Transcript Ethernet - ECE Users Pages

Ethernet
Source for this lecture: http://www.ethermanage.com/ethernet/ethernet.html
Read Forouzan Pages 43-49
``The diagram ... was drawn by Dr. Robert M. Metcalfe in 1976 to present
Ethernet ... to the National Computer Conference in June of that year. On
the drawing are the original terms for describing Ethernet. Since then other
terms have come into usage among Ethernet enthusiasts.''
The Ethernet Sourcebook, ed. Robyn E. Shotwell (New York: NorthHolland, 1985), title page.
1
Invention of Ethernet
“In late 1972, Metcalfe and his Xerox PARC colleagues developed the
first experimental Ethernet system to interconnect the Xerox Alto, a
personal workstation with a graphical user interface. The experimental
Ethernet was used to link Altos to one another, and to servers and laser
printers. The signal clock for the experimental Ethernet interface was
derived from the Alto's system clock, which resulted in a data
transmission rate on the experimental Ethernet of 2.94 Mbps.
Metcalfe's first experimental network was called the Alto Aloha Network.
In 1973 Metcalfe changed the name to "Ethernet," to make it clear that
the system could support any computer--not just Altos--and to point out
that his new network mechanisms had evolved well beyond the Aloha
system. He chose to base the name on the word "ether" as a way of
describing an essential feature of the system: the physical medium (i.e.,
a cable) carries bits to all stations, much the same way that the old
"luminiferous ether" was once thought to propagate electromagnetic
waves through space. Thus, Ethernet was born.”
2
Ethernet Standards
•
Formal specifications for Ethernet were published in 1980 by a multi-vendor
consortium that created the DEC-Intel-Xerox (DIX) standard. This effort
turned the experimental Ethernet into an open, production-quality Ethernet
system that operates at 10-Mbps. Ethernet technology was then adopted for
standardization by the LAN standards committee of the Institute of Electrical
and Electronics Engineers (IEEE 802).
•
The IEEE standard was first published in 1985, with the formal title of "IEEE
802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
Access Method and Physical Layer Specifications." The IEEE standard has
since been adopted by the International Organization for Standardization
(ISO), which makes it a worldwide networking standard.
•
The IEEE standard provides an "Ethernet like" system based on the original
DIX Ethernet technology. All Ethernet equipment since 1985 is built
according to the IEEE 802.3 standard, which is pronounced "eight oh two
dot three." To be absolutely accurate, then, we should refer to Ethernet
equipment as "IEEE 802.3 CSMA/CD" technology. However, most of the
world still knows it by the original name of Ethernet, and that's what we'll call
it as well.
3
Ethernet System Elements
The Ethernet system consists of three basic
elements:
1. the physical medium used to carry Ethernet
signals between computers,
2. a set of medium access control rules embedded
in each Ethernet interface that allow multiple
computers to fairly arbitrate access to the shared
Ethernet channel, and
3. an Ethernet frame that consists of a
standardized set of bits used to carry data over
the system.
4
CSMA/CD Protocol
•
•
•
•
Access to the shared channel is determined by the medium access control
(MAC) mechanism which is based on a system called Carrier Sense
Multiple Access with Collision Detection (CSMA/CD).
Each interface must wait until there is no signal on the channel, then it can
begin transmitting. If some other interface is transmitting there will be a
signal on the channel, which is called carrier. All other interfaces must wait
until carrier ceases before trying to transmit, and this process is called
Carrier Sense.
No one gets a higher priority than anyone else, and democracy reigns. This
is what is meant by Multiple Access
Since signals take a finite time to travel from one end of an Ethernet system
to the other, the first bits of a transmitted frame do not reach all parts of the
network simultaneously. Therefore, it's possible for two interfaces to sense
that the network is idle and to start transmitting their frames simultaneously.
When this happens, the Ethernet system has a way to sense the "collision"
of signals and to stop the transmission and resend the frames. This is called
Collision Detect.
5
Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
 Monitor to ensure wire free before transmitted
 Monitor during transmission to make sure not garbled by a
collision
 Once collision detected stop transmission
6
Collision
• If more than one station happens to transmit on the Ethernet
channel at the same moment, then the signals are said to collide.
The stations are notified of this event, and instantly reschedule their
transmission using a specially designed backoff algorithm. As part of
this algorithm the stations involved each choose a random time
interval to schedule the retransmission of the frame, which keeps
the stations from making transmission attempts in lock step.
• Repeated collisions for a given packet transmission attempt indicate
a busy network. The expanding backoff process, formally known as
"truncated binary exponential backoff," is a clever feature of the
Ethernet MAC that provides an automatic method for stations to
adjust to traffic conditions on the network. Only after 16 consecutive
collisions for a given transmission attempt will the interface finally
discard the Ethernet packet.
