Question : List the essential elements of communication system. Also, draw and explain the Shannon model of communication system.
Following are the essential elements of communication system.
- Information source: Source that produces a message
- Transmitter: An element that functions on the message to generate a signal which can be delivered through a medium/channel
- Communication Channel: that is the medium over which the signal (carrying the information that composes the message) is sent.
- Receiver: An element converts the signal back into the intended message.
- Destination: It can be a person/machine, for whom / which the message is intended.
Diagram: Shannon’s diagram of a general communications system
Here, the noise is considered as an error or undesired disturbance occurs during the transmission (before receiver and after transmitter), from natural and sometimes man-made sources.
Question : Write any four differences between analog and digital communication.
- Signals are records waveforms as they are. Signal occupies the same order of spectrum as the analog data.
- In analog systems electronic circuits are used for transformation of signals.
- About Noise analog signals are more likely to get affected and results in reducing accuracy
- Data transmission is not of high quality
- Converts analog waveforms into set of numbers and records them. The numbers are converted into voltage stream for representation. In case of binary it is converted in 1’s and 0’s.
- In this transformation is done using logic circuits.
- Digital signals are less affected, because noise response are analog in nature.
- Data transmission has high quality.
Question : Differentiate between Synchronous and asynchronous transmission.
Synchronous transmission has following advantages and disadvantages:
- In comparison to asynchronous communication it has higher speeds, because the system has lesser possibility of error. But, if an error takes place, the complete set of data is lost instead of a single character.
- Serial synchronous transmission is principally used for high-speed communication between computers but is unsuitable where the characters are transferred at irregular intervals.
- Lower overhead and thus, greater throughput.
- Process is more complex
- It is not very cost effective as hardware are more expensive
Asynchronous transmission has following advantages and disadvantages:
- Each individual character is complete unit, hence if there is an error in a character, other sequence of characters are not affected. However, Error in start and stop bit(s) may cause serious problems in data transfer.
- Doesn’t require synchronization of both communication sides.
- It is cost effective
- The speed of transmission is limited.
- Large relative overhead, a high proportion of the transmitted bits are uniquely for control purposes.
Question : Give an example of each communication system based on: • Simplex communication, • half-duplex communication, • full duplex communication.
Following are the example for each:
- Simplex communication: Radio/ Television Broadcasting System
- half-duplex communication: walky-talky System
- full duplex communication: Mobile or telephone system
Question: Explain the need of layering in the data communication protocols stack.
The data communication follows protocols or protocols stack like OSI reference model. Since it is difficult to deal with complex set of rules, and functions required for computer networking, these rules and functions are divided with logical groups called layers. Each layer can be implemented interdependently with an interface to other layers providing with services to it or taking its services like data, connection and error control functions are grouped together into a layer. Speech in telephone conversation is translated, with electrical segments and vice versa. Similarly in computer system the data or pattern are converted into signals before transmitting and receiving. These function and rules are grouped together into a layer.
Question: List and explain any two functions of each OSI layer.
The seven layers of ISO OSI reference model are: i) Physical Layer ii) Data Link Layer iii) Network Layer iv) Transport Layer v) Session Layer vi) Presentation Layer vii) Application Layer.
a) The Physical Layer
- Physical Layer defines electrical and mechanical specifications of cables and connectors.
- Specify signaling options for sending control information between two nodes on a network.
b) The Data Link Layer
- The main task of the data link layer is to provide error free transmission.
- The data link layer creates and recognises frame boundaries. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
c) The Network Layer
- The network layer ensures that each packet travels from its sources to destination successfully and efficiently. It determining how packets are routed from source to destination.
- Addressing is another important task of this layer. The addressing used by the second network may be different from the first one. The second network may not accept the packet at all because it is too large. The protocols may differ, and so on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected.
d) The Transport Layer
- The basic function of the transport layer is to accept data from the session layer, split it up into smaller units if need be, pass these to the network layer, and to ensure that the pieces all arrive correctly at the other end.
- Transport Layer provides location and media independent end-to-end data transfer service to session and upper layers.
e) The Session Layer
- Session Establishment and Session Release – Orderly or abort
- Synchronization, Data Exchange and Expedited Data Exchange.
f) The Presentation Layer
- Presentation layer is concerned with the syntax and semantics of the information transmitted.
