4/1/2015
Ethernet Basics
based on Chapter 4 of CompTIA
Network+ Exam Guide, 4th ed., Mike
Meyers
Topics
✦
History
✦
Ethernet Frames
✦
CSMA/CD
✦
Obsolete versions
✦
10Mbps versions
✦
Segments
✦
Spanning Tree Protocol
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Ethernet – Early History
✦
1970: ALOHAnet, first wireless packet-switched
network
‣
‣
‣
✦
1973: Ethernet prototype developed at Xerox PARC
‣
‣
✦
Norman Abramson, Univ. of Hawaii
Basis for Ethernet’s CSMA/CD protocol
1972: first external network connected to ARPANET
(Palo Alto Research Center)
2.94 Mbps initially
1976: "Ethernet: Distributed
Packet Switching for Local
Computer Networks" published
in Communications of the ACM.
‣
‣
Bob Metcalfe and David Boggs
sometimes considered “the beginning of Ethernet”
Ethernet goes Mainstream
✦
1979: DEC, Intel, Xerox collaborate on a commercial
Ethernet specification
‣
‣
✦
1983: IEEE 802.3 specification formally approved
‣
✦
✦
Ethernet II, a.k.a. “DIX” Ethernet
(Digital Equipment Corporation)
Differs from Ethernet II in the interpretation of the third
header field
1987: alternatives to coaxial cables
‣
IEEE 802.3d: FOIRL, Fiber Optic Inter-Repeater Link
‣
IEEE 802.3e: 1 Mbps over Twisted Pair wires (whoopee!)
1990: Twisted-Pair wiring takes over
‣
IEEE 802.3i: 10 Mbps over Twisted-Pair – 10Base-TX,
10Base-T4
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the Future is Now (next chapter)
(and Now is so Yesteryear…)
1995 – Now: speed and cabling improvements
✦
1995: 100Mbps varieties
✦
1999: 1Gbps on twisted-pair
✦
2003-2006: 10Gbps on optical fiber and UTP
✦
2010: 40Gbps, 100Gbps (802.3ba)
‣
‣
optical fiber or twinaxial cable
point-to-point physical topology; for backbones
Ethernet (802.3) relation to OSI
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the Ethernet frame format
Preamble:
10101010
..
10101011
✦
Dest.
MAC
6 octets
Src. MAC
6 octets
Type /
length
2 octets
Ethernet II header contains:
Payload
Padding?
FCS
0..1500
octets
46..0
octets
4
octets
✦
6 octets: destination MAC address
‣ 6 octets: source MAC address
‣ 2 octets: payload-type field
‣
✦
802.3 differs in the third field:
‣
✦
payload length instead of type
0-1500 octets: Payload,
supplied by a higher protocol layer
Could be 802.2
could be layer 3
‣ 46-0 octets: Padding w/ 0-bytes to insure
minimum frame length
‣
‣
✦
4 octets: Ethernet footer contains
FCS (Frame Check Sequence)
‣
Physical frame starts with an
8-octet preamble consisting of
1010…10101011
10Mbs versions only
‣
✦
Maximum frame length is 1518
octets
‣
‣
✦
including the FCS
excluding the preamble
Minimum length is 64 octets
‣
✦
IFG:
96 bittimes
assures collision detection
Physical frame is followed by an IFG,
InterFrame Gap
‣
a CRC checksum
‣
no signal transitions
96 bit-times in duration
Ethernet Addresses
✦
✦
Also called MAC
addresses, hardware or
physical addresses, or
Layer 2 addresses
6 octets long
‣
✦
✦
✦
‣
✦
an “octet” refers to a
“byte” and is used in
networking
First three octets refer to
the manufacturer or
vendor
Last three octets must be
unique within a
mfr/vendor
Written as 6 pairs of
hexadecimal digits
✦
separated by colon or dash
Examples:
‣
00:1a:6b:4e:3f:1b
• Linux
‣
40-A8-F0-A2-DD-CE
• Windows
Broadcast address:
ff:ff:ff:ff:ff:ff
‣
As a destination, this
means “send to all
available nodes”
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wireshark activity
✦
start wireshark
✦
Display filter eth.type
‣
‣
✦
Display filter: eth.len
‣
‣
✦
Any types other than 0x0800?
What layer-3 protocol(s)?
Observe layer-2 protocol(s)
What payload(s)?
