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DATA LINK LAYER
Topics: Data Link layer design issues, Error Detection and Correction, Elementary Data Link protocols:
Unrestricted simplex protocol, Simplex stop-and-wait protocol, Simplex protocol for a noisy channel;
Sliding Window protocols: One-bit sliding window protocol, Protocol using Go back N, Example.
Data link protocol: Higher Level Data Link Control, Data link layer in the internet. Internetworking and
Advanced Internetworking Switching and Bridging, Basic Internetworking (IP), Routing, The Global
Internet, Routing among Mobile Devices.
Design Issues of Layers of OSI Model
A number of design issues exist for the layer-to-layer approach of computer networks. Some of the main
design issues are as follows:
• Reliability: Network channels and components may be unreliable, resulting in loss of bits while
data transfer. So, an important design issue is to make sure that the information transferred is not
distorted.
• Scalability: Networks are continuously evolving. The sizes are continually increasing leading to
congestion. Also, when new technologies are applied to the added components, it may lead to
incompatibility issues. Hence, the design should be done so that the networks are scalable and can
accommodate such additions and alterations.
• Addressing: At a particular time, innumerable messages are being transferred between large
numbers of computers. So, a naming or addressing system should exist so that each layer can
identify the sender and receivers of each message.
• Error Control: Unreliable channels introduce a number of errors in the data streams that are
communicated. So, the layers need to agree upon common error detection and error correction
methods so as to protect data packets while they are transferred.
• Flow Control: If the rate at which data is produced by the sender is higher than the rate at which
data is received by the receiver, there are chances of overflowing the receiver. So, a proper flow
control mechanism needs to be implemented.
• Resource Allocation: Computer networks provide services in the form of network resources to
the end users. The main design issue is to allocate and deallocate resources to processes. The
allocation/deallocation should occur so that minimal interference among the hosts occurs and
there is optimal usage of the resources.
• Statistical Multiplexing: It is not feasible to allocate a dedicated path for each message while it
is being transferred from the source to the destination. So, the data channel needs to be
multiplexed, so as to allocate a fraction of the bandwidth or time to each host.
• Routing: There may be multiple paths from the source to the destination. Routing involves
choosing an optimal path among all possible paths, in terms of cost and time. There are several
routing algorithms that are used in network systems.
• Security: A major factor of data communication is to defend it against threats like eavesdropping
and surreptitious alteration of messages. So, there should be adequate mechanisms to prevent
unauthorized access to data through authentication and cryptography.
Synchronous Transmission
In synchronous transmission, data moves in a complete paired approach in the form of chunks or frames.
Synchronization between the source and target is required so that the source knows where the new byte

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begins since there is no space between the data. Synchronous transmission is effective, dependable and is
utilized for transmitting a large amount of data. It offers real-time communication between linked
devices.
A comparison of synchronous transmission would be the transfer of a large text file. Before the file is
transmitted, it is first dissected into blocks of sentences. The blocks are then transferred over the
communication link to the target location. Because there is no beginning and end bits the data transfer rate
is quicker but there’s a possibility of more errors to occur. Over time, clocks will get out of sync and the
target device would have the incorrect time, so some bytes could become tampered due to lost bits. To
resolve this issue, there is a need for regular re-synchronization of the clocks as well as the use of check
digits to make sure that the bytes are correctly received and translated.
Characteristics of Synchronous Transmission
• There are no spaces in between characters being sent.
• Timing is provided by modems or other devices at the end of the transmission.
• Special syn characters goes before the data being sent.
• The syn characters are applied between chunks of data for timing functions.
Examples of Synchronous Transmission
• Chatrooms
• Video conferencing
• Telephonic conversations
• Face-to-face interactions
Asynchronous Transmission
In asynchronous transmission data moves in a half-paired approach, 1 byte or 1 character at a time. It
sends the data in a constant current of bytes. The size of a character transmitted is 8 bits where a parity
bit is added each at the beginning and at the end which makes it a total of 10 bits. It doesn’t need a clock
for integration; rather it utilizes the parity bits to inform the receiver how to translate the data.
It is straightforward, quick, cost-effective and doesn’t need a 2-way communication.
Characteristics of Asynchronous Transmission
• Each character is headed by a beginning bit and superseded by one or more end bits.
• There may be gaps or spaces in between characters.
Examples of Asynchronous Transmission
• Emails
• Forums
• Letters
• Radios
• Televisions
Synchronous and Asynchronous Transmission
Point of Comparison Synchronous Transmission Asynchronous Transmission