- The presentation layer manages these abstract data structure and converts from the representation used inside the computer to the network standard representation and back.
g) Application Layer
- Application Layer supports functions that control and supervise OSI application processes, such as start/maintain/stop application, allocate/deallocate OSI resources, accounting, check point and recovering.
- It also supports remote job execution, file transfer protocol, message transfer and virtual terminal.
Unit 2 : Modulation and Encoding
Question: What is the need for modulation?
To send transmitting message signals effectively for long distances, we use Modulation.
Question: What are Analog modulation techniques?
The Analog modulation techniques are: a) Amplitude modulation b) Angle modulation i) Frequency modulation ii) Phase modulation.
Question: Define amplitude modulation.
This is defined as the modulation, in which amplitude of carrier is changed in accordance to the amplitude of the modulating signal.
Question: What are the limitations of amplitude modulation?
The amplitude modulation suffers from the following limitations
i) The useful power is contained in the sidebands and even at 100% modulation the and contain only 33% of the total power and hence the modulation efficiency is poor. ii) Due to poor efficiency, the transmitters employing amplitude modulation have very poor range. iii) The reception in this modulation is noisy. The radio receiver picks up all the surrounding noise along with the signal.
Question: Define Frequency Modulation.
Frequency Modulation is the technique in which, the frequency of the carrier wave is changed in accordance with the Amplitude of the modulating signal.
Question: What do you know by angle modulation?
It is possible to convey or transmit information by varying its frequency as well as angle of phase. These are known as frequency and phase modulations respectively and both collectively are known as “Angle Modulation”. The frequency and phase modulation systems have similar characteristics with minor differences. Briefing we can say angle modulations of two types: i) Frequency modulation ii) Phase modulation
Question: What are limitations of AM?
Limitations of AM: i) Power of carrier and of one side band is useless. ii) The AM reception is noisy. iii) The BW is much less. iv) Only two S.Bs are available.
Question: What are the different digital modulation techniques?
There are three major classes of digital modulation techniques used for transmission of digitally represented data: • Amplitude Shift Keying • Frequency Shift Keying(FSK) • Phase-shift keying (PSK)
Question : How many different phase states are used in BPSK and QPSK?
BPSK uses two different phase states and each one differs by 180o , whereas the QPSK uses four different phases and each one differs by 90o .
Question: Why digital modulation is better than the Analog Modulation?
i) It is easy to process the digital information. ii) Digital systems are less prone to noise. iii) Digital signals can be easily re-transmitted.
UNIT – 3
Question: Define multiplexing.
Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.
Question: State the importance of multiplexing.
To make efficient use of high speed telecommunications lines, some form of multiplexing is used. Multiplexing allows several transmission sources to share a larger transmission capacity.
A common application of multiplexing is in long-haul communications. Trunks on long-haul networks are high capacity fiber, coaxial or microwave links. These links can carry large numbers of voice and data transmission simultaneously using multiplexing.
Question: What are the multiplexing techniques?
Four basic multiplexing techniques are frequency division multiplexing (FDM), Time division Multiplexing (TDM), Code division Multiplexing (CDM) and Space-division Multiplexing (SDM).
Question: Write differences between FDM and TDM.
Frequency-Division Multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal. Time-Division Multiplexing (TDM) is a type of digital multiplexing in which two or more signals or bit streams are combined into different slots of a frame. Transmission of frame carries simultaneously data from sub-channels in one communication channel, but are physically taking turns on the channel.
Question: What is CDMA?
Code Division Multiple Access (CDMA) is a sort of multiplexing that facilitates various signals to occupy a single transmission channel. It optimizes the use of available bandwidth.
Question: What is Circuit Switching?
Circuit switching is defined as a mechanism applied in telecommunications hereby the user is allocated the full use of the communication channel for the duration of the call and hence a physical connection is set-up between the caller and the receiver.
Question: Define the difference between switched and leased lines.
In switched line communications, a link that is established in a switched network, such as the international dial-up telephone system. A leased line is a symmetric dedicated service (the same upstream and downstream bandwidth) creating a permanent connection between your premises and the Internet.
Question: What are switched communications networks?
In the switched communications networks data entering the network from a station are routed to the destination by being switched from node to node. For example in the Figure 16 data from station A intended for station F are send to node 4. They may then be routed via nodes 5 and 6 or nodes 7 and 6 to the destination. This is called switched communication networks.
