Naming%20

1
Naming Framing

• Ch. 6Layer 2 Concepts

2
Layer 2s 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?
• Layer 1 cannot decide which computer to transmit
• How does layer 2 makes that decision?

3
IEEE

• IEEE has had the greatest impact on Layer 2
  standards.
• IEEE divided Layer 2 into two sublayers.
• Why?
• Protocol-Dependant VS Protocol-Independent
• What are the two sublayers?

4
IEEELayers 1 2 (or 2-layers Model)
How does LLC affect Encapsulation?
IEEE 802.2
These sample technologies include Layer 1 the
MAC sublayer of Layer 2
Ethernet
IEEE 802.3
10 Base T
IEEE 802.5
FDDI

• The Logical Link Control sublayer was created to
  provide different Layer 2 technologies a single
  method of accessing the higher layers.

5
4 Points you must learn

• 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.

6
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?

7
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
• [phone]

8
Assigning MAC Addresses

• MAC Addresses are split in two
• 1st Half is the OUI. Organizational Unique
  Identifier
• 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?

9
Assigning MAC Addresses

• How many bits are assigned to the vendor as its
  OUI?
• How many bits can the vendor assign?
• Lets look at a MAC address
• Two formats are used
• In octets 44-AB-5F-DF-C1-FB
• In double octets 44AB.5FDF.C1FB
• Well use the octet format.

10
MAC Address Details
44-AB-5F-DF-C1-FB

• Our sample MAC address is
• How many bits?
• How many bytes?

• What portion of this MAC is the OUI?

• What portion of this MAC is vendor assigned?

11
MAC Address Numbering
44-AB-5F-DF-C1-FB

• What numbering system is used for MAC address?
• What is 4F6A in decimal?
• Why Hex.?
• 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

Now do you see why?
12
MAC Address Numbering
44-AB-5F-DF-C1-FB

• 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!!
• 11111111.11111111.11111111.11111111.11111111.11111
  111

13
Hexadecimal Assignment
Hexadecimal Assignment This assignment can be
found at egroups. Complete before you take the
Chapter 6 test.
14
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.

15
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?

16
The Generic Frame
The frame can be any number of bytes in length,
depending on the specific technology used.
17
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.
18
The Generic Frame
Lets look at each field
The Start Frame field tells other devices on the
network that a frame is coming down the wire.
19
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)
20
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.
21
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 dataincluding end-user data (i.e.
email text).
22
The Generic Frame
The Frame Check Sequence field is used for error
checking. The source calculates a number based on
the frames data and places that number in the
FCS field. The destination then recalculates the
data to see if the FCS matches. If they dont
match, the destination deletes the frame.
23
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.
24
Media Access Control

• Specified by the technology being used.
• Dictates who can transmit and when.
• Two types
• Deterministic Lets take turns
• What LAN technology is deterministic?
• Opportunistic First come, first serve
• What LAN technology is opportunistic?

25
Token Ring

• Token Ring uses deterministic Media Access
  Control
• Weve talked about Token Ring some and well
  revisited it again in Chapter 7.

26
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.

27
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!!
28
See You Next Meeting