Transcript Naming & Framing

Naming & Framing
Ch. 6–Layer 2 Concepts
Layer 2’s Job
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For every limitation of Layer 1, Layer 2 has a
solution.
Layer 1 cannot name computers.
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Layer 1 cannot talk to Layers 3-7.
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How does Layer 2 talk to higher layers?
Layer 1 cannot organize bits.
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How does Layer 2 name computers?
What does Layer 2 organize bits into?
Layer 1 cannot decide which computer to
transmit
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How does layer 2 makes that decision?
IEEE
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IEEE has had the greatest impact on
Layer 2 standards.
IEEE divided Layer 2 into two sublayers.
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Why?
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Protocol-Dependant VS Protocol-Independent
What are the two sublayers?
IEEE—Layers 1 & 2
(or 2-layers Model)
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10 Base T
IEEE 802.5
FDDI
Ethernet
IEEE 802.3
IEEE 802.2
How does LLC
affect
Encapsulation?
LLC sublayer of Layer 2
These sample
technologies include
Layer 1 & the MAC
sublayer of Layer 2
The Logical Link Control sublayer was created
to provide different Layer 2 technologies a
single method of accessing the higher layers.
4 Points you must learn
1.
2.
3.
4.
Layer 2 communicates with the upper-level
layers through Logical Link Control (LLC).
Layer 2 uses a flat addressing convention
(Naming refers to the assignment of unique
identifiers - addresses).
Layer 2 uses framing to organize or group the
data.
Layer 2 uses Media Access Control (MAC) to
choose which computer will transmit binary
data, from a group in which all computers are
trying to transmit at the same time.
MAC Address
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The MAC address has been mentioned
many times already this semester.
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What are some other names for the MAC
address?
What does the acronym MAC stand for?
Each interface on a router has a MAC
address.
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Where else do you find MAC addresses?
MAC Addressing Scheme
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MAC addresses are flat.
What do we mean by flat?
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Your Social Security number is a flat number
(987-65-4321).
The different number sections, divided by
hyphens do not mean anything.
Each section of a phone number,
however means something:
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(361) 853-0151
Assigning MAC Addresses
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MAC Addresses are split in two:
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1st Half is the OUI. “Organizational Unique
Identifier”
2nd Half is Vendor Assigned.
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Vendor is another way of saying “the
manufacturer of devices with MAC address.”
How many bits are in a MAC address?
How many (bytes) octets?
Assigning MAC Addresses
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How many bits are assigned to the
vendor as its OUI?
How many bits can the vendor assign?
Let’s look at a MAC address
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Two formats are used:
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In octets
In double octets
44-AB-5F-DF-C1-FB
44AB.5FDF.C1FB
We’ll use the octet format.
MAC Address Details
44-AB-5F-DF-C1-FB
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Our sample MAC address is:
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How many bits?
How many bytes?
What portion of this MAC is the OUI?
What portion of this MAC is vendor
assigned?
MAC Address Numbering
44-AB-5F-DF-C1-FB
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What numbering system is used for MAC
address?
What is 4F6A in decimal?
Why Hex.?
Look at our sample MAC address in decimal
format:
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68-171-95-223-193-251
And then in binary format:
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01000100.10101011.10111111.
11011111.11000001.11111011
Now do you
see why?
MAC Address Numbering
44-AB-5F-DF-C1-FB
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Hexadecimal Numbers are easier to represent and
type into lines of code:
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You only need 12 fields.
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The same number in decimal requires 18 fields.
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The biggest number you can have is FF-FF-FF-FF-FF-FF
255.255.255.255.255.255
And 48 fields in binary!!
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2 hex = 8 bits
11111111.11111111.11111111.11111111.11111111.11111111
FF
Hexadecimal Assignment
Hexadecimal Assignment
This assignment can be found at egroups.
Complete before you take the Chapter 6 test.
Framing Overview
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What is the PDU for Layer 2?
Without frames, streams of bits would
mean nothing but a bunch of 1s & 0s.
Framing is the first step in reorganizing
the bits into a form higher layers can
recognize or...
Framing is the last encapsulation step
before data is transmitted down the
wire as bits.
Framing Overview
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Different technologies (802.3, Token
Ring, FDDI) use different types of
frames at the data link layer.
However, all frames have certain
aspects in common.
What are the individual sections of a
frame called?
The Generic Frame
The frame can be any number of
bytes in length, depending on the
specific technology used.
Start
Frame
Address
Type/
Length
Data
FCS
Stop
Frame
The Generic Frame
For example, the frame of a token
in Token Ring is only 3 bytes.
But an Ethernet frame can be as
long as 1522 bytes.
Start
Frame
Address
Type/
Length
Data
FCS
Stop
Frame
The Generic Frame
Let’s look at each field:
The Start Frame field tells other devices
on the network that a frame is coming down
the wire.
Start
Frame
Address
Type/
Length
Data
FCS
Stop
Frame
The Generic Frame
The Address field stores the source and
destination MAC addresses. Can you guess
how many bytes this field would be?
(Remember: A MAC address is 48 bits)
Start
Frame
Address
Type/
Length
Data
FCS
Stop
Frame
The Generic Frame
The Type/Length field is an optional field
used by some protocols to either state what
type of data is coming or possibly the length
of the frame.
Start
Frame
Address
Type/
Length
Data
FCS
Stop
Frame
The Generic Frame
The Data field is the actual information being sent
by the upper layer protocols. Therefore, it will
include the network layer addresses and all upper
layer data—including end-user data (i.e. email text).
Start
Frame
Address
Type/
Length
Data
FCS
Stop
Frame
The Generic Frame
The Frame Check Sequence field is used for error
checking. The source calculates a number based on
the frame’s data and places that number in the FCS
field. The destination then recalculates the data to
see if the FCS matches. If they don’t match, the
destination deletes the frame.
Start
Frame
Address
Type/
Length
Data
FCS
Stop
Frame
The Generic Frame
The Stop Frame field, also called the Frame Trailer,
is an optional field that is used when the length of the
frame was not specified in the Type/Length field.
Start
Frame
Address
Type/
Length
Data
FCS
Stop
Frame
Media Access Control
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Specified by the technology being used.
Dictates who can transmit and when.
Two types:
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Deterministic: “Let’s take turns”
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What LAN technology is deterministic?
Opportunistic: “First come, first serve”
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What LAN technology is opportunistic?
Token Ring
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Token Ring uses deterministic Media
Access Control
We’ve talked about Token Ring some
and we’ll revisited it again in Chapter 7.
Ethernet & CSMA/CD
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Ethernet technologies are opportunistic.
They use CSMA/CD
Briefly Carrier Sense Multiple Access
with Collision Detection means...
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All devices listen for transmissions
If no transmissions, then device can
transmit
If a collision is detected (spike in voltage),
every device backs off a random amount of
time.
Three Major Technologies
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Ethernet
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Token Ring
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Logical bus (information flow is linear)
Physical star or extended star
Logical ring (information flows in a circle)
Physical star or extended star
FDDI
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Logical ring
Physical dual ring
More on this
in Chapter 7!!