Question: Discuss the advantages of packet switching over circuit switching.
- Line efficiency is greater, because single node to node link can be dynamically shared by many packets over time. in other hand in circuit switching time on a node to node link is pre-allocated using synchronous time division multiplexing.
- A packet switching network can perform data rate conversion.
- When traffic becomes heavy on a circuit switching network, some caller are blocked, on the packet switching network, packets are still accepted, hut delivery delay increases.
- Priorities can be used. Thus it can transmit higher priority packet first.
Question: Define parallel transmission.
In parallel transmission, multiple bits (usually 8 bits or a byte/character) are sent simultaneously on different channels (wires, frequency channels) within the same cable
Question: List guided transmission mediums?
Following are the guided transmission mediums i) twisted pair, ii) coaxial cable, iii) optical fiber
Question: What are the advantages of STP over UTP?
STP is similar to UTP in that the wire pairs are twisted around each other. STP also has shielding around the cable to further protect it from external interference. The maximum segment length of STP cable is 100 meters. Shielded twisted-pair (STP) cable combines the techniques of shielding, cancellation, and wire twisting. Each pair of wires is wrapped in a metallic foil.
Question: List the applications of Coaxial cable.
Following are the main applications of Coaxial cable.
- Television distribution
- Long-distance telephone transmission
- Short-run computer system links
- Local Area Networks
Question: What is Single mode optical fiber?
Single mode uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. The fiber itself is manufactured with a much smaller diameter than that of multimode fibers, and with substantially lowers density (index of refraction). The decrease in density results in a critical angle that is close enough to 90 degrees to make the propagation of beams delays are negligible.
Question: What are microwaves? Explain their properties.
Frequencies between 1 and 300 GHz are called microwaves. Microwaves are unidirectional. When an antenna transmits microwave waves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. Its advantage is that a pair of antennas can be aligned without interfering with another pair of aligned antennas.
The propagation of microwave is line-of-sight. The problem with this propagation is that towers that are far apart from each other need to be very tall. The curvature of the earth as well as other blocking obstacles does not allow two short towers to communicate. For long distance communication, repeaters are often needed. Another disadvantage is that very high frequency microwaves cannot penetrate walls.
Question: What is BNC connector?
The BNC connector (Bayonet Neill–Concelman) is miniatures quick connect/disconnect RF connector used for coaxial cable.
Question: Explain the use of SC Connectors.
This is a fiber optics cable connector. Push/pull connector that can also be used with duplex fiber connection. The SC connector comprises a polymer body with ceramic ferrule barrel assembly plus a crimp over sleeve and rubber boot. These connectors are suitable for, 900µm and 2-3mm cables. The connector is precision made to demanding specifications.
BLOCK 2 UNIT 1
Question: What are various types of networks?
There are basically two types of networks:
- Point to point network or switched networks
- Broadcast Networks.
Question: What is the difference between Broadcasting and Multicasting?
Broadcasting refers to addressing a packet to all destinations in a network whereas multicasting refers to addressing a packet to a subset of the entire network.
Question: Write an advantage and one disadvantage of star topology?
Advantages of a Star Topology: — The advantages of star topologies are as follows:
i) Easy to add new stations as each station has its own direct cable connection to the switch. If a cable is cut, it only affects the computer that was attached to it.
ii) It can accommodate different wiring. It can be installed using twisted pair, coaxial cable or fiber optic cable. Disadvantages of a Star Topology: –The advantages of star topologies are as follows:-
i) Depending on where the switches are located, star networks can require more cable length than a linear topology. ii) If the switch / concentrator/switches fail, nodes attached are disabled.
Question: What is the difference Considerations while Choosing a Topology for a network?
2. The considerations while choosing topologies are as follows: — i) Cost: A linear bus network may be the least expensive way to install a network; you do not have to purchase concentrators ii) Length of cable needed: The linear bus network uses shorter lengths of cable. iii) Future growth: With a star topology, expanding a network is easily done by adding another switch. iv) Cable type: The most common cable is unshielded twisted pair, which is most often used with bus, star topologies.
Question: Write the advantage of peer-to-peer over client-server.
