Data Link Layer

Data link layer is the second layer in OSI reference model and lies above the physical layer. The
data link layer performs the following functions.

 Data link layer receives the data from the network layer & divide it into manageable units
called frames.
 It then provides the addressing information by adding header to each frame. Physical
addresses of source & destination machines are added to each frame.

 It provides flow control mechanism to ensure that sender is not sending the data at the
speed that the receiver cannot process.
 It also provide error control mechanism to detect & retransmit damaged, duplicate, or lost
frame, thus adding reliability to physical layer.
 Another function of data link layer is access control. When two or more devices are
attached to the same link, data link layer protocols determine which device has control
over the link at any given time.

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In LAN data link layer is divided in the 2 layers:

Logical Lick Control Sub-layer

Medium Access Layer

Logical Link Control Sublayer: The uppermost Sublayer is Logical Link Control (LLC). This
Sublayer multiplexes protocols running atop the data link layer, and optionally provides flow
control, acknowledgment, and error recovery. The LLC provides addressing and control of the
data link. It specifies which mechanisms are to be used for addressing stations over the
transmission medium and for controlling the data exchanged between the originator and recipient
machines.

Media Access Control Sublayer: The Sublayer below it is Media Access Control (MAC).
Sometimes this refers to the Sublayer that determines who is allowed to access the media at any
one time. Other times it refers to a frame structure with MAC addresses inside. There are
generally two forms of media access control: distributed and centralized. Both of these may be
compared to communication between people: The Media Access Control Sublayer also
determines where one frame of data ends and the next one starts. There are four means of doing
that: a time based, character counting, byte stuffing and bit stuffing.

Services Provided To Network Layer

• Network layer is the layer 3 of OSI model and lies above the data link layer. Data link layer
provides several services to the network layer.

• The one of the major service provided is the transferring the data from network layer on the
source machine to the network layer on destination machine.

• On source machine data link layer receives the data from network layer and on destination
machine pass on this data to the network layer as shown in Fig.

• The path shown in fig (a) is the virtual path. But the actual path is Network layer -> Data link
layer -> Physical layer on source machine, then to physical media and thereafter physical layer -
> Data link layer -> Network layer on destination machine

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The three major types of services offered by data link layer are:

1. Unacknowledged connectionless service.

2. Acknowledged connectionless service.

3. Acknowledged connection oriented service.

1. Unacknowledged Connectionless Service

(a) In this type of service source machine sends frames to destination machine but the destination
machine does not send any acknowledgement of these frames back to the source. Hence it is
called unacknowledged service.

(b) There is no connection establishment between source and destination machine before data
transfer or release after data transfer. Therefore it is known as connectionless service.

(c) There is no error control i.e. if any frame is lost due to noise on the line; no attempt is made
to recover it.

(d) This type of service is used when error rate is low.

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(e) It is suitable for real time traffic such as speech.

2. Acknowledged Connectionless Service

(a) In this service, neither the connection is established before the data transfer nor is it released
after the data transfer between source and destination.

(b) When the sender sends the data frames to destination, destination machine sends back the
acknowledgement of these frames.

(c) This type of service provides additional reliability because source machine retransmit the
frames if it does not receive the acknowledgement of these frames within the specified time.

(d) This service is useful over unreliable channels, such as wireless systems.

3. Acknowledged Connection - Oriented Service

(a) This service is the most sophisticated service provided by data link layer to network layer.

(b) It is connection-oriented. It means that connection is establishment between source &

destination before any data is transferred.

(c) In this service, data transfer has three distinct phases:-

(i) Connection establishment

(ii) Actual data transfer

(iii) Connection release

(d) Here, each frame being transmitted from source to destination is given a specific number and
is acknowledged by the destination machine.

(e) All the frames are received by destination in the same order in which they are sent by the
source.

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Data Communications Equipment (DCE)

Definition - What does Data Communications Equipment (DCE) mean?

Data communications equipment (DCE) refers to computer hardware devices used to establish
maintain and terminate communication network sessions between a data source and its
destination. DCE is connected to the data terminal equipment (DTE) and data transmission
circuit (DTC) to convert transmission signals.

IT vendors may also refer to data communications equipment as data circuit-terminating

equipment or data carrier equipment

Techopedia explains Data Communications Equipment (DCE)

A modem is a typical example of data communications equipment. In general, data

communications equipment is used to perform signal exchange, coding and line clocking tasks
as a part of intermediate equipment or DTE.

Some additional interfacing electronic equipment may also be needed to pair the DTE with a
transmission channel or to connect a circuit to the DTE. DCE and DTE are often confused with
each other, but these are two different device types that are interlinked with an RS-232 serial
line.

DTE and DCE connectors are wired differently if a single straight cable is employed. DCE
generates internal clock signals, while DTE works with externally provided signals. Without
employing a modem, the DCE and DTE can be connected through a crossable cable medium like
a null modem for Ethernet or typical RS-232 serial line. Many modems are DCE, while the
computer terminal is DTE.

RS-232C - What is RS-232C?

BY DINESH THAKUR

RS-232C is a long-established standard ("C" is the current version) that describes the physical
interface and protocol for relatively low-speed serial data communication Networks between
computers and related devices.

RS-232C is the interface that your computer uses to talk to and exchange data with your modem
and other serial devices. RS-232C is the interface between your Communication networks and
other communication networks.

