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Sem 1 Version 2 Chapter 6
Layer 2 - Concepts
Layer 1 involves media, signals, bit streams that travel on media, components that
put signals on media, and various topologies. Each of its functions has its limitations.
Layer 2 addresses these limitations.
For each limitation in Layer 1, Layer 2 has a solution.
Layer 1 cannot communicate with the upper-level layers; Layer 2 does that
with logical link control (LLC).
Layer 1 cannot name or identify computers; Layer 2 uses an addressing (or
naming) process.
Layer 1 can only describe streams of bits; Layer 2 uses framing to organize
or group the bits.
Layer 1 cannot choose which computer will transmit binary data, from a
group in which all computers are trying to transmit at the same time; Layer 2
accomplishes this by using a system called Media Access Control (MAC).
IEEE LAN standards (including IEEE 802.3 and IEE 802.5) are the IEEE
communications standards and are the predominant LAN standards in the
world today.
IEEE 802.3 specifies the physical layer, Layer 1, and the channel-access
portion of the data link layer, Layer 2.
Recognized IEEE sublayers are:
Media Access Control--- MAC (transitions down to media)
Logical Link Control--- LLC (transitions up to the network layer)
IEEE standards involve only the two lowest layers, therefore the data link
layer is broken into two parts:
the technology-independent 802.2 LLC standard
the specific, technology-dependent parts that incorporate Layer 1, and
the rest of the OSI layers
These standards are active, vital agreements that make technology
compatible and computer communication possible.
Technically speaking, the IEEE standard appears to violate the OSI model in two ways. First,
they define their own layer, the LLC, complete with its own PDU, interfaces, etc. Second, it
appears that the MAC layer standards 802.3 and 802.5 cross over the Layer 2/Layer 1 interface.
However, 802.3 and 802.5 define the naming, framing, and Media Access Control Rules around
which specific technologies were built.
The first seeming contradiction is that Ethernet, a term many of the students will have heard, is
both a Layer 2 AND a Layer 1 technology.
The second seeming contradiction is that a sublayer, the Logical Link Control 802.2 Sublayer, has
been 'carved out' of Layer 2.
The NIC Card is where the Layer 2 MAC address resides, but in many technologies the NIC card
also has the transceiver (a Layer 1 device) built into it and connects directly to the physical
medium. So it would be accurate to characterize the NIC as both a Layer 1 and Layer 2 device.
The emphasis should be on the following:
1) LLC is defined according to IEEE standard 802.2
2) LLC is independent of the specific LAN technology used and
3) LLC serves to communicate upward to layer 3 and downward to the
technology-specific MAC sublayer.
IEEE created the Logical Link Sublayer to satisfy the need to
have part of the datalink layer function independent of
existing technologies.
This layer provides versatility in services to network layer
protocols above it, while communicating effectively with the
diversity of technologies below it. The LLC, as a sublayer,
participates in the encapsulation process. The LLC PDU is
sometimes also called an LLC packet, but this is not a widely
used term.
LLC takes the network protocol data, an IP packet, and adds a more control
information to help deliver that IP packet to its destination. The information it adds
is a DSAP, an SSAP, and Control information to the IP packet data. This re-packaged
IP packet is then sent to the MAC sublayer for handling by the specific technology for
further encapsulation and data. An example of this
specific technology might one of the varieties of Ethernet, or Token Ring, or FDDI.
If we are going to have multiple computers accessing the networking, then some
provision for orderly access to that medium must be made. This is the job of the
Media Access Control, or MAC, sublayer.
Layer 2 into 4 basic concepts:
the LLC
the issue of naming, (addressing),
the issue of framing, (grouping the bits)
the issue of Media Access Control (orderly access to the medium)
There are three more key concepts that you must learn about
Layer 2. These concepts are:
1.LLC is technology independent.
2.It uses a flat naming convention (Naming refers to the
assignment of unique identifiers).
3.Data are carried by frames.
While it may seem abstract and of questionable use, the
students will need to read hexadecimal numbers when
troubleshooting LANs and when configuring routers. Also,
hexadecimal is used extensively in other computer fields.
Hexadecimal (hex) system is a shorthand method for representing the
8-bit bytes that are stored in the computer system. It was chosen to represent
identifiers because it can easily represent the 8-bit byte by using only two
hexadecimal symbols.
MAC addresses are sometimes referred to as burned-in addresses (BIAs) because they
are burned into read-only memory (ROM) and are copied into random-access memory
(RAM) when the NIC initializes.
Hexadecimal is a Base 16 numbering system that is used to represent MAC
addresses. It is referred to as Base 16 because it uses sixteen symbols;
combinations of these symbols can then represent all possible numbers. Since
there are only 10 symbols that represent digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9), and
the Base 16 requires 6 more symbols, the extra symbols are the letters A, B, C,
D, E, and F.