7
Binary exponential backoff retransmission algorithm
 Station encounters first collision randomly select number
from set {0,1}
 Other station(s) involved in collision select a random value
from a set corresponding to number of collisions this other
station has experienced.
 These randomly chosen values represent how many “slot
times” a station must wait before attempting a
retransmission.
 An Ethernet slot time is 512 bits (64 bytes). For 10 mbit/s
Ethernet one bit takes 0.1 microseconds to transmit => slot
time is 51.2 microseconds.
Note: Minimum size Ethernet frame is 64 bytes. This includes
source and destination MAC, type, and CRC fields
8
The First 10 Rounds of Binary
Exponential Backoff Algorithm
Round
Size
Elements
of set
1
2
{0,1}
2
4
{0,1,2,3}
3
8
{0,1,2,3….,7}
4
16
{0,1,2,3………..15}
5
32
{0,………………………..31}
6
64
{0,……………………………….{63}
7
128
{0,……………………………………..{127}
8
256
{0,…………………………………………….{255}
9
512
{0,……………………………………………………511}
10
1024
{0,…………………………………………………………{1023}
For round 11 through 16 {0,…………………………………………………….{1023}
17 => Excess collision error
9
Every time a station successfully transmits a frame, it gets to reset
its backoff choice set to {0,1}.
half duplex -- only one end station may transmit at the same time.
full duplex – both end stations may transmit at the same time if
desired.
10
Ethernet Frame Format
Original Metcalfe protocol adopted by Digital Equipment, Intel,
Xerox. => “DIX” Ethernet.
Ethernet 2 is updated same DIX frame format.
IEEE Standard 802.3 Not same format
Ethernet: <8> <6>
Preamble
<6>
DA SA
46 to 1,500
<2> bytes
<4>
T
DATA
<6> <2>
If Fail
Discard
<bytes>
<12>
CRC Interframe GAP
802.3 <8>
<6>
<4>
Preamble
DA SA L DATA CRC
<12>
Interframe GAP
}
**Some more stuff here!
11
Minimum Length
• Require a minimum length for CSMA/CD to work
correctly
• If complete frame is sent before a collision is
detected by sender, too late for sender to repeat
because MAC layer has discarded the
completed frame.
• Smallest frame length allowed by standard is a
frame of length 51.2 microseconds which is 51.2
microseconds x 10 mbits/seconds = 512 bits (64
bytes)
• For 18 bytes of header and the 4 CRC bytes of
trailer, 64-18=46 data bytes minimum
12
Slot Time
• Slot time in Ethernet is defined as the
round trip time plus the time to send the
jam sequence
• Slot time is defined in bits and is defined
as 512 bits
• Slot time in 10 mbit/sec ethernet is 51.2
microseconds
13
Jam sequence
• Assuming a minimum size ethernet frame of 512
bytes
• If a sender at one end sends a frame and
another sender at the far end also sends a
frame, need for the senders to be able to see the
collision and to send a jam sequence to inform
other stations of the collision.
• The round trip time plus the time to send out the
jam sequence should be less than the time for a
sender to send the minimum frame. Sender
must be able to know about the collision before it
sends the entire frame.
14
Maximum Network Length
• Maximum length = propagation speed x (slot
time / 2)
• In wire, electricity travels at 2/3 the rate for travel
in air => 2 x 108 m/s
• Maximum length for 10 mbit/sec ethernet is
(2x108) x ((51.2 x 10-6)/2) = 5120 meters
• Taking into account the delay needed for
repeaters, jammers, etc the standard uses only
2,500 meters as the maximum cable length
allowed.
15
Maximum length
• The standard defines the maximum
allowed frame length to be 1518 bytes.
Removing the 18 bytes of header and
trailer, data may be no longer than 1,500
bytes.
• Maximum limit was due to 1) memory
expense so wanted a small buffer and to
2) prevent one computer from dominating
the medium
16
MAC Addresses
• The first two fields in the frame carry 48-bit addresses, called the
destination and source addresses. The IEEE controls the
assignment of these addresses by administering a portion of the
address field. The IEEE does this by providing 24-bit identifiers
called "Organizationally Unique Identifiers" (OUIs), since a unique
24-bit identifier is assigned to each organization that wishes to build
Ethernet interfaces. The organization, in turn, creates 48-bit
addresses using the assigned OUI as the first 24 bits of the address.
This 48-bit address is also known as the physical address, hardware
address, or MAC address.
• A unique 48-bit address is commonly pre-assigned to each Ethernet
interface when it is manufactured, which vastly simplifies the setup
and operation of the network.
