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Ethernet
Oakton Community College
CIS 238
Ethernet
• Developed as Xerox Network System (XNS) by
PARC
• Original form is “DIX”, later Ethernet II
• Standardized by IEEE as 802.2, 802.3 (wired),
802.11 (wireless)
• 802.4 (Token Ring), 802.5 (Token Bus) no longer
used
Media Types
• Thicknet Coax (10 Base 5): several kilometers
• Thinnet Coax (10 Base): thousands of meters
• Unshielded twister pair:
(10/100BaseT/1000BaseTX) – 100 M.
10GBaseT – 15M (CX) – 100M (10GBaseT)
• Shielded twisted pair. Same a UTP.
• Fiber Optic – multi-mode: 220-550M (50-62.5
micron), single mode: 10-70KM.
• Wireless – B/G (2.4GHZ) < 100M, A (5GHZ) <<
100M
Media Types
• Coax almost never used, except maybe by the cable company.
Replaced by fiber optic which uses less power, less susceptible
to interference, same distance advantages at higher speeds.
• Twisted pair is an “unbalanced” electrical circuit - no absolute
ground like coax, so attenuation leads to severe distance
limitations (100M).
• “Twisted” to minimize electrical crosstalk caused by (counter)
EMF. “Shielded” if external EMF is a factor.
• Multiple fiber optic type from 10BaseFX to 100GBaseER4. Too
many standards to list.
Media Access Control Address
• 6 byte field burned into the Ethernet chip
• Can be overridden as “local” address
• Has meaning only on local network (OSI Layer 2)
• Represented as 12 hexadecimal digits.
• The first 3 bytes (6 hex numbers) represent the
manufacturer (OUI) as assigned by IEEE.
Digital Encoding
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Coax Manchester encoding: encoding a logic 0 is indicated by a 0 to 1
transition at the centre of the bit and a logic 1 is indicated by a 1 to 0
transition at the centre of the bit. This is used on coax media.
Original Data: Value Sent
- Logic 0: 0 to 1 (upward transition at bit centre)
- Logic 1: 1 to 0 (downward transition at bit centre)
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Example: data (1,1,0,1,0,0) being sent:
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Shielded twisted pair uses transmission using a 4b/5b MLT code with three
signal levels (as opposed to just two above). So 100 MBS uses 33 MHZ
over 2 pair. Gigabit Ethernet utilizes five levels and 8b/10b encoding,
sending 1 Gbps within 100 MHz of bandwidth over 4 pair..
Segments
•
Base Ethernet is a broadcast medium, every computer on a network section
(segment) shares the same wire(s)/electrical connection.
•
Multiple sections can be connected together by digital “repeaters” to extend the
range of a segment by repeating digital signals from one side to the other. This is
an OSI layer 1 device.
•
Repeaters are passive devices (no MAC Address) but are always powered to repeat
digital signals.
•
Coax Ethernet repeaters are subject to the 5/4/3 rule – 5 segments connected by 4
repeaters with three segments active - a limit of 30 active workstations.
•
A multi-port repeater is referred to as a “hub”. Usually only used with twisted pair
wiring.
•
Hubs have their own version of the repeater rule when daisy-chained called the
Class I (10 MBS, 4 hubs) or Class II (100 MBS 2 hubs) that define how they can be
connected within a collision domain.
Access Method
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Ethernet is a “broadcast” on each “segment” where multiple host(s) try to gain
control of a single media.
Access is gained by sending a “sense” packet 64 bytes long on each segment (511
“bit times”).
Signal propagation times and attenuation determine the distance limit for each
media type.
If another host transmits at the same time a “collision” is detected in a “jam”
packet.
• Each workstation then backs off a pseudo-random amount of time before
re-trying.
• This process is referred to a Collision Sense, Multiple Access with Collision
Detection – CSMA/CD
LAN Segment – Collision Domain
•
The area within which CSMA/CD takes place is called a “collision domain”. With multiple
workstations accessing the medium in this way, queueing theory states full utilization with
this half-duplex protocol is 33%.
•
To connect two collision domain segments, a “bridge” is used connecting a collision domain
segment to a port. This is an OSI Layer 2 device.
