When a data message transmits to outside, usually, it will go through multiple
layers and each layer will encapsulate the data into a bigger datagram, data
link layer will encode the bits into packets before it transmitted to the
physical layer. To encapsulate a frame is to add a header and a tail before and
after a piece of data. The receiving end is to recognize the beginning and end
of the frame from the received bitstream, and the frame demarcation is the
necessary requirement for packet switching. Also, in data link layer frame end
placed check code (parity, sum, CRC) to check for the substance, which tries to
find out all the potential errors of connection with the physical layer into
logical error-free data link by encapsulate the data.

data encapsulation in the data link layer refers to the encapsulated data
information, including address segments and data segments. Simply
understanding, the data link layer put the data from the network layer with the
head and tail forming frames to be delivered to the physical layer.  The address segment contains the address of
the point-to-point sender node and the receiving node, for example, Media Access
Control.  The control segment is used to
represent the type of the number grid connection frame, and the data segment
contains the actual data to be transmitted.

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2. Is the link layer
incapable of detecting an error in a received frame?


link layer is able to detect the error in the received frame. In fact, Link
layer can not only encode the message from the network layer but also be able
to do the logical link control and flow control which can detect an error and
recover when the error happened in transmission. When sender side sends a
message, it adds a flag to the datagram to represent error detection code. Once
the message obtained by the receiver, it will recalculate the datagram to get
an error detection code, this error detection code has to be same as the error
code sent from the sender. For example, when the sender sends the message, the
sender uses an algorithm to generate the error detection code based on the
message bits, any changes to the message will cause the error detection code
becomes different. The receiver side will use the same algorithm to calculate
the error detection code to make sure these two error detection codes are same.
Common detection methods include parity check code (PCC) and cyclic redundancy
check (CRC). PCC is the final set of parity bits. Cyclic redundancy check is a
method of generating fixed number verification code based on data transmitted
or saved, which is mainly used to detect or verify data transmission or error
after storage. The generated number is calculated before transmission or
storage and appended to the data, and then the receiver tests to determine
whether the data has changed.


3. Is flow control
one of the services provided by the link layer?


flow control is one of the services that the link layer provided. It is one
responsibility of data link layer and is also a very important part of data
link layer. The functionality of flow control is managing the amount of data
which can be sent from a sender before receiving acknowledgment. The speed of data
sending from the sender has to match the speed of receiving data side,
otherwise the receiver can cause data loss in the transmission process, Also,
the sending speed of the sender has to be matched with the loading rate on the
line (related to the channel bandwidth), otherwise the data will be lost in the
transmission process.  In the data
communication, due to the difference between the working rate of the equipment
used by the two parties and the space of buffer storage, it is possible that
the sending capability of sending end is greater than the receiving capacity of
the receiving end. If it is not controlled, it will cause the loss of the
frame. The “flow control” function on the data link layer is actually
the control of the data transmission rate of the transmitting terminal, so that
the data transmission rate cannot exceed the data receiving capacity of the
receiver. For example, when school opens the class registration server access,
the first day, students usually experience the slow loading of the website, the
reason mostly is because the server is processing the huge amount of request
from all students. The receiver will inform the sender before the limitation
happened and allow fewer data to be sent before the limits reached. In general
there are two ways to do the flow control in the data link layer:

based flow control scheme:

receiving end, after receiving a data frame, sends a confirmation frame to the
sender, indicating that the sender can continue to send data to him. The
sending end must wait for the receiving end to return the acknowledgement
response message after sending a frame of data, and then send the next frame of
data. The receiving end is by checking the check sequence (FCS) of the frame
and sending the confirmation frame without error. Otherwise, it will not send
the return message, indicating that the frame has gone wrong and requires the

– based flow control scheme:

control scheme based on rate” is a rate control scheme based on sliding
window mechanism, it provides the sender how can send a data frame, limits the
send the data transmission rate, without the receiver back to confirm the


4. What operations
does the link layer support?


data link layer supports the datagram processing between the network layer and
physic layer, which includes encapsulating the datagram from bits to packet before
it transmitting to outside and decode the datagram from the encapsulated packet
back to bits when receiver side received the data packet. Link layer can work
as an adapter between It also contains the operations for the network layer and
physical layer, for example, if the physical layer is radio, the link layer
will provide the compatible data which can be resolved by the radio.  Also, it can do logical link control which is
to control the amount and the speed rate of data sending to limit the data flow
of the sender so that the transmission rate does not exceed the receiving
capacity of the receiver.

addressing which is to obtain the MAC address when it transmits data frame to
the destination, media access control which is mainly to avoid and recover from
congestion and collusions, error detection which is to verify the data message
during communication.  In network transmission, because of noise and
signal attenuation, it is often used in the link to flip, lose, etc. For this
reason, the information receiver often needs to check and correct the
information received. The data link layer can also handle and define physical
layer standards. All these operations supported by link layer is to make sure
the data the reliable when it sends out to destination or received the data
from outside.