17
Multicast and Broadcast
• A multicast address allows a single Ethernet frame to be
received by a group of stations. Network software can
set a station's Ethernet interface to listen for specific
multicast addresses. This makes it possible for a set of
stations to be assigned to a multicast group which has
been given a specific multicast address. A single packet
sent to the multicast address assigned to that group will
then be received by all stations in that group.
• There is also the special case of the multicast address
known as the broadcast address, which is the 48-bit
address of all ones. All Ethernet interfaces that see a
frame with this destination address will read the frame in
and deliver it to the networking software on the
computer.
18
Preamble:
AA AA AA AA AA AA AA AB
(64 bits)
Interfame GAP 96 bits 10 Mbits => 9.6 s GAP helps improve
fairness of collision detection algorithm.
Ethernet
T => Type Field Identifies Protocol
IE 0x0800 => IP
How can we tell which type (DIX or 802.3) frame we received?
Look at 2 bytes after MAC fields in the received Ethernet frame:
•If this 2 byte field  1500 this is a valid length and we have an
802.3 frame. Since this is 802.3 we have a “Logical Link Control
(LLC) subnetwork attachment point (SNAP) Header” which may
be used for other purposes with other protocols (like SNA).
•If this 2 byte field  1501 This is DIX frame format. No type
fields in DIX are ever less than 1501. Thus if you see  1501, it is
a DIX frame. Example 0x0800 is a value  1501.
19
802.3 Special Case => Repackage Old DIX Type Field
<1>
AA
<1>
AA
<1>
03
<3>
000000
<2>
DIX Type
IE 0x0800
**This is the some
other stuff from 2
slides previous.
20
Purpose of Ethernet
• A given Ethernet system can carry several
different kinds of high-level protocol data.
For example, a single Ethernet can carry
data between computers in the form of
TCP/IP protocols as well as Novell or
AppleTalk protocols. The Ethernet is
simply a trucking system that carries
packages of data between computers; it
doesn't care what is inside the packages.
21
What is the maximum number of Ethernet frames per second on a 100,000,000 bit per second
(One Hundred Megabit per Second) Ethernet network when each Ethernet frame carries 46
bytes of IP data?
The size of the Ethernet frame is
6 bytes of Destination MAC Address + 6 Bytes of Source MAC Address + 2 Bytes of Type +
46 Bytes of IP data + 4 Bytes of CRC = 64 Byte Ethernet Frames
Each Ethernet frame must have 8 Bytes of Preamble and 12 Bytes of Interframe Gap resulting
in 8 + 12 + 64 = 84 bytes transmitted on the wire for each Ethernet frame transmitted.
100,000,000 Bits/sec x (1 Byte/8 Bits) x (1 Frame/84 Bytes) = 148,809.5
From Spirent SmartBits 2000
Port-Pair Throughput
=====================
Frame size
64
100
200
300
1000
Ethernet Frame Size in Bytes (Includes overhead byts but not Preamble or Interframe Gap
100Mb MaxRate 148810 104167 56818 39063 12255
Theoretical Maximum Possible Number Frames per second
Avg % passed
17.53 25.35 43.80 65.02 99.68
-----------------------------------------------------5 to 6
26089 26406 24888 25400 12216
Percent of the Max number possible that actually passed
Number of frames that passed our Linux Router
22
Trivia: Ethernet transmits the most significant byte first but in each
byte it transmits the least significant bit first.
Important: Ethernet Cables
Computer 1
HUB
Computer 2
Straight-through Patch
Cables
Computer 1
Computer 1
Need to use a cross over
cable.
23
Because one pair of wires used to transmit, another pair used to
receive transmissions. Must swap transmit/receive.
Trivia: In 1979 Metcalfe founded 3 COM Corporation:
computers communicate compatibility
Neighbor Address Discovery
For IP over Ethernet, need a way to discover MAC-Layer
addresses of its IP neighbors.
24
We use an address resolution protocol (ARP) cache. IP uses this
helper protocol ARP that runs directly over Ethernet and does
not include an IP header.
ARP is in RFC-826
ARP sends a frame to the broadcast address (of all 48 bits one).
Included in this is the target’s protocol address (IP address). This
is the (IP) address of the end station for which we want the MAC
address.
ONLY the target machine sends its MAC address back to
requester.
One requester receives ARP reply it stores it for some hold time.
Common time is from 15 minutes to 24 hours. (No standard
time in RFC-826)
25
MAC DA = Broadcast
MAC SA =
my hardware address
Ethermet
Frame
Header
ARP Etype ( 0806 )
Enet HW Type (1)
Prot Type ( 0800 )
OpCode = 1 (Req)
hLen = 6
pLen = 4
Source’s
Hardware
Address
Source’s Protocol Address
Target’s Hardware Address
Target’s Pro-tocol Address
ARP
Packet
Payload
Padding
Padding
Padding
Padding
Frame Check Sequence (CRC - 32)
FIGURE 8. 3 Ethernet frame carrying ARP packet.