The advantages of peer-to-peer over client-server based networks are: i) No need for a network administrator ii) Network is fast/inexpensive to setup & maintain iii) Each PC can make backup copies of its data to other PCs for security. By far the easiest type of network to build, peer-to-peer is perfect for both home and office use.
Question: List any three WAN access methods.
WAN access methods are as follows:
- Packet Switching: –Packet switching is used to overcome from limitations of circuit switching, packet switching has emerged as the standard switching technology for computer-to-computer communications, and therefore, used by most of the communication protocols such as X.25, TCP/IP, Frame Relay, ATM, etc.
- Lease Line: A permanent telephone connection between two points set up by a telecommunications common carrier.
- ISDN: Integrated Services Digital Network (ISDN) was developed by ITU- Tin 1976. It is a set of protocols that combines digital telephony and data transport services. iv) DSL: Digital subscriber line (DSL) is a family of technologies that provide Internet access by transmitting digital data over the wires of a local telephone network.
Block 2, Unit 2
Question: How transport layer of OSI model provide flow control to improve the issue of congestion in the data transfer?
The Transport Layer is responsible for providing flow control to alleviate the issue of congestion in the data transfer. Two main methods for flow control include:
Buffering: Buffering is a form of data flow control regulated by the Transport Layer as depicted. It is responsible for ensuring that sufficient buffers (Temporary Memory) are available at the destination for the processing of data and that the data is transmitted at a rate that does not exceed what the buffer can handle.
Windowing: Windowing is a flow control scheme in which the source computer will monitor and make adjustments to the amount of information sent based on successful, reliable receipt of data segments by the destination computer. The size of the data transmission, called the “window size”, is negotiated at the time of connection establishment, which is determined by the amount of memory or buffer that is available.
Question: Write the main similarities between the TCP/IP and OSI reference models.
The main similarities between the OSI and TCP/IP models include the following:
- They share similar architecture. – Both of the models share a similar architecture. This can be illustrated by the fact that both of them are constructed with layers.
- They share a common application layer.- Both of the models share a common “application layer”. However in practice this layer includes different services depending upon each model.
- Both models have comparable transport and network layers.- This can be illustrated by the fact that whatever functions are performed between the presentation and network layer of the OSI model similar functions are performed at the Transport layer of the TCP/IP model.
- Both models assume that packets are switched.- Basically this means that individual packets may take differing paths in order to reach the same destination
Question: How does the HTTP protocol transfer the information on the World Wide Web?
Hypertext Transfer Protocol (HTTP) is a communications protocol for the transfer of information on the intranet and the World Wide Web. Its original purpose was to provide a way to publish and retrieve hypertext pages over the Internet.
HTTP is a request/response standard between a client and a server. A client is the end-user, the server is the web site. The client making an HTTP request – using a web browser, spider, or other end-user tool – is referred to as the user agent. The responding server – which stores or creates resources such as HTML files and images – is called the origin server. In between the user agent and origin server may be several intermediaries, such as proxies, gateways, and tunnels. HTTP is not constrained to using TCP/IP and its supporting layers, although this is its most popular application on the Internet. Indeed HTTP can be “implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used.”
Typically, an HTTP client initiates a request. It establishes a Transmission Control Protocol (TCP) connection to a particular port on a host (port 80 by default; see List of TCP and UDP port numbers). An HTTP server listening on that port waits for the client to send a request message. Upon receiving the request, the server sends back a status line, such as “HTTP/1.1 200 OK”, and a message of its own, the body of which is perhaps the requested file, an error message, or some other information.
Question: Explain the working of Address Resolution Protocol (ARP).
The address resolution protocol is a protocol used by the Internet Protocol (IP), specifically IPv4 (IP version 4), to map IP network addresses to the hardware addresses used by a data link protocol. It is used when IPv4 is used over Ethernet. ARP works on Ethernet networks as follows. Ethernet network adapters are produced with a physical address embedded in the hardware called the Media Access Control (MAC) address.
Manufacturers take care to ensure these 6-byte (48-bit) addresses are unique, and Ethernet relies on these unique identifiers for message delivery. When any device wishes to send data to another target device over Ethernet, it must first determine the MAC address of that target given its IP address These IP-to-MAC address mappings are derived from an ARP cache maintained on each device.