4F6A = (4 x 163)+ (F[15] x 162)+ (6 x 161)+ (A[10] x 160)
Convert decimal numbers to hex numbers.
As with binary numbers, converting from decimal to binary is done with an
approach called the remainder method. This method uses successive
divisions of the base number of the system, which in this case are base 16.
Convert the decimal number 24032 to hex
Example:
24032 divided by 16 – 16 into 24032 will give us 1502 with a remainder of 0
1502 divided by 16 – 16 into 1502 will give us 93 with a remainder of 14 or E
93 divided by 16 – 16 into 93 will give us 5 with a remainder of 13 or D
13 divided by 16 – 16 into 13 will give us 0 with a remainder of 3
By collecting all the remainders backward, we have the hex number 3DE0.
Converting hexadecimal numbers to
decimal numbers can be done by
multiplying the hex digits by the base
number of the system, base 16, raised to
an exponent based on its position.
Example (working from right to left)
Convert the hex number 3F4B to
decimal
3 x 163 = 12288
F(15) x 162 = 3840
4 x 161 = 64
B(11) x 160 = 11
16203 = decimal equivalent
There are two methods of converting
binary to hex.
The first is to convert binary to decimal
and then convert decimal to hex, using the
methods you have learned so far. The
second method is to use a scientific
calculator.
It is important to be able to do decimal,
binary, and hexadecimal conversions
without a calculator.
In this course, the largest decimal number
you have to deal with is 255; the longest
binary number you have to deal with is 8
bits (1111 1111); and the largest
hexadecimal number is 2 hex bits FF.
Explain how the data link layer requires identifiers for computers
At the data link layer, a header, and possibly a trailer, is added to upper layer data.
The header and trailer contain control information intended for the data link layer
entity in the destination system. Data from upper layer entities is encapsulated in
the data link layer header and trailer.
MAC addresses are 48 bits in length and are expressed as 12 hexadecimal digits.
The first six hexadecimal digits, which are administered by the IEEE, identify the
manufacturer or vendor and thus comprise the Organizational Unique Identifier
(OUI). The last six hexadecimal digits comprise the interface serial number, or
another value administered by the specific vendor:
Every computer has a unique
way of identifying itself.
Each computer, whether it is
attached to a network or not,
has a physical address. No two
physical addresses are ever
alike.
Referred to as the Media Access
Control Address or MAC
Address, the physical address is
located on the Network
Interface Card or NIC Card.
Explain how the NIC is constantly sampling the cable
for destination MAC addresses that match its own
On an Ethernet network, when one device wants to send
data to another device, it can open a communication
pathway to the other device by using its MAC address.
Here's how: when a source sends data out on a network, the
data carries the MAC address of its intended destination.
As this data travels along the network media, the NIC in
each device on the network checks to see if its MAC address
matches the physical destination address carried by the data
packet.
If no match is made, the NIC ignores the data packet and it
continues along the network to the next station.
An important part of both encapsulation and decapsulation is the
addition of source and destination MAC addresses. Information
cannot be properly sent or delivered on a network without
these addresses. Specifically, some of the header information
involved in encapsulating data are the source and destination
MAC addresses.
Explain the main limitation of MAC addressing
MAC addresses are vital to the functioning of a computer network. They provide a
way for computers to identify themselves; they give hosts a permanent, unique name,
and they are not going to run out anytime soon, since there are xxxx possible MAC
address names.
There is one major disadvantage to MAC addresses. There have no structure and are
considered flat address space. Different vendors have different OUI’s, but they are like
social security numbers or personal identification numbers. Once your network gets
beyond a few computers in size, this disadvantage becomes a problem.
The primary limitation of MAC addressing: it is
a flat, non-hierarchical naming system which
does not scale well to large numbers of
computers.
Since we are interested in internetworking large
numbers of computers, another addressing
scheme -- imposed at Layer three -- is necessary.
Encoded bit streams on physical media represent a tremendous technological
accomplishment, but they, alone, are not enough to make communication happen.
Framing capability helps obtain essential information, that could not, otherwise, be
obtained with coded bit streams alone:
which computers are communicating with one another
when communication between individual computers begins and when it
terminates
a record of errors that occurred during the communication
whose turn it is to "talk" in a computer "conversation"
Once you have a way to name computers, you can move on to framing, which is the
next step.
Framing is the Layer 2 encapsulation process; a frame is the Layer 2 protocol data unit
(PDU).
Compare diagrams of bits and frames.
When you are dealing with bits, the most accurate diagram to visualize is a voltage
versus time graph.
However, since you are dealing with larger units of data and addressing and control
information, voltage versus time graphs could become ridiculously large and
confusing. Another type of diagram, the frame format diagram, is based on voltage
versus time graphs. They are read from left to right, just like an oscilloscope graph.