5. A brief
description of the IEEE (Institute of Electrical and Electronics Engineers)802
standard development.


IEEE 802 committee was formed in February 1980 and its mission is to set
standards for LANs and MANs. The services and protocols defined in IEEE 802 are
limited to the lowest two layers of the OSI model In fact, the IEEE 802 divides
the OSI data link layer into two sub-layers: Logical Link Control (LLC) and
Media Access Control (MAC). The IEEE 802 series of standards are local area
network and metropolitan area network standards developed by the IEEE 802 LAN /
MAN Standards Committee. One of the most widely used are Ethernet, Token Ring,
Bridging, Virtual Bridged LANs, Wireless LAN and more. Each of these series of
standards is covered by a dedicated working group within the committee. With
the development of internet technology, there are a lot of new communication
technology brought into the market and some environment got changed meanwhile,
there is a need to provide a standard for those new communication. IEEE 802 has
been developed 25 working groups at present, each working group is to provide
the standard in specific communication environment. For example, IEEE 802.1 is
the standard for higher layer LAN protocols. IEEE 802.3 is the standard for
Ethernet. IEEE 802.24 is the new one which was introduced in 2012 for smart
Grid Tag.


6. A brief
description of WiFi (Wireless Fidelity).


(Wireless Fidelity)is a wireless network communication technology which
provides the functionality to connect the electronic devices to a wireless
local area network (WLAN). Usually it uses 2.4G UHF or 5G SHF ISM radio
frequency. Wi-Fi wireless connection has distance limitation which allows the
device in specific the range to connect to the wi-fi network. The father the
wifi brand spread out, the weak the strength of the signal. Also, to connect to
a wireless LAN can be password protected, or authentication protected, or
public opened. The communication protocol is based on the IEEE 802.11 series
protocol standard. The category of wireless network in wireless local area network
(LAN) refers to the “wireless compatibility certification” is
essentially a commercial certification, at the same time is also a kind of
wireless network technology, computer via Ethernet cable connection before,
while the wi-fi is connected by radio waves; In common is a wireless router,
then cover the range of the wireless router waves can be connected to the
Internet using wi-fi connection, if connected to a wireless router ADSL line or
other Internet access routes, also known as hot spots. Wireless network access
to the Internet can be simple to understand for wireless Internet, almost all
smartphones, tablets and laptops support wi-fi access to the Internet, is the
one of the most widely used a wireless network transmission technology. It’s
actually converting a wired network signal to a wireless signal. However, wifi
connection is usually not very secure especially in public, the hacker can
easily mock the wifi hotspot or intercept the packet during the communication.
When using WiFi hotspots in public places, try not to operate the network
shopping and online banking, avoid the leakage of important personal sensitive
information, and even be transferred by the hacker bank.


7. A brief
description of the link layer sub-layers.


link layer contains two sub layers: logical link control layer and media access
control layer.

logical link control sub-layer, LLC, is a sublayer on the MAC, which blocks the
difference between the various macs and provides a unified data link service to
its upper layer.


main function of LLC is to handle the exchange of frames between two sites, and
realize the error-free frame transmission and response function and flow
control function of end-to-end. Because of LAN’s media sharing characteristics,
broadcast communication can also be realized.

provides two services for network users: unrecognized connectionless and
connection-oriented services. Unconfirmed connectionless service: it provides
the means for network layer entities to exchange link service data units
without establishing data link level connections. Data transfer can be
point-to-point, point-to-point, or broadcast. This is a datagram service.
Connection-oriented service: in this mode of service, link connection must be
established before frame transmission can be carried out. It provides the
sequence, flow control and error recovery of the data link layer, which is a
virtual circuit service.


is mainly responsible for controlling and connecting physical media of physical
layer. At the time of sending data, MAC protocol can determine whether can send
data in advance, if you can send to add some control information to the data,
finally send data and control information in the form of regulations to the
physical layer; When receiving data, the MAC protocol first determines the
input information and whether the transmission error occurs. If there is no
error, the control information is removed to the LLC layer.


8. A brief
description of NICs (Network Interface Cards).


Interface Card (NIC). It is the essential basic equipment in computer network,
it provides physical connection for data communication between computers. Once
every computer is connected to the network, it is necessary to install the
network card. Most of the time, the network card is installed on the computer
motherboard expansion slot, there is very little part through the computer’s
other interfaces such as PCMCIA to install. The built-in network card can be
used for PC, MAC, graphics workstation and other computer systems. External
network CARDS are usually used for laptops, and some are installed directly on
the computer’s motherboard, without requiring additional installation. Whether
it is an ordinary computer or high-end server, as long as the connection to the
LAN, you need to install a network card. If necessary, a computer can also
install one or more network card NIC inserted in the computer or server
expansion slot, through the network cable and network Exchange data sharing
resources. In addition, after installing the network card, often also need to
configure the protocol. Nic work flow start with receiving data from the
computer, and then the data encoded into a specific format through the network
cable (cable) sent to the other card in the network, while at the other end of
the network card to receive the data, it is the specific format of data into
the computer understand the data formats to computer processing.

complete the working process of the network interface car, it generally
includes a few sub processes: 1. It firstly communication with the host to get
the data packet from the computer. 2. Store the data received from computer to
a cache, which is called data caching. 3. Data frame formatting to format the
data. 4. Parallel data and serial data conversion. 5. Data module encoding and
decoding. 6. Network cable communication with signal handshakes.

cards are usually classified as below.

to the supported bandwidth division, there are 10Mbit / s network card, 10 /
100Mbit / s adaptive network card and 1000Mbit / s network card.

