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!!