If the given IP address does not appear in a device’s cache, that device cannot direct messages to that target until it obtains a new mapping. To do this, the initiating device first sends an ARP request broadcast message on the local subnet. The host with the given IP address sends an ARP reply in response to the broadcast, allowing the initiating device to update its cache and proceed to deliver messages directly to the target.
Question: What are the sub-layers of data link layer? Explain.
Data link layer is divided into two sublayers LLC and MAC. Logical Link Control (LLC) concerned with providing a reliable communication part between two devices. It is also involved with flow control and sequencing. The LLC is non-architecture-specific and is the same for all IEEE defined LANs. Medium Access Control (MAC) focuses on methods of sharing a single transmission medium.
Question: List the services of data link layer.
Following are services provided by data link layer:
i) Framing: Encapsulation of network layer data packets into frames, and Frame synchronization
ii) Flow Control: Flow control deals with how to keep the fast sender from overflowing a slow receiver by buffering and acknowledgement procedures. This flow control at data link layer is provided in addition to the one provided on the transport layer.
iii) Error detection and correction codes: Various methods used for error detection and corrections are – Parity bit, cyclic redundancy check, checksum, Hamming code, etc.
iv) Multiple access protocols for channel-access control v) Physical addressing (MAC addressing) vi) Quality of Service (QoS) control
Question: What is parity bit method? Explain its use with the help of an example.
Parity bit method is very simple error detection method in the digital communication. A binary digit called “parity” is used to indicate whether the number of bits with “1” in a given set of bits is even or odd. The parity bit is then attached to original bits. Assume sender want to send some bit streams like 001 0101 and 101 0011. If we are using even parity bit method, we will add “0” with the bit steam having even number of 1’s otherwise add “1”. So our bit steams will be changed after adding parity bit as 1001 0101 and 0101 0011. At the receiver again the number of 1’s are counted in the original message, if the parity bit is mismatched we can say an error has occurred in the message. Just like the even parity we may have odd parity bit method.
Question: Explain the use of Automatic Repeat Request in error correction.
It is an error control method for data transmission that makes use of error detection codes, acknowledgment and/or negative acknowledgment messages, and timeouts to get reliable data transmission. Generally, when the sender does not receive the acknowledgment before the timeout occurs, it retransmits the frame until it is either correctly received or the error persists beyond a predetermined number of retransmissions. Three types of ARQ protocols are Stop-and-wait ARQ, Go-Back-N ARQ, and Selective Repeat ARQ, these mechanisms we will study further in this unit.
Question: Explain the importance of Sliding Window protocol. Also, List the types of sliding window techniques.
In Sliding Window data and control frames flow from sender to receiver in a more continuous manner and several frames can be outstanding at any one time. Allow multiple outstanding (un-ACKed) frames. Upper bound on un-ACKed frames, called window. Sender needs to buffer data so that if data is lost, it can be resent. Receiver needs to buffer data so that if data is received out of order, it can be held until all packets are received Flow control. The transmitting station maintains a sending window that maintains the number of frames it is permitted to send to the receiving station and the receiving station also maintains a receiving window that performs complementary functions. The two sides use the window to coordinate the flow of frames between each other. The window wrap around is used to reuse the same set of numbers for different frames. There are sliding window techniques: Go Back N Selective Repeat
Question: Discuss the working of selective Repeat method. Also, compare it with GO Back N.
This method provides for a more refined approach. In contrast to the Go back N, the only messages retransmitted are those for which negative acknowledgement is received. In this the sending process continues to send a number of frames specified by a window size even after a frame loss. Unlike Go-Back-N, the receiving process will continue to accept and acknowledge frames sent after an initial error; this is the general case of the sliding window protocol with both transmit and receive window sizes greater than 1. The receiver process keeps track of the sequence number of the earliest frame it has not received, and sends that number with every acknowledgement (ACK) it sends. If a frame from the sender does not reach the receiver, the sender continues to send subsequent frames until it has emptied its window. The receiver continues to fill its receiving window with the subsequent frames, replying each time with an ACK containing the sequence number of the earliest missing frame. Once the sender has sent all the frames in its window, it re-sends the frame number given by the ACKs, and then continues where it left off. Now if we compare Selective Repeat behaves in the same way like Go-Back-N , it except when the receiver receives a frame which is out of sequence, it sends a SREJ(Selective Reject) message. Sender retransmits only the rejected packet and continues with other packets. Here in Selective Repeat method the both the Sender’s and Receiver’s buffer size are equal to the window size.