Display filter: eth.addr==<your MAC address>
‣
What traffic is coming from, going to your
machine?
Ethernet II versus
802.3 with 802.2
✦
802.3
length < 1500
left: an Ethernet II
frame specifies a type,
and leaves the next
layer to find the data’s
end
‣
✦
Ethernet II
type > 1500
all types are values greater
than 1500 (0x0600)
• viz., IP is type 0x0800
right: an 802.3 frame
specifies the payload
length, and includes
802.2 headers
‣
length is always 1500 or
less
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Ethernet II frame
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802.3 frame, with 802.2 headers
✦
This frame shows
802.3 and 802.2
headers.
✦
It also shows the
FCS (checksum)
field, which
happens to be
incorrect.
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So, how does the NIC determine where a
frame ends?
✦
✦
Ethernet II frame doesn’t specify its overall
length
10Base5, 10Base2 standards:
‣
✦
10BaseT:
‣
✦
NIC detects end of signal - absence of current
NIC listens for a special TP_IDL signal on the wire,
followed by InterFrame Gap
100BaseT, GigE, 10GigE:
‣
4B/5B encoded “start-of-frame signals” and “endof-frame signals” replace preamble and TP_IDL
Introduction
10BaseT: The FCS and TP_IDL signal
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Ethernet and Cabling – the Coaxial Era
✦
✦
✦
✦
10Base5 (standard IEEE
802.3, 1983)
10Base2 (standard IEEE
802.3a, 1985)
Physical bus topology
required a logical bus
topology
CSMA/CD protocol used in
the Collision Domain
10Base5
✦
1983: IEEE 802.3
‣
✦
The original form
“Thick coax” cable
‣
‣
RG-8/U or RG-11 specified
half-inch diameter
✦
“Vampire tap” connection
punctures insulation to make
electrical connections
✦
“10Base5”:
‣
‣
‣
10 Mbps
Baseband signaling
500 meters maximum length
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10Base2
✦
1985: IEEE 802.3a
‣
✦
“Thin coax” cable
‣
‣
‣
✦
✦
Physical update for cheaper
cabling
RG-58a/u
5mm diameter
electrically compatible with thick coax
BNC connectors allow easy disconnection,
reconnection
“10Base2”:
‣
‣
‣
10 Mbps
Baseband signaling
200 meters maximum length
(actually 185m)
CSMA / CD
✦
Carrier-Sense Multiple Access with Collision
Detection
✦
Multiple Access: more than one node can transmit on
the shared medium
✦
Carrier-Sense: a NIC that wants to transmit must first
listen for an active transmission
‣
if it doesn’t hear an idle “carrier signal” it backs off and waits
before trying again
✦
Collision Detection: if a NIC hears interference while it
is transmitting, it knows that a collision with another
transmission has occurred
✦
Colliding nodes attempt to re-transmit using an
“Exponential Backoff” approach
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Exponential Backoff
✦
When a NIC detects a collision, it:
‣
continues transmitting until at least 64 octets are sent, then
stops
selects a k equal to 0 or 1, and waits for k "slot times"
• slot time equals 512 bit times for 10Mbps, 100Mbps; 4096 bit
times for 1Gbps
‣
repeats the CSMA/CD attempt
‣
✦
If a second collision occurs, it selects k = 0,1,2,or 3;
on a third collision, k = 0,1,2,3,4,5,6, or 7… and so on
‣
‣
‣
this gives exponentially increasing maximum waiting time,
and exponentially decreasing chance of two NICs repeatedly
colliding.
After 10 collisions the range of k stays at 0..1023
After 16 collisions, the attempt is abandoned
Segments and Collision Domains
✦
All the nodes sharing a
cable form a segment
✦
The segment defines a
collision domain
‣
Frames on this segment
may collide with each
other…
These two segments form
separate collision domains
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Extended Collision Domains
✦
A repeater, such as this
one, connects two
segments into a single
collision domain
‣
✦
✦
Frames on either
segment can
collide with others.