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Synchronous vs. Asynchronous Transmission
1. In synchronous transmission data is transmitted in the form of chunks, while in asynchronous
transmission data is transmitted one byte at a time.
2. Synchronous transmission needs a clock signal between the source and target to let the target
know of the new byte. While in asynchronous transmission, a clock signal is not needed because
of the parity bit attached to the data sent which serves as a start indicator of the new byte.
3. Data transfer rate of synchronous transmission is faster since it transmits in chunks of data,
compared to asynchronous transmission which transmits one byte at a time.
4. Asynchronous transmission is straightforward and cost-effective while synchronous transmission
is complicated and pricey.
5. Synchronous transmission is systematic and needs lower overhead compared to asynchronous
transmission.
Both synchronous and asynchronous transmissions have their benefits and limitations. Asynchronous
is used for sending a small amount of data while synchronous transmission is used for sending bulk of
data. Therefore both synchronous and asynchronous transmissions are essential for data transmission.
Definition
Transmits data in the form of chunks
or frames
Transmits 1 byte or character at a
time
Speed of
Transmission
Quick Slow
Cost Expensive Cost-effective
Time Interval Constant Random
With gap between
the data?
Yes None
Examples
Chat Rooms, Telephonic
Conversations, Video Conferencing
Email, Forums, Letters

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SWITCHING
• The networking device which performs switching is known as switch.
• It operates on hardware part based on look up table to perform switching operation.
• Switch checks for destination MAC address in the packet and switches the packet to appropriate
destination host.
• It operates on layer-2 i.e. data link layer.
FORWARDING
• It does not perform any check on the packet; just forward it to next hop.
• It checks for MPLS label in the packet verify it in the flow table and consecutively forward the
packet.
• If the destination is in the same subnet, it forwards the packet to corresponding port of destination
host.
• If it is in the other subnet, it forwards the packet to next MPLS router.

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ROUTING
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• The networking device which performs routing is known as router.
• It operates based on routing table.
• It operates on OSI layer-3.
• It works based on IP address.
• It follows different protocols such as RIP, OSPF etc. for routing.
DATA LINK LAYER DESIGN ISSUES
Data-link layer is the second layer after the physical layer. The data link layer is responsible
for maintaining the data link between two hosts or nodes.
Before going through the design issues in the data link layer. Some of its sub-layers and their
functions are as following below.
The data link layer is divided into two sub-layers:
1. Logical Link Control Sub-layer (LLC) –
Provides the logic for the data link, Thus it controls the synchronization, flow
control, and error checking functions of the data link layer. Functions are –
• (i) Error Recovery.
• (ii) It performs the flow control operations.
• (iii) User addressing.
2. Media Access Control Sub-layer (MAC) –
It is the second sub-layer of data-link layer. It controls the flow and multiplexing for
transmission medium. Transmission of data packets is controlled by this layer. This
layer is responsible for sending the data over the network interface card.
Functions are –

