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world is the CSMA/CD access method.
The sublayers of the Data Link layer
Remember The Data Link layer is divided into two sublayers by the 802 standards: the Logical Link Control
(LLC) and Media Access Control (MAC) sublayers. The LLC sublayer is defined in 802.1 and 802.2. The
MAC sublayer is defined in the 802.1, 802.3, 802.5, and 802.12. Be sure you know the general subject of each
of these standards for the exam.
Connecting on the LLC sublayer
The LLC (Logical Link Control) layer creates connections between networked devices. If you want to send
data from your workstation client to a server on the same network segment, itÇs the LLC that creates and
manages the connection required to transmit your data.
Remember Conceptually, the LLC sublayer sits on top of the MAC sublayer. ItÇs defined by the 802.2
standard to be topology independent. The LLC functions include:
" Managing frames to upper and lower layers
"
Error control
"
Flow control
The LLC works with the transport layer by providing connection-oriented and connectionless services. It
manages and creates the communication link.
The LLC sublayer transfers data in two ways:
" Connectionless services: Messages are not acknowledged by the receiving device, which speeds up
the processing. Although it sounds unreliable, this type of transfer is commonly used at this level
because the upper OSI layers implement their own error-checking and control.
"
Connection-oriented services: Because each message is acknowledged, this service is much slower
than connectionless services, but itÇs much more reliable.
See Chapter 6 for more information on connectionless and connection- oriented services.
Going with the flow control
Remember Another communications control defined on the LLC sublayer is flow control. Flow control
meters the flow of data between network devices that may not be running at the same speeds. Please donÇt
think that flow control occurs on the Data Link layer. The Transport layer of the OSI model actually manages
the mechanisms used to control the flow of data between two hosts. The Data Link layer defines the data
values used in the flow control signaling between two transmitting hosts. See Chapter 6 for more information
on flow control.
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In situations where one communicating device is sending information at either a faster or a slower rate than
the other device, some form of control is necessary to meter the flow of data between the devices to avoid a
loss of data. Flow control prevents the slower device from being swamped, and, more importantly, prevents
data from being lost or garbled. It works by pausing the faster device to enable the slower device to catch up.
Remember There are two types of flow control implemented in data communications software and
hardware. Software flow control, common to networking, involves a process called XON/XOFF, which
roughly stands for transmission on/transmission off. This process involves the sending device continuing to
send data until the receiving device signals (by sending a control character) that transmissions need to stop
until the receiving device can catch up. When the receiving device is ready to go, it sends another control
signal for the sending device to begin the data flow again.
Hardware flow control, also called RTS/CTS (Ready To Send/Clear to Send), uses two wires in a cable, one
for RTS and one for CTS. The sending device uses the RTS signal to indicate when itÇs ready to send. The
receiving device uses the CTS to indicate itÇs ready to receive. When either is turned off, the flow is
interrupted.
Detecting errors in the flow
Remember Another function of the Data Link layer is error detection. Error detection is the process of
detecting whether errors occurred during the transmission of the bits across the wire. The Data Link layer uses
a calculated value called the CRC (Cyclic Redundancy Check) thatÇs placed into the Data Link trailer thatÇs
added to the message frame before itÇs sent to the Physical layer. The receiving computer recalculates the
CRC and compares it to the one sent with the data. If the two values are equal, itÇs assumed that the data
arrived without errors. Otherwise, the message frame may need to be retransmitted under control of an upper
layer. Although the Data Link layer implements error detection, it does not include a function to perform error
recovery. This is left for the upper layers to deal with, primarily on the Transport layer.
Communicating on the MAC sublayer
The MAC sublayer of the Data Link layer provides a range of network services, including controlling which
network device has access to the network and providing for physical addressing.
Remember The MAC sublayer carries the physical address of each device on the network. This address is
more commonly called a deviceÇs MAC address. The MAC address is a 48-bit address thatÇs encoded on
each network device by its manufacturer. This works on the same principle that each domicile on your street
has a unique address assigned to it by the Postal Service. ItÇs the MAC address that the Physical layer uses to
move data between nodes of the network.
A MAC address is made up of two parts: the manufacturerÇs ID number and a unique serialized number
assigned to the device by its manufacturer. The 48-bits (6 bytes) of the MAC address are divided evenly
between these two numbers. The first three bytes of the MAC address contain a hexadecimal manufacturer
code that has been assigned by the IEEE. For example, CiscoÇs IEEE MAC ID is 00 00 0C (each byte holds
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