welcome to the Cisco netacad introduction to networks video series by
Jason Johnson this video is module six data link layer the data link layer we
will be covering the purpose of the data link layer topologies and the data link
frame the purpose of the data link layer
the data link layer is responsible for communications between in device network
interface cards it does allow for upper layer protocols to access the physical
media and in Capulin capsule eights having trouble saying that the last few
days encapsulate s-- layer 3 packets ipv4 and ipv6 into layer 2 frames and it
also performs error detection and rejects corrupt frames and we can see an
example here of our NIC cards needing to be connected they're connected by v4
addresses here and the layer 2 and layer 3 you have the layer 2 header which is
your destination NIC and your source NIC at the MAC address and your layer 3 IP
packet and your ipv4 IP address the I Triple E 802 land and man standards are
specific to the type of network whether it's Ethernet Debu LAN W pan personal
area network the data link layer consists of two sub layers the logical
link control or the LLC and the media access control Mac and that's different
than the machine address code of the MAC address just so from a distinction there
the LLC sub layer communicates between the networking software at the upper
layers and the device hardware at the lower layers and the Mac sub layer is
responsible for data encapsulation and media access control and we can see an
example of this right here we have this is a packet we've got our network our
network layer protocol our data link the LLC sub layer
802 proto 2 2 protocol LLC sublayers that's governed by the I Triple E which
is one of those governing bodies the Mac sub-layer and then we have our and I
need to move this real quick our I Triple E 802 3 802 11 or 802 15
depending on which one we're looking at they're providing access to media
packets packets exchanged between nodes may experience numerous data link layer
and media transitions so at each hop along the path each time it stops at a
device whether it's a router or switch the router performs four basic layer 2
functions that one it accepts a frame from the network medium it D
encapsulates the frame to expose the encapsulated packet so it can read it
and see where it needs to be know the routing and then it Rhian capsule eights
the packet into a new frame and then that new frame gets sent onto the medium
to the new network segment data link layer protocols are defined by the
engineering organizations I Triple E ITU ISO and ANSI and I'm not gonna read all
these out you've seen these before in previous modules and I have encourage
you to go and check these out I will put links to these in the description below
on the video Wikipedia link so you can go check those out and find more
information out about them topologies we have two different type of topologies
we've touched briefly on these in previous modules we have the physical
topology and the logical topology the physical topology shows physical
connections and how devices are interconnected and the logical topology
identifies the virtual connections between devices using device interfaces
and IP addressing schemes here we have an example of the physical topology it
does show the physical connections and how the devices are interconnected so
here we have the internet and then we have broken off sometimes on your charts
you will not have this box showing the lands you won't have this back box which
help is a good identifier to show you where the lands are and where your
different areas are and your physical break up but up so you just have to kind
of look at these without the box sometimes but this is a good example
for what we're learning here this is showing us the physical servers the
switches the routers where they're located and it's giving us the physical
locations so if we have this chart and we know we need to go get classroom
three room 2127 we should have a series of laptops in there we won't actually
list out all of them on this chart here we just list out that we have endpoints
in there and then we would then on a separate on a separate list we would
have how many different devices and then more information about them but that
would be more on a that that's more of a list that you would keep for what your
assets are that you're responsible for or that you're tracking this is the
logical topology it'd have dent afire the virtual connections between devices
using device interfaces and IP addressing schemes we can see here that
it changes just a little bit that we still show the servers and the routers
and the switches and the end devices but now we have the actual connections so
we're showing that that's a you know gigabit 0/0 here we're showing our F 0
ones connections F 0 2 here so we know what the connections are on that
particular switch so if we go to that room off of our physical topology and go
find that switch we should be able to find a switch in that room and we should
be able to go to F 0 2 and find out where that that connection is made that
should be the connection that's made on that switch so this n is this gives us
the IP addressing in this case the ipv4 addressing now there are three common
physical LAN topologies we have a point-to-point a hub-and-spoke and a
mesh the point-to-point is the simplest and most common way in topology it
consists of a permanent link between two and the points the hub-and-spoke is
similar to a star topology where there's a central site and it interconnects
branch sites through point-to-point links and then we have the mesh which
provides high availability but requires every in system to be connected to every
other end system and it's a lot more expensive to do that because you have to
have more equipment and more cabling or not cabling but you have to have have
everything connected together
now the physical point-to-point topologies directly connect to nodes the
nodes may not share the media with other hosts and because of all frames on the
media can only travel to or from the two nodes point-to-point way and protocols
can be very simple so you have your network and then you have your nodes and
those are just point-to-point way in topology some LAN topologies are in
devices on lands are typically interconnected using a star or extended
star topology star and extended star topologies are easy to install
they're very scalable and are easy to troubleshoot so this is a star topology
here where you have a switch or a router usually you're gonna have a switch here
because a switch gives you more ports interfaces and you have all your end
devices on there this is an extended star topology where you may have two
routers connected to each other or two switches connected to each other and
then in Devices off of those we also had bus topologies and ring topologies but