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• (i) To perform the control of access to media.
• (ii) It performs the unique addressing to stations directly connected to
LAN.
• (iii) Detection of errors.
Design issues with data link layer are:
1. Services provided to the network layer – The data link layer act as a service
interface to the network layer. The principal service is transferring data from
network layer on sending machine to the network layer on destination machine.
This transfer also takes place via DLL (Data link-layer).
2. Frame synchronization – The source machine sends data in the form of blocks
called frames to the destination machine. The starting and ending of each frame
should be identified so that the frame can be recognized by the destination machine.
3. Flow control – Flow control is done to prevent the flow of data frame at the
receiver end. The source machine must not send data frames at a rate faster than the
capacity of destination machine to accept them.
4. Error control – Error control is done to prevent duplication of frames. The errors
introduced during transmission from source to destination machines must be
detected and corrected at the destination machine.
ELEMENTARY DATA LINK PROTOCOLS
Protocols in the data link layer are designed so that this layer can perform its basic functions:
framing, error control and flow control. Framing is the process of dividing bit - streams from
physical layer into data frames whose size ranges from a few hundred to a few thousand bytes.
Error control mechanisms deals with transmission errors and retransmission of corrupted and
lost frames. Flow control regulates speed of delivery and so that a fast sender does not drown a
slow receiver.
TYPES OF DATA LINK PROTOCOLS
Data link protocols can be broadly divided into two categories, depending on whether the
transmission channel is noiseless or noisy.

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SIMPLEX PROTOCOL
The Simplex protocol is hypothetical protocol designed for unidirectional data transmission over
an ideal channel, i.e. a channel through which transmission can never go wrong. It has distinct
procedures for sender and receiver. The sender simply sends all its data available onto the
channel as soon as they are available its buffer. The receiver is assumed to process all incoming
data instantly. It is hypothetical since it does not handle flow control or error control.
In this protocol we assume that data is transmitted in one direction only. No error occurs; the
receiver can only process the received information at finite rate. These assumptions imply that
the transmitter cannot send frames at rate faster than the receiver can process them.
The main problem here is how to prevent the sender from flooding the receiver. The general
solution for this problem is to have the receiver send some sort of feedback to sender, the process
is as follows −
Step1 − The receiver sends the acknowledgement frame back to the sender telling the sender that
the last received frame has been processed and passed to the host.
Step 2 − Permission to send the next frame is granted.
Step 3 − The sender after sending the sent frame has to wait for an acknowledge frame from the
receiver before sending another frame.
This protocol is called Simplex Stop and wait protocol, the sender sends one frame and waits for
feedback from the receiver. When the ACK arrives, the sender sends the next frame.
The Simplex Stop and Wait Protocol is diagrammatically represented as follows −
STOP – AND – WAIT PROTOCOL
Stop – and – Wait protocol is for noiseless channel too. It provides unidirectional data
transmission without any error control facilities. However, it provides for flow control so that a
fast sender does not drown a slow receiver. The receiver has a finite buffer size with finite

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processing speed. The sender can send a frame only when it has received indication from the
receiver that it is available for further data processing.
STOP – AND – WAIT ARQ
Stop – and – wait Automatic Repeat Request (Stop – and – Wait ARQ) is a variation of the
above protocol with added error control mechanisms, appropriate for noisy channels. The sender
keeps a copy of the sent frame. It then waits for a finite time to receive a positive
acknowledgement from receiver. If the timer expires or a negative acknowledgement is received,
the frame is retransmitted. If a positive acknowledgement is received then the next frame is sent.
Data transfer is only in one direction, consider separate sender and receiver, finite processing
capacity and speed at the receiver, since it is a noisy channel, errors in data frames or
acknowledgement frames are expected. Every frame has a unique sequence number.
After a frame has been transmitted, the timer is started for a finite time. Before the timer expires,
if the acknowledgement is not received, the frame gets retransmitted, when the
acknowledgement gets corrupted or sent data frames gets damaged, how long the sender should
wait to transmit the next frame is infinite.
The Simplex Protocol for Noisy Channel is diagrammatically represented as follows −