Hubs (a.k.a. multi-port
repeaters) do the same
thing, with multiple
segments
Switches don't – they
keep collision domains
separate
Hubs and Extended Collision Domains
✦
A repeater (or
hub) joins two (or
more) segments
‣
✦
Layer-1 device
These segments
share a common
collision domain
‣
The hub will
broadcast all
frames, as if the
two segments
were one
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The (Obsolete) 5-4-3 Rule
✦
The “5-4-3” rule
(or “5-4-3-2-1” rule)
‣
‣
‣
‣
≤ 5 segments (cables)
connected by ≤ 4
repeaters
≤ 3 active segments (i.e.,
with transmitting nodes)
2 passive segments
✦
Single Collision Domain
✦
(Not important in
switched networks…)
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how big can a collision domain be?
✦
✦
✦
5-4-3 rule limits amount of cable in use
How far apart could two computers be, using
10Base5 cable?
How far apart could two computers be, using
10Base2 cable?
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Ethernet on Twisted-Pair
✦
Physical star, but Ethernet is still a bus-oriented
protocol
✦
10Base-T (IEEE 802.3i, 1990)
✦
‣
Twisted-Pair cables
• Cheaper, easier to use than coax
‣
100m maximum length
100Base-T (IEEE 802.3u, 1995)
‣
‣
‣
✦
“Fast Ethernet”
100Base-T4: Cat3, 4 pairs used
100Base-TX: Cat5, 2 pairs used
Full Duplex (IEEE 802.3x, 1997)
‣
Applies to 100BaseT and later
Ethernet on Optical Fiber
✦
✦
Alternate, longer-distance media extend
Ethernet's reach
10Base-FL (IEEE 802.3j, 1993)
‣
‣
‣
✦
10Mbps, multimode optical fiber
2000m maximum length
Not common
100Base-FX (IEEE 802.3u, 1995)
‣
‣
“Fast Ethernet”
100Base-FX: multimode optical fiber
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10Base-T and Hubs
✦
✦
✦
10Base-T cables
connect one host
to a hub
Hubs can have
dozens of ports,
each connected
to a separate host
Ports are completely interconnected
‣
Hubs are OSI layer-1 devices, no smarts built in
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CSMA/CD and a Hub – Simulation from Tomsho
✦
Web Link:
‣
http://montcs.bloomu.edu/Networking/Simulations/Tomsho/csmacd.swf
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10Base-T and Collision Domains
✦
Hubs are multiport
repeaters
‣
‣
5-4-3 rule still applies
Shorter cables mean
smaller collision domains
✦
All nodes in the network
on the right are in the
same collision domain
✦
Optical fiber (10BaseFL) permits much larger
collision domains
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(other definitions of “segment”)
✦
"Segment" may have other meanings…
✦
Related meanings in Ethernet:
‣
‣
✦
"Segment" and "Collision Domain" are sometimes
used interchangeably.
A "Segment" can refer to a "Broadcast Domain".
Unrelated meanings in the TCP/IP world:
‣
"Segment" refers to a "protocol data unit" at the
Transport layer of the OSI or TCP/IP stack.
•
‣
versus "frame" which refers to a protocol data unit at
the Datalink and Physical layers
– viz., Ethernet frame
"Segment" can mean an IP subnetwork.
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Ethernet Switches
✦
✦
Switches are physically similar to hubs
Each cable between a host and a switch is a
separate Ethernet segment
‣
✦
Only two nodes (host and switch) on each segment,
so collisions are not much of a problem
Switch must be smart enough – and fast
enough – to act like a separate node on each
of its ports
‣
‣
More expensive than a hub
Switches were very uncommon in 10Base-T
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Switch Simulation, from Tomsho
✦
Web Link:
‣
http://montcs.bloomu.edu/Networking/Simulations/Tomsho/switch.swf
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Switches versus Hubs
✦
Switches don't echo all frames to all
segments, so each segment is a separate
collision domain
‣
✦
Broadcast frames are sent to all segments
‣
✦
Switches are Layer-2 devices
Segments connected to the switch form a
Broadcast Domain
Collisions don't occur between broadcast
frames, because the switch sends them one
at a time
Hubs, Switches, and Collision Domains
✦
Switches form separate collision domains
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Hubs, Switches, and Broadcast
Domains
✦
Switches maintain a single broadcast domain
‣
Hubs and switches both copy broadcast frames to all other
ports
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The Switch In a Wireless Router
✦
Wireless router includes
a router, a switch, and
a Wireless Access Point
(WAP)
✦
Router functionality in
the CPU
✦
✦
Virtual LAN (VLAN)
connects WAP and
switch's internal ports
Another VLAN connects
WAN port to router
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