Ieee 802 Standard

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  • Topic: Ethernet, IEEE 802, IEEE 802.11
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IEEE 802
CSC 343·643

WAKE FOREST
U N I V E R S I T Y Department of Computer Science

Fall 2010

IEEE 802 Standards
• Previously we discussed abstract channel allocation protocols • IEEE has produced several LAN standards, known as IEEE 802 • IEEE 802 standards cover – Logical Link Control (LLC, IEEE 802.2) – CSMA/CD (IEEE 802.3) – Token bus (IEEE 802.4) – Token ring (IEEE 802.5) – Wireless (IEEE 802.11 and 802.16) – IEEE 802.1 serves as an introduction to IEEE 802

E. W. Fulp

CSC 343·643

Fall 2010

1

IEEE 802.3
• Standard describes a 1-persistent CSMA/CD LAN What are the transmission rules? • 802.3 History – Pure ALOHA for communication between Hawaiian Islands – CSMA/CD developed at Xerox PARC, connected 100 stations, first demo November 11, 1973 by Robert Metcalfe – System called Ethernet Why the name Ethernet? – DEC and Intel later created standard for 10 Mbps Ethernet, the basis for IEEE 802.3 • Remember 802.3,4,5 will describe the physical and MAC layers E. W. Fulp CSC 343·643 Fall 2010 2

IEEE 802.3 Physical Layer
• Four types of cabling
Name 10Base5 10Base2 10Base-T 10Base-F Cable thick coax thin coax twisted pair fiber optics Max segment 500m 200m 100m 2000m Nodes/seg 100 30 1024 1024 Advantages good for backbone cheap easy setup long distance

• Names have the form xBasey • 10Base5 (thick Ethernet) – Requires thick (size of a garden hose) yellow coax – Connections via vampire taps • 10Base2 (thin Ethernet) – Smaller diameter coax (cable TV size) – Connection via T-junctions E. W. Fulp CSC 343·643 Fall 2010 3

• Thick and thin Ethernet suffer from cable breaks, bad taps, and loose connectors real bus topology (no hub) • 10Base-T – Twisted pair (telephone), all connect via hub – Limited distance (100m) • 10Base-F – Fiber optics, long distances possible (low s/n ratio)

Controller

Controller Transceiver cable Vampire tap Transceiver 10Base5 Transceiver + controller

Twisted pair

Core

Connector Hub 10Base2 10Base-T

E. W. Fulp

CSC 343·643

Fall 2010

4

• Manchester encoding used by 802.3 baseband systems – Manchester and Differential Manchester Advantages and disadvantages – High is +0.85 volts and low is -0.85 volts Bit stream Binary encoding 1 0 0 0 0 1 0 1 1 1 1

Manchester encoding

Differential Manchester encoding

Transition here indicates a 0

Lack of transition here indicates a 1

E. W. Fulp

CSC 343·643

Fall 2010

5

802.3 MAC (Frame Structure)
• Frame structure is
Bytes 7 Preamble 1 2 or 6 Destination address 2 or 6 Source address Length of data field 2 0-1500 Data 0-46 Pad 4 Checksum

Start of frame delimiter

• Frame starts with preamble 7 bytes of [10101010...] – Results in a 10 MHz square wave for 5.6 µsec – Allows clock synchronization • Start delimiter is next [10101011] indicates - data is next • Source and destination addresses follow (MAC address) – 6 byte standard for 10 Mbps – First bit is 0 for ordinary address 1 for group address E. W. Fulp CSC 343·643 Fall 2010 6

– Group address - All stations in the group receive – If all bits are 1’s then broadcast frame • Bit 46 (adjacent to high-order-bit) distinguishes between local and global addresses – Local - Assigned by administrator, no meaning outside LAN – Global - Assigned by IEEE, stations have the same address • Length field identifies number of bytes in data field – 64 to 1500 bytes possible Why 64? Why 1500? – As transmission speed increases, frame size must increase (or cable length decrease)... • Checksum (CRC) of the data is added to the end

E. W. Fulp

CSC 343·643

Fall 2010

7

IEEE 802.3 MAC (Medium Access)
• CSMA/CD requires a random wait after collision • Binary exponential backoff algorithm – After collision time divided into discrete slots (equal to 2tprop ) – After first collision each station waits 0 or 1 slots (random) – After second (consecutive) collision each station waits either 0, 1, 2, or 3 slots (randomly selected) – Therefore, after...
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