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GO – BACK – N ARQ
Go – Back – N ARQ provides for sending multiple frames before receiving the
acknowledgement for the first frame. It uses the concept of sliding window, and so is also called
sliding window protocol. The frames are sequentially numbered and a finite number of frames
are sent. If the acknowledgement of a frame is not received within the time period, all frames
starting from that frame are retransmitted.
Working of Go-Back-N ARQ
Suppose there are a sender and a receiver, and let's assume that there are 11 frames to be sent.
These frames are represented as 0,1,2,3,4,5,6,7,8,9,10, and these are the sequence numbers of the
frames. Mainly, the sequence number is decided by the sender's window size. But, for the better
understanding, we took the running sequence numbers, i.e., 0,1,2,3,4,5,6,7,8,9,10. Let's consider
the window size as 4, which means that the four frames can be sent at a time before expecting the
acknowledgment of the first frame.
Step 1: Firstly, the sender will send the first four frames to the receiver, i.e., 0,1,2,3, and now the
sender is expected to receive the acknowledgment of the 0th
frame.
Let's assume that the receiver has sent the acknowledgment for the 0 frame, and the receiver has
successfully received it.

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The sender will then send the next frame, i.e., 4, and the window slides containing four frames
(1,2,3,4).
The receiver will then send the acknowledgment for the frame no 1. After receiving the
acknowledgment, the sender will send the next frame, i.e., frame no 5, and the window will slide
having four frames (2,3,4,5).

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Now, let's assume that the receiver is not acknowledging the frame no 2, either the frame is lost,
or the acknowledgment is lost. Instead of sending the frame no 6, the sender Go-Back to 2,
which is the first frame of the current window, retransmits all the frames in the current window,
i.e., 2,3,4,5.

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Important points related to Go-Back-N ARQ:
o In Go-Back-N, N determines the sender's window size, and the size of the receiver's
window is always 1.
o It does not consider the corrupted frames and simply discards them.
o It does not accept the frames which are out of order and discards them.
o If the sender does not receive the acknowledgment, it leads to the retransmission of all
the current window frames.
SELECTIVE REPEAT ARQ
This protocol also provides for sending multiple frames before receiving the acknowledgement
for the first frame. However, here only the erroneous or lost frames are retransmitted, while the
good frames are received and buffered.
AN UNRESTRICTED SIMPLEX PROTOCOL
Before learning the protocol, we must assume some facts like, Data is transmitted in only one
direction. Both the network layers are always ready for transmitting and receiving. we will
ignore the processing time and most of all the communication channel between the data link
layer will never damage or loose the frames.
This protocol consists of two procedures: sender and receiver. The sender sends the data to the
receiver and runs on the data link layer of the sender’s machine and receiver receives the data
from the sender and it runs on the data link layer of the receiver’s machine. And here a frame
arrival is used which sends the information that an undamaged frame has arrived.
This is a very simple protocol in which the sender uses a while loop and it continuously sends the
data to the receiver. The initial work of the receiver is to wait for the frame. as soon as it starts
receiving the frame it set the event to frame_arrival or else event is set to wait for frame.

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HIGH-LEVEL DATA LINK CONTROL (HDLC)
High-level Data Link Control (HDLC) is a group of communication protocols of the data link
layer for transmitting data between network points or nodes. Since it is a data link protocol, data
is organized into frames. A frame is transmitted via the network to the destination that verifies its
successful arrival. It is a bit - oriented protocol that is applicable for both point - to - point and
multipoint communications.
Transfer Modes
HDLC supports two types of transfer modes, normal response mode and asynchronous balanced
mode.
• Normal Response Mode (NRM) − Here, two types of stations are there, a primary
station that send commands and secondary station that can respond to received
commands. It is used for both point - to - point and multipoint communications.
• Asynchronous Balanced Mode (ABM) − Here, the configuration is balanced, i.e. each
station can both send commands and respond to commands. It is used for only point - to -
point communications.