The frame format diagrams show different groupings of bits, or fields, that perform
other functions.
Picture frames delineate
the extent of a picture.
Pallets make goods ready
for transport.
Movie frames carry a
sequence of visual
information.
All of these analogies apply
to the framing of bits of
information for transport
on the physical medium.
The student to read a wide range of frame, packet, and segment diagrams without
being overwhelmed.
The generic frame is a theoretical construct, and abstraction not unlike the OSI
model, which can help with the introduction and retention of the technology and
protocol specific frames (802.3, 802.5, FDDI), packets (IP), and segments (TCP and
UDP) which the student will encounter these in later chapters.
Out of the chatter and noise and abyss of the medium, a clear signal to other hosts that
something important is to follow is the clarion call of the start frame delimiter.
Different technologies handle this with different bit patterns, but the idea is the same.
All frames contain naming information, such as the name of the source computer (in the
form of its MAC address) and the name of the destination computer (in the form of its
MAC address).
All frames have some specialized fields. In some technologies, a "length" field specifies
the exact length of a frame.
Some have a "type" field, which specifies the Layer 3 protocol making the sending
request. There is also a set of technologies where no such fields are used.
Emphasize the idea that encapsulated data from the upper layers is what constitutes
the data for Layer 2. For example, complete or fragmented IP datagrams are placed in
this frame data field.
Instructor Note:
The purpose of this target indicator is to introduce students to error correction. While
this is a massive topic in its own right, at this point in the curriculum the students
should be exposed to the notion that special numbers -- the frame check sequences -- are
generated as kind of a packing slip to indicate what the contents of the frame are and to
allow checks to see if damages occur.
There are three ways to calculate the frame check sum:
*** cyclic redundancy check (CRC) - performs polynomial calculations on the data
*** two-dimensional parity - adds an 8th bit that makes an 8-bit sequence have an
odd or even number of binary 1s
*** Internet checksum - adds the numbers to determine a number
Instructor Note:
The purpose of this target indicator is to emphasize that just as
the start frame delimiter announced the beginning ( Here comes the
frame) of a frame, an end frame delimiter announces that the bit
stream that makes up one particular frame has ended.
This is intimately tied to the contention issues of which machine
next has "control" of transmitting on the medium.
Interestingly, in Ethernet the end frame delimiter is simply
silence; other technologies uses particular bit patterns.
Define media access control (MAC).
Media Access Control (MAC) refers to protocols that
determine which computer on a shared-medium environment
(collision domain) is allowed to transmit the data.
Media Access Control, with LLC, comprises the IEEE version
of Layer 2, that is, MAC and LLC are both sublayers of Layer
2.
There are two broad categories of Media Access Control.
deterministic (taking turns),
non-deterministic (first come, first served).
The vehicle is the frame, the bridge is the shared medium, and paying the fee
at the tollbooth is the protocol that allows access to the bridge.
The people are the data, the cars are the frames, the roller coaster tracks are
the shared medium, and the protocol is the waiting in line and presentation of the ticket.
There is one shared medium - the space above the meeting table (air) through which signals, or spoken words, are communicated. The protocol for
determining access to the medium is that the first person that speaks, when everyone
quiets down, can talk as long as he/she wishes, until finished. In this analogy, the words
of the individual coworker are packets, the air above the meeting table is the medium,
and the "first person to speak" group rule is the protocol.
The purpose of this target indicator is to introduce one of the two basic categories of
MAC algorithms. Deterministic approaches to media access control guarantee a
regulated sequence of opportunities to transmit. The Token passing approach is
presented as the main deterministic algorithm. This approach to media access control
may seem preferable to students, as every computer is guaranteed its turn to transmit.
Thus, theoretically, collisions are impossible. However, there are built-in inefficiencies
(waiting for the token to come to a particular station wanting to transmit even when no
other stations want to transmit) that make other media access control strategies
desirable.
The purpose of this target indicator is to introduce one of the two basic categories of
MAC algorithms. Opportunistic approaches to media access control rely on random
number backoff algorithms to allocate slot times and opportunities to transmit. While
seemingly chaotic, they are actually extremely efficient at allocating access to the
medium. The CSMA/CD algorithm is introduced. Students should memorize the
meaning of the acronym. Students should be encouraged to put the algorithm in their
own words. Students should be required to flowchart the algorithm in their own
words.
The
End
The purpose of this target indicator is to foreshadow the next chapter. Three popular
Layer 2 technologies are going to investigated in detail in Chapter 7 -- Token Ring
(deterministic, Token Passing), FDDI (deterministic, token passing), and Ethernet
(opportunistic, CSMA/CD).
Token-ring - logical ring and physical bus topology.
FDDI - logical ring and physical dual-ring topology
Ethernet - physical star, extended star, and logical bus topology