•
A bridge functions by listen, learn, forward. Separate CSMA/CD on each port - populating an
internal MAC Address table assigning each MAC address to a port it responds on.
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A multi-port bridge is called a switch.
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Since each port on a switch terminates a collision domain; if only one device is connected to
the port, CSMA/CD can be dropped. This allows the port to function in full-duplex
(simultaneous send/receive) mode.
•
Simple switches only function as one Layer 2 LAN segment. Configurable switches can
separate ports into logical semgment known as V(virtual)LANs.
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Finally, a Layer 2 segment is terminated by a OSI Layer 3 device – a router.
Ethernet Frames
•
All information placed into Ethernet “packets” or frames. Multiple frame types: Ethernet
II/DIX, 802.3, 802.2, 802.2 inside 802.3, 802.2 + Ethernet II or SNAP - used by multiple LAN
protocols.
•
Ethernet frames preceded by an 8 byte “preamble” of 7 bytes alternating 1 and 0 for timing
and one “flag” byte ending in 0x7e.
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Ethernet packet has a header with Destination (MAC) Address, Source MAC Address and 2byte Ethertype/Length field and terminated by a 32 bit Frame Check Sequence (FCS)
representing a “hash” of the packet contents – excluding preamble.
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1500 Byte frame (data) limit.
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A common network problem used to be Ethernet attached devices on the same LAN not
“seeing” each other due use of different frame type (Netware IPX). As TCP/IP becomes the
de-facto LAN protocol, these type of connectivity issues become rare since TCP/IP uses
DIX/EII frame type by default. Though this is configurable on most NICs (but don’t do it).
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If the EtherType/Length value is greater than 0x05DC (decimal 1500), then the frame is
interpreted / processed as an Ethernet II packet.
Ethernet Frame Types
• “Raw” Ethernet (Xerox format)
• -------------------- Data direction
• Type field values include:
• 0x0600 XNS (Xerox)
0x0800 IP (Internet protocol)
0x6003 DECNET
0x8137 IPX
Ethernet Frame Types
• IEEE Standard
• ----------------------- Data direction
Ethernet Frame Types
• 802.2 (protocol encapsulation header after 802.3
header)
• ----------------------- Data direction
• Used primarily for non-routed LAN protocols:
0x0404 SNA
0xF0F0 NETBEUI
0x0A0A for LLC2
Ethernet Frame Types
• SNAP
• ----------------------- Data direction
• 802.2 header for DIX frame type
• Primarily used for TCP/IP, IPX
802.11 Specification
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802.11b (channels 1-11 or 14)
- Speeds of 5.5 or 11 Mb/sec
- 2.4 GHZ spread spectrum
- 100M inside, 300M outside
802.11g (channels 1-11 or 14)
- Speeds of 6-54 Mb/sec
- 2.4 GHZ spread spectrum or QAM
- 100M
802.11n (MIMO) (channels 1-11 or 14)
- Speeds to 150 Mb/sec
- 2.4 GHZ QAM or other method
- 100M, Full Duplex
802.11a (23 channels, channel numbering varies)
- Speeds of 6-54 Mb/sec
- 5 GHz band
- 100M Distance inside or outside
Coming soon: 802.11ac - full duplex gigabit wireless
Wireless Security
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WEP (Wireless Enryption Protocol):
- 64-bit key (40-bit secret code, 24-bit “init” vector)
- 104 bit key (13 byte key as 26 hex digits)
- symmetric key with CRC -32 check
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WPA (WiFi Protected Access):
- 3DES, 128 bit key, per packet, TKIP
- seed “key phrase” 8-63 bytes
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WPA2 (WiFi Protected Access 2):
- AES, 256 bit key, per packet, CCMP
- seed “key phrase” 8-63 byte
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EAP (Extensible Authentication Protocol) for user based authentication in combination with
802.1X – port/mac security.
- EAP types: EAP-TLS, EAP-TTLS/MSCHAPv2, PEAPv0/EAP-MSCHAPv2, PEAPv1/EAP-GTC,
PEAP-TLS, EAP-SIM, EAP-AKA, EAP-FAST
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Wireless frame types use double Ethernet headers to allow “hopping” from wireless access
point to access point.