Naming & Framing Ch. 6–Layer 2 Concepts

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

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?

IEEE—Layers 1 & 2 (or 2-layers Model) The Logical Link Control sublayer was created to provide different Layer 2 technologies a single method of accessing the higher layers. IEEE Base TIEEE 802.5FDDI IEEE Ethernet These sample technologies include Layer 1 & the MAC sublayer of Layer 2 LLC sublayer of Layer 2 How does LLC affect Encapsulation?

4 Points you must learn 1. Layer 2 communicates with the upper-level layers through Logical Link Control (LLC). 2. Layer 2 uses a flat addressing convention (Naming refers to the assignment of unique identifiers - addresses). 3. Layer 2 uses framing to organize or group the data. 4. 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 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 ( ). The different number sections, divided by hyphens do not mean anything. Each section of a phone number, however means something: (361)

Assigning MAC Addresses MAC Addresses are split in two: 1 st Half is the OUI. “Organizational Unique Identifier” 2 nd 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?

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? 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? What is 4F6A in decimal? Why Hex.? Look at our sample MAC address in decimal format: And then in binary format: 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 And 48 fields in binary!! AB-5F-DF-C1-FB 2 hex = 8 bits FF

Hexadecimal Assignment This 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 Address Start Frame Type/ Length DataFCS Stop Frame The frame can be any number of bytes in length, depending on the specific technology used.

The Generic Frame Address Start Frame Type/ Length DataFCS Stop 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.

The Generic Frame Address Start Frame Type/ Length DataFCS Stop 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.

The Generic Frame Address Start Frame Type/ Length DataFCS Stop 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)

The Generic Frame Address Start Frame Type/ Length DataFCS Stop 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.

The Generic Frame Address Start Frame Type/ Length DataFCS Stop 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. text).

The Generic Frame Address Start Frame Type/ Length DataFCS Stop 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.

The Generic Frame Address Start Frame Type/ Length DataFCS Stop 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.

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!!