Naming & Framing
Ch. 6–Layer 2 Concepts
By: Allan Johnson
Layer 2’s Job For every limitation of Layer 1,
Layer 2 has a solution. Layer 1 cannot name computers.
How does Layer 2 name computers? Layer 1 cannot talk to Layers 3-7.
How does Layer 2 talk to higher layers?
Layer 1 cannot organize bits. What does Layer 2 organize bits into?
IEEE IEEE has had the greatest impact
on Layer 2 standards.
IEEE divided Layer 2 into two sublayers. Why?
What are the two sublayers?
IEEE—Layers 1 & 2
The Logical Link Control sublayer was created to provide different Layer 1 technologies a single method of accessing the higher layers.
IEEE
802.3
10 B
ase T
IEEE
802.5
FD
DI
IEEE 802.2Eth
ern
et
These sample technologies
include Layer 1 & the MAC sublayer
of Layer 2
LLC sublayer of Layer 2
MAC Address The MAC address has been
mentioned many times already this semester. 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. Where else do you find MAC addresses?
MAC Addressing Scheme MAC addresses are flat. What do we mean by flat?
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: (361) 853-0151
Assigning MAC Addresses MAC Addresses are split in two:
1st Half is the OUI. What does OUI mean? 2nd Half is Vendor Assigned.
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? How many nibbles?
Assigning MAC Addresses 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
Two formats are used: In octets 44-AB-5F-DF-C1-FB In double octets 44AB.5FDF.C1FB
We’ll use the octet format.
MAC Address Details
Our sample MAC address is: How many bits? How many bytes? How many nibbles?
44-AB-5F-DF-C1-FB
What portion of this MAC is the OUI? What portion of this MAC is vendor assigned?
MAC Address Numbering
What numbering system is used for MAC address?
Why? Look at our sample MAC address in
decimal format: 68-171-95-223-193-251
And then in binary format: 01000100.10101011.10111111. 11011111.11000001.11111011
44-AB-5F-DF-C1-FB
Now do you see
why?
MAC Address Numbering
Hexadecimal Numbers are easier to represent and type into lines of code: You only need 12 fields.
The biggest number you can have is FF-FF-FF-FF-FF-FF The same number in decimal requires 18 fields.
255.255.255.255.255.255 And 48 fields in binary!!
11111111.11111111.11111111.11111111.11111111.11111111
44-AB-5F-DF-C1-FB
2 hex = 8 bits
FF
Hexadecimal Assignment
Hexadecimal AssignmentThis assignment can be found at egroups.
Complete before you take the Chapter 6 test.
Framing Overview 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 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
AddressStartFrame
Type/Length
Data FCS StopFrame
The frame can be any number of bytes in length, depending
on the specific technology used.
The Generic Frame
AddressStartFrame
Type/Length
Data FCS StopFrame
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.
The Generic Frame
AddressStartFrame
Type/Length
Data FCS StopFrame
Let’s look at each field:
The Start Frame field tells other devices on the network that a frame is coming down the wire.
The Generic Frame
AddressStartFrame
Type/Length
Data FCS StopFrame
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)
The Generic Frame
AddressStartFrame
Type/Length
Data FCS StopFrame
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.
The Generic Frame
AddressStartFrame
Type/Length
Data FCS StopFrame
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).
The Generic Frame
AddressStartFrame
Type/Length
Data FCS StopFrame
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.
The Generic Frame
AddressStartFrame
Type/Length
Data FCS StopFrame
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.
Media Access Control Specified by the technology being
used. Dictates who can transmit and when. Two types:
Deterministic: “Let’s take turns” What LAN technology is deterministic?
Opportunistic: “First come, first serve” What LAN technology is opportunistic?
Token Ring
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 Ethernet technologies are
opportunistic. They use CSMA/CD Briefly Carrier Sense Multiple Access
with Collision Detection means... 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 Ethernet
Logical bus (information flow is linear) Physical star or extended star
Token Ring Logical ring (information flows in a
circle) Physical star or extended star
FDDI Logical ring Physical dual ring
More on this in Chapter
7!!
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