Chapter 5. Data Link Layer. O BJECTIVES. Understand the concept of the hop-to-hop delivery compared to host-to-host delivery and application-to-application delivery. Understand the concept of access method and define different access methods used in LANs and WANs.
Understand the concept of the hop-to-hop delivery compared to
host-to-host delivery and application-to-application delivery.
Understand the concept of access method and define different
access methods used in LANs and WANs.
Understand the addressing mechanism used in the data link
layer and how network layer addresses are mapped to data
Understand how error control is handled at the data link layer.
Understand the functions of the data link layer.
After reading this chapter, the reader should be able to:
DUTIES OF THE
DATA LINK LAYER
Data link layer in the Internet model
Data-link layer duties
Technical Focus:Addresses in Local Area Networks
The physical address for most computers on local area networks
is imprinted on the network card that is installed in the computer.
If the user or network manager changes the network card
(because of a failure, for example), the physical address
of the computer is changed. In most cases, changing
the network card requires reconfiguration of the computer.
Data can be corrupted during transmission. For reliable communication, errors must be prevented, or detected and corrected.
In a single-bit error, only 1 bit in the data unit has changed.
A burst error means that two or more bits in the data unit have changed.
Burst error of length five
Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.
In vertical redundancy check (VRC), a parity bit is added to every data unit so that the total number of 1s becomes even.
Even parity VRC concept
In longitudinal redundancy check (LRC), a block of bits is divided into rows and a redundant row of bits is added to the whole block.
Stop and wait ARQ
The sending device keeps a copy of the last frame transmitteduntil it receives an acknowledgment for that frame.
Both data frames and ACK frames are
numbered 0 and 1 alternately. A data 0 frame is acknowledged
by an ACK 1 frame.
If an error is discovered in a data frame, a negative
acknowledgment (NAK) frame is returned.
If an expected acknowledgment is not received within an
allotted time period, the sender assumes that the last data frame was lost in transit and sends it again.
Technical Focus:Procedure for Stop-And-Wait ARQ
The sending device keeps copies of all transmitted frames until
they have been acknowledged.
In addition to ACK frames, a receiver can return a NAK frame
if the data have been received damaged. The NAK frame
tells the sender to retransmit a damaged frame.
Like stop-and-wait ARQ, the sending device in sliding-window
ARQ is equipped with a timer to enable it to handle lost
Technical Focus:Procedure for Sliding-Window ARQ
Flow control refers to a set of procedures used to restrict the amount of data the sender can send before waiting for acknowledgment.
Medium access methods
The poll/select method is mostly used in time-sharing systems when a central computer is used to control other computers.
Token passing network
Token passing is used mostly by local area networks (LANs). We discuss LANs in Chapter 9.
CSMA/CD is used in the Ethernet LAN discussed in Chapter 9.
CSMA/CA is used in the wireless LANs discussed in Chapter 10.
A data link protocol is a set of specifications used to implement the data link layer.
In a character-oriented protocol, the frame is interpreted as a series of characters. In a bit-oriented protocol, the frame or packet is interpreted as a series of bits.
All bit-oriented protocols are relatedto high-level data link control(HDLC), a bit-oriented protocol.