they're not in use very often anymore they're not confident more to terms that
you need to know one is half duplex communication this only allows one
device to send a receive at a time on a shared medium and it's used on
Debu LANs and legacy bus topologies with ethernet hubs so think about half duplex
communication so you have two people trying to talk to each other but only
one can talk at a time so one person is gonna say information the other person
is gonna say yes I got your information and here's your information back and
they're gonna talk but only one person can talk at a time that's a half duplex
communication a full duplex communication allows both devices to
simultaneously transmit and receive on a shared medium Ethernet switches operate
in full duplex mode and so full duplex is you can get multiple devices talking
or both devices and it's much better than half duplex but there are instances
where you do have half duplex running
another term that you need to know is contention contention based access all
nodes operating in half duplex competing for use of the medium and some examples
are carrier sense multiple access with collision detection csma/cd
and that's used on legacy bus topology Ethernet and then you have carrier sense
multiple access with collision avoidance csma/ca and those as used on the
wireless LANs and I will tell you that you need to know the difference between
these two you will most likely see these on an exam at some point so you want to
make sure that you understand the csma csma/cd and the csma/ca
now I don't memorize what the carrier sense multiple access detection a
collision detection I know the detection versus the avoidance and what I do is
when I see a question with that I can I can't do that on them I probably
couldn't just write that out from memory but I do know what I see on an exam what
the difference between detection is and avoidance is the other term is
controlled access deterministic access where each node has its own time on the
medium and it's used on legacy networks such as a token ring or an arc net and
you won't see those very often today but you still need to know the term of
controlled access now the csma/cd that's used by legacy Ethernet lands operates
in half duplex mode and it uses collision detection and the csma/cd has
collision detection process devices transmitting simultaneously will result
in a single collision on the shared medium devices detect the collision and
devices wait a random period of time and then retransmit the data ca is used by
Triple E 802 11 W LANs it operates in half duplex mode where only on devices
sends and receives at a time and the collision avoidance process when
transmitting devices also include the time duration needed for the
transmission and other devices on the shared medium receive the time duration
information and know how long the medium will be unavailable data link frame the
frame is data is encapsulated by the data link layer with the header and a
trailer to form a frame and that's one of the words you do need to be familiar
with you're gonna see that on an exam a frame the data link frame has three
parts a header data in the trailer so the fields of the header and the trailer
vary according to the data link layer protocol and the
out of control information carried within the frame varies according to the
access control information and the logical topology so we'll take a look at
a visual here so we have our header we have our packet of data and we have our
trailer and within that within the header we have our frame start we have
our addressing we have the type and our control you have your data and then you
have your error detection and your frame stop so your frames start and stop just
identifies the beginning and ending of the frame so it says hey here's where
I'm starting here's where I'm ending when it gets when it gets those when it
gets that data the addressing indicates the source and destination nodes
you know sources that you have to know where it's coming from you need to know
where it's going the type identifies the encapsulated layer 3 protocol the
control identifies flow control services the data is your data and the error
detection is used to determine transmission errors because built in
it'll say you know it'll have a number and if that number isn't accurate on the
other end based upon all this information if some of these pack some
of these bits did not come through the error detection will pick up on that and
the receiving end will be like wait a minute I did not get the same
information that was sent based on the error detection error area here and it
will reject that frame the layer 2 addresses they're also referred to as
physical addresses they're contained in the frame header they're used only for
local delivery of a frame on the link they're not routed they're not used for
routing and updated by each device that forwards the frame so if we're going on
local see the layer 2 right here is just your MAC address that's the destination
NIC and source NIC and it is in within the same local area network or in the
same physical address or referred to as a physical address
it's in the same area now the logical topology and physical media determine
the data link protocol used so Ethernet 802 11 Wireless point-to-point high
level data link control or frame relay and each protocol performs media access
control for specified logical topologies these you need to be familiar with but
you're not going to be working with these in depth in this course you'll get
into those later horses this has been module six
and some of the things that we've learned in this model the data link
layer of the OSI model or layer two prepares network data for the physical
network the data link layer is responsible for network interface card
to network interface card communications the I Triple E 802 land man data link
layer consists of the followings two sub layers LLC and Mac those are important
the two types of topologies are land and land three common types of physical LAN
topologies are point-to-point hub-and-spoke and mesh half duplex
communications exchange data in one direction at a time in full duplex sends
and receives data simultaneously in contention based multi Nexus networks
all nodes are operating in full hour and half duplex examples of contention based
are csma/cd for bus and csma/ca 402 b lands the data link frame has three
basic parts the header the data and the trailer and frame fields include frame
start stop indicator flags addressing type control data and error detection
and the data link addresses are also known as physical addresses and last
data link addresses are only used for link local delivery of frames okay so
this has been module 6 I hope this information was helpful for you and I
hope you have a great day