 By bus type, there are ISA network card, PCI
network card, USB network card and PCMCIA network card specially used for
notebook computers.

application areas, there are workstation network card and server network card.

to the port type of the network card, there are RJ-45 port (twisted pair)
network card, AUI port (thick coaxial cable) network card, BNC port (thin
coaxial cable) network card and fiber port network card.

to the number of ports connected with different transmission media, there are
single-port network card, dual-port network card and three-port network card
such as RJ-45 + BNC, BNC + AUI, RJ- 45 + BNC + AUI Network card.

according to the setting of the network cable, the network card can be divided
into two kinds of wired network card and wireless network card.

9. Types of data link


data link control protocol is also known as the link communication protocol,
which is the data link layer protocol in the OSI reference model. Link control
protocol can be divided into asynchronous protocol and synchronous protocol.


point-to-point Protocol is mainly used to establish connection and send data
between two network nodes by dialing or special line. PPP is a solution for
simple connections between various types of hosts, Bridges and routers. PPP
protocol is one of the most widely used protocols in wan. Its advantages are
simple, user authentication, IP allocation and so on.


protocol is simple and efficient technology of data link layer, combines with
other physical layer technologies to form an Ethernet technology access system.
Ethernet access refers to the combination of Ethernet technology and integrated
wiring, as the access network of the public telecommunication network, which
directly provides users with the transmission channels of multiple services
based on IP.


Level Data Link Protocol is a set of agreement is used to transfer data between
network nodes, is by the international organization for standardization (ISO)
issued a high reliability, high efficiency of the data link control procedure,
its characteristic is the data and control information to bits, adopt the
“frame” the format of the transmission.


 Frame Relay is a grouping – oriented
communication method used to connect computer systems. It is mainly used in
public or private LAN connection and wide area network connection. Most public
telecommunications bureaus provide frame relay services as a way to build
high-performance virtual wide-area connections.


Transmission: The Asynchronous Transmission divides the bits into small groups
that can be 8 bits long or longer. The sender can send the bits at any time,
and the receiver never knows when they will arrive. A common example is the
communication between a computer keyboard and a host. By pressing an alphabetic
key, a numeric key, or a special character key, an 8-bit ASCII code is sent.
The keyboard can send code at any time, depending on the user’s input speed,
and the internal hardware must be able to receive a typed character at any


10. MAC (Media Access
Control) design and CRC (Cyclic Redundancy Check) error detection scheme.


access control (MAC). It is to solve the problem of how to allocate the channel
when the use of channel in local area network is generated. It defines how the
data frames are transmitted over the medium. In a link that shares the same
bandwidth, access to the connection medium is “first served”. The
physical addressing is defined here and the logical topology is also defined
here. Line control, error notification, frame delivery order, and optional flow
control are also implemented in this sub-layer. The two media access control methods
widely used in LAN are: Contact-type medium access control, also referred to as
medium access control protocol of the model, such as CSMA/CD mode. Deterministic
media access control, also referred to as an ordered access control protocol,
such as a Token approach.


redundancy check (CRC) is a data transmission error detection function, the
data is polynomial calculation, and will be the result of the attached to the
back of the frame and receiving equipment also performs a similar algorithm, to
ensure the accuracy and integrity of data transmission. CRC is a kind of
checksum, which is two bytes of data stream using binary division (no carry,
use XOR instead of subtraction) to divide the remainder. The divisor is the
binary representation of the information data flow that needs to be calculated
and verified. The divisor is a predefined (short) binary number of (n+1), which
is usually represented by the coefficient of polynomials. Before you do
division, you have to add n times 0 to the data information. Generally
speaking, the form is:

(x) * x ^ {n} (x) = Q(x) * K (x) – R (x) M (x)

M(x) is the original information polynomial. K(x) is the “key” polynomial
of n order. M (x) * x ^ {n} represents the original information followed by n times
0. R(x) is the remainder polynomial, which is the CRC “checksum”. In
communication, the sender attaches the R (replace the additional 0) to the
original information data M and then sends it back. The receiver, after receiving
the M and R, it checks if   M (x) * x ^
{n} + R (x) can be (x) can be devidec by K (x). If it can be divided, the
recipient thinks the message is correct. Error correction coding are often
closely related to CRCs, it can be used to correct the error produced in the
process of transmission. CRC’s error detection ability depends on the order of
key polynomials and the specific key polynomials used. Error polynomial E(x) is
the difference or result of the message code word received and the correct
message code word. The CRC algorithm cannot detect errors when the error code
polynomial can be divisible by CRC polynomials. CRC can detect the two – bit
error of all interval distance less than polynomial order. In this case, the
error code polynomial is: E(x) = x^i + x^k, i