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HDLC Frame
HDLC is a bit - oriented protocol where each frame contains up to six fields. The structure varies
according to the type of frame. The fields of a HDLC frame are −
• Flag − It is an 8-bit sequence that marks the beginning and the end of the frame. The bit
pattern of the flag is 01111110.
• Address − It contains the address of the receiver. If the frame is sent by the primary
station, it contains the address(es) of the secondary station(s). If it is sent by the
secondary station, it contains the address of the primary station. The address field may be
from 1 byte to several bytes.
• Control − It is 1 or 2 bytes containing flow and error control information.
• Payload − This carries the data from the network layer. Its length may vary from one
network to another.
• FCS − It is a 2 byte or 4 bytes frame check sequence for error detection. The standard
code used is CRC (cyclic redundancy code)
Types of HDLC Frames
There are three types of HDLC frames. The type of frame is determined by the control field of
the frame −

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• I-frame − I-frames or Information frames carry user data from the network layer. They
also include flow and error control information that is piggybacked on user data. The first
bit of control field of I-frame is 0.
• S-frame − S-frames or Supervisory frames do not contain information field. They are
used for flow and error control when piggybacking is not required. The first two bits of
control field of S-frame is 10.
• U-frame − U-frames or Un-numbered frames are used for myriad miscellaneous
functions, like link management. It may contain an information field, if required. The
first two bits of control field of U-frame is 11.
DATA LINK LAYER IN THE INTERNET
INTERNETWORKING AND ADVANCED INTERNETWORKING
Internetworking is combined of 2 words, inter and networking which implies an association
between totally different nodes or segments. This connection area unit is established through
intercessor devices akin to routers or gateway. The first term for associate degree internetwork
was catenet. This interconnection is often among or between public, private, commercial,
industrial, or governmental networks. Thus, associate degree internetwork could be an
assortment of individual networks, connected by intermediate networking devices, that

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function as one giant network. Internetworking refers to the trade, products, and procedures
that meet the challenge of making and administering internet works.
To enable communication, every individual network node or phase is designed with a similar
protocol or communication logic, that is Transfer Control Protocol (TCP) or Internet Protocol
(IP). Once a network communicates with another network having constant communication
procedures, it’s called Internetworking. Internetworking was designed to resolve the matter of
delivering a packet of information through many links.
There is a minute difference between extending the network and Internetworking. Merely
exploitation of either a switch or a hub to attach 2 local area networks is an extension of LAN
whereas connecting them via the router is an associate degree example of Internetworking.
Internetworking is enforced in Layer three (Network Layer) of the OSI-ISO model. The
foremost notable example of internetworking is the Internet.
There is chiefly 3 units of Internetworking:
1. Extranet
2. Intranet
3. Internet
Intranets and extranets might or might not have connections to the net. If there is a connection
to the net, the computer network or extranet area unit is usually shielded from being accessed
from the net if it is not authorized. The net isn’t thought-about to be a section of the computer
network or extranet, though it should function as a portal for access to parts of the associate
degree extranet.
1. Extranet – It’s a network of the internetwork that’s restricted in scope to one
organization or entity however that additionally has restricted connections to the
networks of one or a lot of different sometimes, however not essential. It’s the very
lowest level of Internetworking, usually enforced in an exceedingly personal area.
Associate degree extranet may additionally be classified as a Man, WAN, or
different form of network however it cannot encompass one local area network i.e.
it should have a minimum of one reference to associate degree external network.
2. Intranet – This associate degree computer network could be a set of interconnected
networks, which exploits the Internet Protocol and uses IP-based tools akin to web
browsers and FTP tools, that are underneath the management of one body entity.
That body entity closes the computer network to the remainder of the planet and
permits solely specific users. Most typically, this network is the internal network of
a corporation or different enterprise. An outsized computer network can usually
have its own internet server to supply users with browseable data.
3. Internet – A selected Internetworking, consisting of a worldwide interconnection of
governmental, academic, public, and personal networks based mostly upon the
Advanced analysis comes Agency Network (ARPANET) developed by ARPA of
the U.S. Department of Defense additionally home to the World Wide Web
(WWW) and cited as the ‘Internet’ to differentiate from all different generic
Internetworks. Participants within the web, or their service suppliers, use IP
Addresses obtained from address registries that manage assignments.
Internetworking has evolved as an answer to a few key problems: isolated LANs, duplication
of resources, and an absence of network management. Isolated LANs created transmission