Somewhere in your PC, typically on a Universal Asynchronous Receiver/Transmitter (UART)

chip on your motherboard, the data from your computer is transmitted to an internal or external

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modem (or other serial device) from its Data Terminal Equipment (DTE) interface. Since data in
your computer flows along parallel circuits and serial devices can handle only one bit at a time,
the UART chip converts the groups of bits in parallel to a serial stream of bits.

As your PC's DTE agent, it also communicates with the modem or other serial device, which, in
accordance with the RS-232C standard, has a complementary interface called the Data
Communications Equipment (DCE) interface. RTS/CTS is the way the DTE indicates that it is
ready to transmit data and the way the DCW indicates that it is ready to accept data

RS232C, a standard interface approved by the Electronic Industries Alliance (EIA) for
connecting serial devices. In 1987, the EIA released a new version of the standard and changed
the name to EIA-232-D. And in 1991, the EIA teamed up with Telecommunications Industry
association (TIA) and issued a new version of the standard called EIA/TIA-232-E. Many people,
however, still refer to the standard as RS-232C, or just RS-232.

Almost all modems conform to the EIA-232 standard and most personal computers have an
EIA-232 port for connecting a modem or other device. In addition to modems, many display
screens, mice, and serial printers are designed to connect to a EIA-232 port. In EIA-232
parlance, the device that connects to the interface is called a Data Communications Equipment
(DCE) and the device to which it connects (e.g., the computer) is called a Data Terminal
Equipment (DTE).

The EIA-232 standard supports two types of connectors -- a 25-pin D-type connector (DB-25)
and a 9-pin D-type connector (DB-9). The type of serial communications used by PCs requires
only 9 pins so either type of connector will work equally well.

Although EIA-232 is still the most common standard for serial communication, the EIA has
recently defined successors to EIA-232 called RS-422 and RS-423. The new standards are
backward compatible so that RS-232 devices can connect to an RS-422 port.

RS-232 Mechanical Specification

There is a standardized pin out for RS-232 on a DB25 connector, as shown in Figure.

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 The essential feature ofRS-232 is that the signals are carried as single voltages referred to a
common ground on pin 7. In its simplest form, the RS-232:C interface consis;9'of only two wires
for data and ground. The ground is the absolute voltage reference for all the interface circuitry,
the point in the circuit from which all voltages are measured. Data on pin 2 of the DTE is
transmitted, while the same data on pin 2 of a DCE (modem) is received data as shown in Figure.

Data is transmitted and received on pins 2 and 3, respectively. Data Set Ready (DSR) is an
indication from the Data set (the modem or DSU/CSU) that it is on. Similarly, DTR indicates
that the DTE is on. Carrier Detect (CD) indicates that carrier for the transmission data is on.

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Pins 4 and 5 carry the Request to Send (RTS) and Clear to Send (CTS) signals. In most
situations, RTS and CTS are constantly on the communication session. However, where the DTE
is connected to a multipoint line, RTS is used to turn the carrier on the modem on and off. On a
multipoint line, it is imperative that only one station is transmitting at a time. When a station
wants to transmit, it raises RTS. The modem turns on carrier, typically waits a few milliseconds
for carrier to stabilize, and raises CTS.

The DTE transmits when it sees CTS up. When the station has finished its transmission, it drops
RTS and the modem drops CTS and carrier together.

Terminals and modems usually communicate bidirectional. Bidirectional interchange between

the two devices is directly analogous to the connection of two telephones. The differences
between the DTE and DCE are that DTEs transmit on pin 2 and receive on pin 3. DCEs transmit
on pin 3 and receive on pin 2. When the modem is communicating with another modem, three
essential links are established. These are:

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  DTE to DCE The DTE or terminal and modem or DCE talk to each other.
 DCE to DCE The two modems on the link talk to each other.
 DTE to DCE The host computer talks with its modem.

When a DCE to DCE connection is established the modem will send a connect message to the
computer. The connect message specifies the computer about the baud rate being used by the two
modems for communication.

In case of modems with dissimilar baud rate, the modem of higher baud rate is set to send the
data at the same rate of the other modem using the computer's communication program.

The output signal level usually swings between +12v and,-12v. RS-232 is simple, universal,
well understood and supported everywhere. However, it has some serious shortcomings as an
electrical interface.

First, the interface presupposes a common ground between the DTE and DCE. This is a
reasonable assumption where a short cable connects a DTE and DCE in the same room, but with
longer lines and connections between devices that may be on different electrical buses, this may
not be true. Second, a signal on a single line is impossible to screen effectively for noise. By
screening the entire cable one can reduce the influence of outside noise, but internally generated
noise remains a problem. As the baud rate and line length increase, the effect of capacitance
between the cables introduces serious cross talk until a point is reached where the data itself is
unreadable. Low capacitance cable can reduce crosstalk. Also, as it is the higher frequencies that
are the problem control of slew rate in the signal (i.e. making the signal more rounded, rather
than square) also decreases the crosstalk. The original specifications for·R$-232 had no
specification for maximum slew rate.

The standards for RS-232 and similar interfaces usually restrict RS-232 to 20 kbps or less and
line lengths of 15m (50 ft) or less. These restrictions are mostly throwbacks to the days when 20
kbps were considered a very high line speed, and cables were thick, with high capacitance,
However, in practice, RS-232 is fur more robust than the traditional specified limits of 20 kbps
over u 15m line would imply. Most 56 kbps DSUs are supplied with both V.35und RS-232 ports
because RS-232 is perfectly adequate at speeds up to 200 kbps. The 15 m limitation for cable
length can be stretched to about 30 m for ordinary cable, if well screened and grounded, and
about 100 m if the cable is Jaw capacitance as well.