Chapter 6 – Layer 2 MAC, Frames, Hex, etc.

Layer 1 / Layer 2 Layer 1 Signals, media, bit streams Layer 2 –Communicates with upper layers using Logical Link Control (LLC) –Identifies computers using MAC address –User framing to organize/group bits –Uses Media Access Control to decide which computer will transmit data

IEEE Specifications for Ethernet –Divides layer 2 into two sub-layers Media Access Control (MAC) Logical Link Control (LLC) –Divided to allow data link layer to be independent of existing technologies NIC is built into some transceivers; so it is accurate to call it a Layer 2 and Layer 2 device –Defines fields in data link frames Allow higher layer protocols to share 1 physical data link for Token Ring

LLC Sublayer of Layer 2 – participates in encapsulation process –LLC PDU is called LLC packet –Adds control information to IP packet Destination Service Access Point (address) Source Service Access Point (address) –Manages communications between devices over a single link – Can be connection-oriented or connectionless

Media Access Control sublayer protocols that a host follows in order to access the physical media –Token –CSMA/CD

Layer 2 Communicates with upper-level layers using LLC Uses flat addressing (MAC address) Uses framing to organize data –Frames divided into fields Uses Media Access Control to choose which computer will transmit binary data

MAC Addresses 48 bits Expressed as 12 Hex digits –First 6 are OUI assigned to id vendor –Second 6 assigned by vendor to id equipment –Burned-in to card; copied into RAM when NIC initialized

Hexadecimal Conversion Decimal 33 = binary Binary = Hex 21 –Divide into group of 4 binary digits –Start from right side to count –Convert each group of 4 binary digits to decimal equivalent Hex 21 = decimal 33 –2(16**1)+1(16**0) = 33

Hexadecimal Fun To convert directly from decimal to Hex –Divide repeatedly by 16 –65/16 = 4 R1 –1/16 – 0 R 4 Use remainders (backward) to from hex equivalent In this example, 41

Binary to Hex and Back Know binary equivalent of 0-15(decimal) Know decimal equivalent of a group of 4 binary digits Be able to convert mentally 0000 = = 13 = D 0001 – = 14 = E 0010 = = 15 = F

Layer 2 Frame Header and trailer added to data (packet) Encapsulation process called Framing Layer 2 uses Frames as PDU –Frames are divided into fields Each field performs a special function –Three analogies to explain framing Picture frame, packaging/shipping, Movies/TV

Frame Fields Frame Start Address-- source and destination computer Length – how long Type – upper layer protocol Data – actual data being transmitted (can be padded) FCS – error detection Frame Stop – end of frame

FCS Calculations (Frame Check Sequence) Cyclic redundancy check (CRC) Two-dimensional parity –Adds an 8 th bit Internet checksum –Adds value of all data bits

Media Access Control (MAC) Deterministic –Take Turns Token Ring –Talking stick analogy Non-deterministic –First come, first served Ethernet –CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

Layer 2 Technologies Ethernet Token Ring FDDI All three specify Layer 2 issues (LLC, framing, naming, MAC)