynchronous communication protocols. File Transfer Protocol (FTP), Apple Filing Protocol (AFP) and Bit Torrent are all examples of file transfer protocols. Typically, they divide data into small packets of bits, which are then sent over a network to a destination one at a time. A packet is not sent until the sender receives confirmation from the recipient that the previous packet has been received.

Email

There are three major protocols for sending and receiving email messages. Simple Mail TransferProtocol (SMTP) is an asynchronous protocol most often used to send email. Post Office Protocol (POP) and Internet Message Access Protocol (IMAP) are both asynchronous protocols most often used for receiving email.

World Wide Web 

The World Wide Web is entirely made up of asynchronous protocols. The most common is Hypertext Transfer Protocol (HTTP), though web sites also use Hypertext Transfer Protocol Secure (HTTPS) among other protocols for exchanging information over the web.

Serial Peripheral Interface Bus
  
The Serial Peripheral Interface Bus (SPI) is a synchronous communication protocol used to link computers within a formal system. Typically, computers are linked into a master-slave relationship where one computer is the "master" controlling the other "slaves."

Inter-Integrated Circuit 

Inter-Integrated Circuit (I2C) is a synchronous protocol for connecting devices such as drives, input/output devices and printers to a motherboard or other computer control system. I2C is a very common method for linking peripheral devices to computers, and has become the basis for a number of other technological systems such as the System Management Bus (SMB) that controls power to computer motherboards.

ISDN:

Integrated Services Digital Network (ISDN) is a set of communication standards for digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network.

Historical outline:

It was first defined in 1988 in the CCITT red book. Prior to ISDN, the telephone system was viewed as a way to transport voice, with some special services available for data. The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. There are several kinds of access interfaces to ISDN defined as Basic Rate Interface (BRI), Primary Rate Interface (PRI), Narrowband ISDN (N-ISDN), and Broadband ISDN (B-ISDN).

Subscriber’s access:

ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kilobit/s. A major market application for ISDN in some countries is Internet access, where ISDN typically provides a maximum of 128 kbps in both upstream and downstream directions. Channel bonding can achieve a greater data rate; typically the ISDN B-channels of three or four BRIs (six to eight 64 kbps channels) are bonded.

ISDN Layers:

Layer 1

ISDN physical layer (Layer 1) frame formats differ depending on whether the frame is outbound (from terminal to network) or inbound (from network to terminal).

In the figure frames are 48 bits long, of which 36 bits represent data. The bits of an ISDN physical layer frame are used as follows:

• F - Provides synchronization
• L - Adjusts the average bit value
• E - Ensures contention resolution when several terminals on a passive bus contend for a channel
• A - Activates devices
• S - Is unassigned
• B1, B2, and D - Handle user data

Figure: ISDN Physical Layer Frame Formats Differ Depending on Their Direction

Multiple ISDN user devices can be physically attached to one circuit. In this configuration, collisions can result if two terminals transmit simultaneously. Therefore, ISDN provides features to determine link contention. When an NT receives a D bit from the TE, it echoes back the bit in the next E-bit position.

The TE expects the next E bit to be the same as its last transmitted D bit.

Terminals cannot transmit into the D channel unless they first detect a specific number of one’s (indicating "no signal") corresponding to a pre-established priority. If the TE detects a bit in the echo (E) channel that is different from its D bits, it must stop transmitting immediately. This simple technique ensures that only one terminal can transmit its D message at one time. After successful D-message transmission, the terminal has its priority reduced by requiring it to detect more continuous ones before transmitting. Terminals cannot raise their priority until all other devices on the same line have had an opportunity to send a D message. Telephone connections have higher priority than all other services, and signaling information has a higher priority than non signaling information.

Layer 2

Layer 2 of the ISDN signaling protocol is Link Access Procedure, D channel (LAPD). LAPD is similar to High-Level Data Link Control (HDLC) and Link Access Procedure, Balanced (LAPB). As the expansion of the LAPD acronym indicates, this layer is used across the D channel to ensure that control and signaling information flows and is received properly.

                   
                       Figure: LAPD Frame Format Is Similar to That of HDLC and LAPB

The LAPD Flag and Control fields are identical to those of HDLC. The LAPD Address field can be either 1 or 2 bytes long. If the extended address bit of the first byte is set, the address is 1 byte; if it is not set, the address is 2 bytes. The first Address-field byte contains the service access point identifier (SAPI), which identifies the portal at which LAPD services are provided to Layer 3. The C/R bit indicates whether the frame contains a command or a response. The Terminal Endpoint Identifier (TEI) field identifies either a single terminal or multiple terminals. A TEI of all ones indicates a broadcast.

Layer 3

Two Layer 3 specifications are used for ISDN signaling: ITU-T (formerly CCITT) I.450 (also known as ITU-T Q.930) and ITU-T I.451 (also known as ITU-T Q.931). Together, these protocols support user-to-user, circuit-switched, and packet-switched connections. A variety of call-establishment, call-termination, information, and miscellaneous messages are specified, including SETUP, CONNECT, RELEASE, USER INFORMATION, CANCEL, STATUS, and DISCONNECT. These messages are functionally similar to those provided by the X.25 protocol.

                Figure: An ISDN Circuit-Switched Call Moves through Various Stages to Its Destination

               Broadband ISDN: Broadband Integrated Services Digital Network (BISDN)

Broadband Integrated Services Digital Network (BISDN or Broadband ISDN) is designed to handle high-bandwidth applications. BISDN currently uses ATM technology over SONET-based transmission circuits to provide data rates from 155 to 622Mbps and beyond, contrast with the traditional narrowband ISDN(or N-ISDN), which is only 64 Kbps basically and up to 2 Mbps.

 

The designed Broadband ISDN (BISDN) services can be categorized as follows:

 Conversational services such as telephone-like services, which was also supported by ISDN. Also the additional bandwidth offered will allow such services as video telephony, video mail, as well as multi-media mail and traditional electronic mail. Retrieval services which provides access to (public) information stores, and information is sent to the user on demand only.

 No user control of presentation. This would be for instance, a TV broadcast, where the user can choose simply either to view or not.

User controlled  presentation. This would apply to broadcast information that the user can partially control.