Question: Compare the Throughput of pure and slotted ALOHA.
Throughput is the percentage of the transmitted frames that arrive successfully (without collisions) or the percentage of the channel bandwidth that will be used for transmitting frames without collisions. The throughput ( S) for pure ALOHA is S = G × e −2G . The maximum throughput is Smax = 0.184 when G= (1/2). Where, G is equal to the traffic load. In case of Slotted ALOHA the throughput is S = G × e−G and the maximum throughput is Smax = 0.368 when G = 1.
Question: Explain the need of RARP.
RARP maps a physical address to its IP address. Where is this needed? A node is supposed to have its IP address stored on its hard-disk. However, there are situations when the host may not have hard disk at all, for example a diskless workstation. But also when a host is being connected to the network for the first time, at all such times, a host does not know its IP address. In that case RARP find out the IP address.
Question: Explain the meaning of repeaters in analog and digital system.
In telecommunication, the term repeater has the following standardized meanings:
- An analog device that amplifies an input signal regardless of its nature (analog or digital).
- A digital device that amplifies, reshapes, retimes, or performs a combination of any of these functions on a digital input signal for retransmission.
Question: What are the advantages and disadvantages of bridges?
Following are some advantages and disadvantages of Bridges:
- A bridge cannot filter out broadcast traffic.
- It introduces 20 to 30 % latency.
- Only 2 networks can be linked with a bridge
Question: Explain the advantages of using switch. Also discuss its disadvantages.
Following are some advantages and disadvantages of switches:
- Reduces the number of Broadcast domains
- Supports VLAN’s (virtual local area network (VLAN) is a logical grouping of hosts on one or more LANs that allows communication to occur between hosts as if they were on the same physical LAN.) that can help in Logical segmentation of ports [physical ports]. Splitting up the broadcast domain.
- Intelligent device [compared to Hub’s] which can make use of CAM table for Port to MAC mapping
- Compared to Bridges, Switches are more H/w oriented therefore operations are less CPU intense [Basic operations]
- The cost to number of ports ratio is best i.e. for a cheaper cost you get switches with more number of ports available than Routers. Disadvantages:
- Not as good as a router in limiting Broadcasts
- Communication between VLAN’s need inter VLAN routing [Router], but these days there are a number of Multilayer switches available in the market.
- Handling Multicast packets needs quite a bit of configuration & proper designing.
Question: What is network gateway? Explain
In a communications network, gateway is a network node equipped for interfacing with another network that uses different protocols. A gateway may contain devices such as protocol translators, impedance matching devices, rate converters, fault isolators, or signal translators as necessary to provide system interoperability. It also requires the establishment of mutually acceptable administrative procedures between both networks. A protocol translation/mapping gateway interconnects networks with different network protocol technologies by performing the required protocol conversions.
Question: Which layer ensures the process-to-process or end-to-end delivery of an entire message? Explain.
Transport layer makes sure that the entire message (not a single packets receives) is delivered to a process that is the end (last) entity participating in message exchange. So it provides process-to-process or end-to-end delivery of an entire message.
Question: List the methods or mechanism provided by Transport layer for reliable delivery of data.
Transport Layer is responsible for reliable delivery of data, it provide following methods to provide reliable delivery of data.
- Error control
- Sequence control
- Loss control
- Duplication control
Question: List three stages of Connection Oriented transmission.
Connection Oriented transmission has following three stages:
- Connection establishment.
- Data transfer
- Connection termination.
Question: Compare and contrast between Upward and Downward Multiplexing.
Upward Multiplexing: Transport layer uses virtual circuits based on the services of the lower three layers. The underlying network charge for each virtual circuit connection. To make well cost-effective use of an established circuit, the transport layer sends several transmissions based for the same destination along the same path by upward multiplexing.
Downward Multiplexing: Allows the transport layer to split a single connection among several different path to improve throughput (speed of delivery). This option is useful when the underlying networks have low or slow capacity. For example, some network layer protocols have restriction on the sequence number that can be handled .X.25 uses a three bit numbering code, so sequence number are restricted to the range of 0 to 7. In this case throughput can be unacceptably low. To counteract this problem, the transport layer can use more than one virtual circuit at the network layer to improve throughput by sending several data segment at once delivery is faster.