802.3 : Ethernet
OVERVIEW
Originally developed at Xerox.
Frames Collide on the Common bus
Consider how collisions can occur, since a station will not begin transmitting until it senses an idle carrier.
If collision occurs, how to avoid repeating it, since both stations will try again? Solution is to have each station wait a random amount of time before retrying. The two stations will likely select a different random value.
Specific Ethernet solution:
NOTE: B.E.B. is compromise solution to conflicting situation:
if range of time slots too small, high probability of repeated collide
if range of time slots too large, throughput suffers (wait too long to retransmit)
Ethernet Frames
Ethernet uses 48-bit (6 byte) addresses. Each frame contains both source and destination address.
Recall that frames are broadcast.
Some frames are truly broadcast, e.g. intended for all stations. A special destination address, usually all 1s, is reserved for this purpose, and all NIC cards will match it.
Frame Formats
Generic: consists of frame header part, with addressing and type information, and frame data part with the bits/bytes being transmitted.
Note: an octet is the same thing as a byte
Ethernet frame
field |
bytes |
description |
preamble |
8 |
alternating 1 and 0 for synch (Manchester sine wave) |
destination |
6 |
all 1s means broadcast, first bit 1 = group address, else individual |
source |
6 |
physical address of source station |
type/length |
2 |
#data bytes, or predefined frame type (mutually exclusive ranges) |
data |
46-1500 |
If less than 46 bytes of data, is padded out to 46 bytes with zeroes. Necessary for minimum 512 bit frame length (relate to time slot) |
CRC |
4 |
Cyclic Redundancy Code |
Ethernet Cabling Nomenclature
Links courtesy of Purdue University web site for textbook: Computer Networks and Internets, by Douglas Comer.
NIC. Network Interface Card. Interface between computer and network. Handles network transmission so that the main CPU need not bother. Consider it a peripheral, like a disk or CD-ROM. Better reasons for NIC:
1. general purpose CPU is not fast enough to handle network speed traffic
2. CPU speeds vary from one computer to the next, whereas network operates at fixed speed
3. OS scheduling policies cannot guarantee the real-time response that network traffic processing requires
Ethernet NIC card being clumsily held (image 1-035)
Thicknet. Uses thick coaxial cable. (.5" O.D. aka 10Base5 500 meters max betwen repeaters). AUI (Attachment Unit Interface) cable connects NIC to transceiver, which is how it attaches to the bus. Connector at NIC and transceiver end called AUI connector. This is the original ethernet technology. Can connect multiple stations to one transceiver using a connection multiplexer.
AUI connection multiplexer (image 1-056)
Thinnet. Uses thin coaxial cable(.25" O.D. aka 10Base2 200 meters max between repeaters). Tranceivers not required (functionality built into NIC). NIC attaches to cable using BNC connector.
BNC connector plus terminator plus T-connector (image 1-54)Typically attach BNC T-connector to NIC, so that cable (the bus) can run directly from one computer to the next.
Three NICs on thinnet with BNC connectors and cables (image 1-017)NIC with AUI can be connected to thinnet cable using adapter/transceiver.
NIC has AUI but cabling is thinnet : inline adapter (image 1-016)NIC has AUI but cabling is thinnet : plug-in adapter (image 1-039)
10Base-T. Uses twisted-pair cable. NIC attaches to cable using RJ-45 connector (like telephone). Stations are connected to hub via the twisted pair. Hub has fixed capacity (e.g. 32 stations). Physically, the topology resembles a star, but logically it is still a bus all stations receive all network traffic. Older NICs can be attached using an adapter.
NIC has AUI but cabling is 10Base-T : inline adapter (image 1-23)
Related Home Pages:
notes | CSC 465 | Peter Sanderson | Computer Science | SMSULast reviewed: 11 March 1998
Peter Sanderson ( pete@csc.smsu.edu )