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problems between totally different offices or departments. Duplication of resources meant that
constant hardware and code had to be provided to every workplace or department, as did a
separate support employee. This lack of network management meant that no centralized
methodology of managing and troubleshooting networks existed.
One more form of the interconnection of networks usually happens among enterprises at the
Link Layer of the networking model, i.e. at the hardware-centric layer below the amount of the
TCP/IP logical interfaces. Such interconnection is accomplished through network bridges and
network switches. This can be typically incorrectly termed internetworking, however, the
ensuing system is just a bigger, single subnetwork, and no internetworking protocol, akin to
web Protocol, is needed to traverse these devices.
However, one electronic network is also reborn into associate degree internetwork by dividing
the network into phases and logically dividing the segment traffic with routers. The Internet
Protocol is meant to supply an associate degree unreliable packet service across the network.
The design avoids intermediate network components maintaining any state of the network.
Instead, this task is allotted to the endpoints of every communication session. To transfer
information correctly, applications should utilize associate degree applicable Transport Layer
protocol, akin to Transmission management Protocol (TCP), that provides a reliable stream.
Some applications use a less complicated, connection-less transport protocol, User Datagram
Protocol (UDP), for tasks that don’t need reliable delivery of information or that need period of
time service, akin to video streaming or voice chat.
Internetwork Addressing –
Internetwork addresses establish devices severally or as members of a bunch. Addressing
schemes differ based on the protocol family and therefore the OSI layer. Three kinds of
internetwork addresses area units are ordinarily used: data-link layer addresses, Media Access
control (MAC) addresses, and network-layer addresses.
1. Data Link Layer addresses: A data-link layer address unambiguously identifies
every physical network association of a network device. Data-link addresses
typically area units cited as physical or hardware addresses. Data-link addresses
sometimes exist among a flat address area and have a pre-established and usually
fastened relationship to a selected device. End systems usually have just one
physical network association, and therefore have just one data-link address. Routers
and different internetworking devices usually have multiple physical network
connections and so eventually have multiple data-link addresses.
2. MAC Addresses: Media Access management (MAC) addresses encompass a set of
data-link layer addresses. MAC addresses establish network entities in LANs that
implement the IEEE MAC addresses of the data-link layer. MAC addresses
different area units distinctively for every local area network interface. MAC
addresses are forty-eight bits long and are expressed in form of twelve hexadecimal
digits. The primary half dozen hexadecimal digits, which are usually administered
by the IEEE, establish the manufacturer or merchant and therefore comprise the
Organizational Unique Identifier (OUI). The last half dozen positional notation

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digits comprise the interface serial variety or another price administered by the
particular merchant. MAC addresses are typically area units referred to as burned-in
addresses (BIAs) as a result of being burned into read-only memory(ROM) and are
traced into random-access memory (RAM) once the interface card initializes.
3. Network-Layer Addresses: Network addresses sometimes exist among a gradable
address area and typically area units referred to as virtual or logical addresses. the
connection between a network address and a tool is logical and unfixed, it usually
relies either on physical network characteristics or on groupings that don’t have any
physical basis. finish systems need one network-layer address for every network-
layer protocol they support. Routers and different Internetworking devices need one
network-layer address per physical network association for every network-layer
protocol supported.
Challenges to Internetworking –
Implementing useful internetwork isn’t at any certainty. There are several challenging fields,
particularly in the areas of dependableness, connectivity, network management, and
adaptability, and each and every space is essential in establishing associate degree economical
and effective internetwork. A few of them are:-
• The initial challenge lies when we are trying to connect numerous systems to
support communication between disparate technologies. For example, Totally
different sites might use different kinds of media, or they could operate at variable
speeds.
• Another essential thought is reliable service that should be maintained in an
internetwork. Individual users and whole organizations depend upon consistent,
reliable access to network resources.
• Network management should give centralized support associate degreed
troubleshooting capabilities in an internetwork. Configuration, security,
performance, and different problems should be adequately addressed for the
internetwork to perform swimmingly.
• Flexibility, the ultimate concern, is important for network enlargement and new
applications and services, among different factors.