Data link layer
The data link layer is the 2nd layer in the OSI
model. its function is to primarily prepare packets for transmission over the
physical media. It also serves to control the flow of data that will traverse
the physical media. The Data Link PDU is the Frame. The two main functions
performed at this layer include.
§
Framing of packets
received from the network layer
§
Control of how data is
handled by the physical media through MAC (Media Access Control) and detection
of errors.
The devices at this
layer are referred to as nodes.
Protocols and services
In the other layers,
most of the protocols were defined by the RFCs. However, in the Data Link
layer, the IEEE defines most of the protocols. We will discuss some of these
protocols in chapters on switching.
Physical layer
The OSI model layers that we have discussed previously mainly
focus on conversion of the data from the user to a form that can be transmitted
over the physical media. The physical layer which is the 1stlayer of the OSI model, is responsible for transmitting the data
over the different types of physical media that may be present.
The frames that are
received from the data link layer are converted into bits for transmission over
the medium in this layer. Depending on the type of physical medium, the
physical layer may represent the bits as either; light signals, electrical
signals or waves for transmission over wireless media.
The transmission of
this bits over the physical media depends on the following:
§
The type of medium and
its connectors
§
The form of
representation of the bits; either electrical, light or wave signals
§
The data encoding and
the control information
§
The types of
transmitters and receivers in the networks
By this stage in
communication, the user data has undergone several processes; segmentation at
the transport layer, packets in the network layer, encapsulation into frames at
the data link layer and finally, the data is converted into one of the three
forms that can be transmitted over the physical media; electrical, light
signals or microwaves in the physical layer.
The three main forms
of transmission media that we use in networks are:
1. Copper cable
2. Fiber
3. Wireless
Just like the data
link layer, the standards in the data link layer are not defined under RFC,
however, other standards do exist.
Data carrying capacity
in the physical layer
The various physical
media offer transfer of data at different speeds. The data transfer rate can be
measured using three metrics.
1. Bandwidth – the capacity of the line. This is
measured in bp/s (bits per second), kb/s kilobit per second and mb/s megabit
per second.
NOTE: you should be careful not to confuse the bandwidth units.
The symbol used to represent bandwidth is bps or b/s. the letter b is in small
letters, this is not the same as the storage speed which is measured in Bytes
per second (Bps) with a capital B.
1. Throughput – this is the actual transferred
data over a certain amount of time, in most cases it is usually less than the
bandwidth.
2. Goodput – the actual useable data that has
been transferred over a certain period of time is known as goodput.
Ethernet
The standards at the
layer 1 and 2 of the OSI model are defined as Ethernet standards. The different
standards used in Ethernet define the different layer 1 and layer 2 protocols,
however, the format of the frame does not change.
As we mentioned in the
previous sections, the data link layer provides mechanisms for converting
packets to frames while the physical layer converts frames to bits which are
then transmitted over the physical media.
The hub
In the yester years,
the main Ethernet standards in many Local area networks was the HUB. The hub
used a technique known as the CSMA/CA (Carrier Sense Multiple Access/ Collision
Avoidance) and CSMA/CD (Carrier Sense Multiple Access/ Collision Detection).
This was implemented using either the bus or hub technologies.
Like the name
suggests, the hub was at the heart of communication in a network segment and
devices could only transmit data one at a time.
The HUB operates at
layer 1, this means that when data is received by a hub, it floods this
information to all the devices in the network. This is a major performance
issue since there may be congestion, network failure among others.
NOTE: the area in which the Hub is the center of communication is
refer to as a collision domain. This is because there is a high possibility of
collisions.
The communication in a
collision domain such as the one by a hub has the following characteristics.
§
Flooding – where
frames are sent to all devices in connected to the hub.
§
Only one device can
communicate at any instance
§
Communication is only
one way
§
One collision domain
The switch
The problems
associated with hubs, such as high failure rate due to collisions were a major
hindrance to growth of networks. As a result, a new solution to combat this was
introduced. The switch, changed communication in networks in many ways, each
port on a switch acted as a single collision domain, therefore, the switch had
as many collision domains as it had ports. This meant that the likelihood of
collisions was reduced. The switch worked at layer 2 as opposed to layer one
which meant that the switch could learn of the devices on its network and
instead of flooding frames, unicast communication was possible.
Further development
lead to bidirectional communication, instead of only one way communication as
seen in hubs, also with switches many devices can communicate simultaneously.
The area where a
switch is center of communication is known as a broadcast domain. In this type
of communication, each port is its own collision domain therefore, there are as
many collision domains as there are switch ports.
Some of the other characteristics of a switch
include:
§
Communication at
data-link layer instead of the physical layer
§
Full duplex
communication
§
Broadcast domains
instead of a single collision domain
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