naming & framing

Institute of Technology Sligo - Dept of Computing

Naming & FramingNaming & Framing

Ch. 6–Layer 2 ConceptsCh. 6–Layer 2 Concepts

Paul Flynn


Layer 2’s JobLayer 2’s Job

For every limitation of Layer 1, Layer 2 For every limitation of Layer 1, Layer 2 has a solution.has a solution.

Layer 1 Layer 1 cannotcannot name computers. name computers. How does Layer 2 name computers?How does Layer 2 name computers?

Layer 1 Layer 1 cannotcannot talk to Layers 3-7. talk to Layers 3-7. How does Layer 2 talk to higher layers?How does Layer 2 talk to higher layers?

Layer 1 Layer 1 cannotcannot organize bits. organize bits. What does Layer 2 organize bits into?What does Layer 2 organize bits into?


IEEEIEEE

IEEE has had the greatest impact on IEEE has had the greatest impact on Layer 2 standards.Layer 2 standards.

IEEE divided Layer 2 into two sublayers.IEEE divided Layer 2 into two sublayers. Why?Why?

What are the two sublayers?What are the two sublayers?


IEEE—Layers 1 & 2IEEE—Layers 1 & 2

The Logical Link Control sublayer was created The Logical Link Control sublayer was created to provide different Layer 1 technologies a single to provide different Layer 1 technologies a single method of accessing the higher layers.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 AddressMAC Address

The MAC address has been mentioned The MAC address has been mentioned many times already this semester.many times already this semester. What are some other names for the MAC What are some other names for the MAC

address?address? What does the acronym MAC stand for?What does the acronym MAC stand for?

Each interface on a router has a MAC Each interface on a router has a MAC address.address. Where else do you find MAC addresses?Where else do you find MAC addresses?


MAC Addressing SchemeMAC Addressing Scheme

MAC addresses are flat.MAC addresses are flat. What do we mean by flat?What do we mean by flat?

Your Social Security number is a flat number Your Social Security number is a flat number (987-65-4321).(987-65-4321).

The different number sections, divided by The different number sections, divided by hyphens do not mean anything.hyphens do not mean anything.

Each section of a phone number, Each section of a phone number, however means something:however means something: (361) 853-0151 (361) 853-0151


Assigning MAC AddressesAssigning MAC Addresses

MAC Addresses are split in two:MAC Addresses are split in two: 11stst Half is the OUI. What does OUI mean? Half is the OUI. What does OUI mean? 22ndnd Half is Vendor Assigned. Half is Vendor Assigned.

Vendor is another way of saying “the Vendor is another way of saying “the manufacturer of devices with MAC address.”manufacturer of devices with MAC address.”

How many bits are in a MAC address?How many bits are in a MAC address? How many (bytes) octets?How many (bytes) octets? How many nibbles?How many nibbles?


Assigning MAC AddressesAssigning MAC Addresses

How many bits are assigned to the How many bits are assigned to the vendor as its OUI?vendor as its OUI?

How many bits can the vendor assign?How many bits can the vendor assign? Let’s look at a MAC addressLet’s look at a MAC address

Two formats are used:Two formats are used: In octets In octets 44-AB-5F-DF-C1-FB44-AB-5F-DF-C1-FB In In doubledouble octets octets 44AB.5FDF.C1FB 44AB.5FDF.C1FB

We’ll use the octet format.We’ll use the octet format.


MAC Address DetailsMAC Address Details

Our sample MAC address is:Our sample MAC address is: How many bits?How many bits? How many bytes?How many bytes? How many nibbles?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 NumberingMAC Address Numbering

What numbering system is used for MAC What numbering system is used for MAC address?address?

Why?Why? Look at our sample MAC address in decimal Look at our sample MAC address in decimal

format:format: 68-171-95-223-193-25168-171-95-223-193-251

And then in binary format:And then in binary format: 01000100.10101011.10111111.01000100.10101011.10111111. 11011111.11000001.1111101111011111.11000001.11111011

44-AB-5F-DF-C1-FB

Now do you see

why?