26
Example ARP table on a Windows PC:
C:\>arp -a
Interface: 130.207.231.158 on Interface 2
Internet Address
Physical Address
Type
130.207.231.1
00-04-de-1b-cf-fc
dynamic
130.207.231.12
00-a0-c9-72-52-23
130.207.231.161
00-90-27-73-c5-74
dynamic
130.207.231.177
00-a0-c9-31-87-04
dynamic
dynamic
C:\>
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LAN interconnection devices operate on “ OSI ” (Open Systems
Interconnection Reference Model) Layers:
Layer 1 :
Layer 2:
Layer 3:
Physical Layer
Data Link Layer
Network Layer
Application
Presentation
Application
Session
Transport
Transport
Internet
Network
Subnetwork
Data Link
Physical
Internet Protocol
Stack
Open Systems
Interconnection
Reference Model
28
OSI Layer 1:
Repeaters
HUBS
( Not too common now)
( Very common )
HUB
To Backbone
Four HUB rule - only 4 hub hops allowed in a
“ Collision Domain ”
Hubs are physical layer devices that just “Repeat” what they see, “errored” frames and all.
Endstation
1 -- TD+ 
2 -- TD- 
3 -- RD+ 
4
5
6 -- RD- 
7
8
10 Base-T pinout and
HUB attachment
Hub Side
1 -TD+
2 -TD3 -RD+
4
5
6 -RD7
8
Endstation
1 -- TD+ 
2 -- TD- 
3 -- RD+ 
4
5
6 -- RD- 
7
8
Endstation
1 
TD+
2
TD3 
RD+
4
5
6
RD7
8
Crossover cable
Functionality
29
OSI Layer 2:
Bridges
Switches
Layer - 2 switches
( Not common now )
( What we call bridges now )
( Way cool marketing term)
Forwarding decisions based only on data link layer header, that is the MAC Destination Address.
Use a table made from observing which addresses are seen on each port.
No more than 7 bridges in diameter.
Today’s ethernet environments use lots of ethernet switches, reducing the collision domain.
Collision domain is the set of ethernet segments that can directly hear each others frames.
Worst case is all inputs want to go out on the same output. Must have some buffering
and some sort of fairness algorithm inside
30
10 megabit/second
•
•
•
•
The original standardized Ethernet system operates at 10-Mbps, and there are four
base band media segments defined in the 10-Mbps Ethernet standard.
The four media types are shown with their IEEE shorthand identifiers. The IEEE
identifiers include three pieces of information. The first item, "10", stands for the
media speed of 10-Mbps. The word "BASE" stands for "baseband," which is a type of
signaling. Baseband signaling simply means that Ethernet signals are the only signals
carried over the media system.
The third part of the identifier provides a rough indication of segment type or length.
For thick coax the "5" indicates the 500 meter maximum length allowed for individual
segments of thick coaxial cable. For thin coax the "2" is rounded up from the 185
meter maximum length for individual thin coaxial segments. The "T' and "F" stand for
"twisted-pair" and "fiber optic," and simply indicate the cable type.
The thick coaxial media segment was the first one defined in the earliest Ethernet
specifications. Next came the thin coaxial segment, followed by the twisted-pair and
fiber optic media segments. The twisted-pair segment type is the most widely used
today for making network connections to the desktop.
31
100 megabit/second
• Compared to the 10-Mbps specifications, the 100-Mbps
system results in a factor of ten reduction in the bit-time,
which is the amount of time it takes to transmit a bit on
the Ethernet channel. This produces a tenfold increase
in the speed of the packets over the media system.
However, the other important aspects of the Ethernet
system including the frame format, the amount of data a
frame may carry, and the media access control
mechanism, are all unchanged.
• The Fast Ethernet specifications include mechanisms for
Auto-Negotiation of the media speed. This makes it
possible for vendors to provide dual-speed Ethernet
interfaces that can be installed and run at either 10-Mbps
or 100-Mbps automatically.
32
100 megabit/second
• The first item, "100", stands for the media speed of 100-Mbps. The
"BASE" stands for "baseband," which is a type of signaling.
Baseband signaling simply means that Ethernet signals are the only
signals carried over the media system.
• The third part of the identifier provides an indication of the segment
type. The "T4" segment type is a twisted-pair segment that uses
four pairs of telephone-grade twisted-pair wire. The "TX" segment
type is a twisted-pair segment that uses two pairs of wires (This is
what we are using in the lab). The "FX" segment type is a fiber optic
link segment based on the fiber optic physical medium standard
that uses two strands of fiber cable. The TX and FX medium
standards are collectively known as 100BASE-X.
33