MAC Address NumberingMAC Address Numbering

Hexadecimal Numbers are easier to represent and Hexadecimal Numbers are easier to represent and type into lines of code:type into lines of code: You only need 12 fields. You only need 12 fields.

The biggest number you can have is FF-FF-FF-FF-FF-FFThe biggest number you can have is FF-FF-FF-FF-FF-FF

The same number in decimal requires 18 fields. The same number in decimal requires 18 fields. 255.255.255.255.255.255255.255.255.255.255.255

And 48 fields in binary!!And 48 fields in binary!! 11111111.11111111.11111111.11111111.11111111.1111111111111111.11111111.11111111.11111111.11111111.11111111

44-AB-5F-DF-C1-FB

2 hex = 8 bits

FF


Hexadecimal AssignmentHexadecimal Assignment

Hexadecimal AssignmentThis assignment can be found at egroups.

Complete before you take the Chapter 6 test.


Framing OverviewFraming Overview

What is the PDU for Layer 2?What is the PDU for Layer 2? Without frames, streams of bits would Without frames, streams of bits would

mean nothing but a bunch of 1s & 0s.mean nothing but a bunch of 1s & 0s. Framing is the first step in reorganizing Framing is the first step in reorganizing

the bits into a form higher layers can the bits into a form higher layers can recognize or...recognize or...

Framing is the last encapsulation step Framing is the last encapsulation step before data is transmitted down the wire before data is transmitted down the wire as bits.as bits.


Framing OverviewFraming Overview

Different technologies (802.3, Token Ring, Different technologies (802.3, Token Ring, FDDI) use different types of frames at the FDDI) use different types of frames at the data link layer.data link layer.

However, all frames have certain aspects However, all frames have certain aspects in common.in common.

What are the individual sections of a frame What are the individual sections of a frame called?called?


The Generic FrameThe Generic Frame

AddressStartFrame

Type/Length

Data FCS StopFrame

The frame can be any number of bytes in length, depending

on the specific technology used.



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.



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.



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)



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.



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).



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.



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 ControlMedia Access Control

Specified by the technology being used.Specified by the technology being used. Dictates who can transmit and when.Dictates who can transmit and when. Two types:Two types:

DeterministicDeterministic: “Let’s take turns”: “Let’s take turns” What LAN technology is deterministic?What LAN technology is deterministic?

OpportunisticOpportunistic: “First come, first serve”: “First come, first serve” What LAN technology is opportunistic?What LAN technology is opportunistic?


Token RingToken Ring

Token Ring uses deterministic Media Token Ring uses deterministic Media Access ControlAccess Control

We’ve talked about Token Ring some We’ve talked about Token Ring some and we’ll revisited it again in Chapter 7.and we’ll revisited it again in Chapter 7.


Ethernet & CSMA/CDEthernet & CSMA/CD Ethernet technologies are opportunistic. Ethernet technologies are opportunistic.

They use CSMA/CD They use CSMA/CD Briefly Briefly CCarrier arrier SSense ense MMultiple ultiple AAccess ccess

with with CCollision ollision DDetection means...etection means... All devices listen for transmissionsAll devices listen for transmissions

If no transmissions, then device can If no transmissions, then device can transmittransmit

If a collision is detected (spike in voltage), If a collision is detected (spike in voltage), every device backs off a random amount of every device backs off a random amount of time.time.


Three Major TechnologiesThree Major Technologies EthernetEthernet

Logical bus (information flow is linear)Logical bus (information flow is linear) Physical star or extended starPhysical star or extended star

Token RingToken Ring Logical ring (information flows in a circle)Logical ring (information flows in a circle) Physical star or extended starPhysical star or extended star

FDDIFDDI Logical ringLogical ring Physical dual ringPhysical dual ring

More on this in Chapter

7!!

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