Ethernet

1
Ethernet

• EECE 542
• Brad Guenther

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History

• Originally developed at Xerox PARC (Palo Alto
  Research Center) in 1973
• First 10 Mbps standard published in 1980 by DEC,
  Intel, and Xerox (DIX). Used thick coax cable.
• This led to IEEE 802.3 Carrier Sense Multiple
  Access with Collision Detection (CSMA/CD) Access
  Method and Physical Layer Specifications in 1985.

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• 802.3a-1985 10Base2 Thin net
• 802.3i-1990 10BaseT twisted-pair
• 802.3u 100Base-T Fast Ethernet w/
  Auto-Negotiation
• 802.3x-1997 Full Duplex standard
• 802.3ab-1999 1000Base-T Gigabit Over Twisted pair

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10 Mbps Frame Requirements

• Minimum frame time 51.2 ms
• Minimum length 51.2 ms x 10 Mbps 512 bits
  64 bytes
• Why? Late Collisions are a bad thing! (Early
  Collisions are normal)
• Max length set to 1518 bytes due to memory cost
  and monopoly laws
• Bytes are sent in normal order, but the bits in
  each byte are sent in LSB order
• 9.6 microsecond interframe gap

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802.3 MAC Frame
1500 max
1
6
6
2
4
7
Destination Address
Source Address
Data
FCS
Pad
Preamble
Length
SD

• Start with 7 bytes of 1010101 to get timing
  syncd (layer 1)
• Then indicate the frame is starting (10101011)
  last chance to sync!
• PDU headers (addresses length type)
• Data (46-1500 bytes -- may contain some control
  info as well)
• Add bits if necessary to get to 46 bytes
• CRC-32 FCS

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Frame Encapsulation

• Different Encapsulation types exist for Ethernet
• Two machines communicating using a given layer
  three protocol must be using the same Ethernet
  encapsulation type
• Different encapsulation types can be used for
  different layer 3 protocols.

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Frame Encapsulation Types

• 802.3 (Raw Ethernet)
• Ethernet II
• 802.2 (802.3 w/ addition of 802.2 headers)
• SNAP (802.3 802.2 SNAP headers)

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Ethernet II

• Standard introduced by DIX.
• Used almost universally by networks using TCP/IP
• Uses type field that denotes the layer 3 protocol
  that is encapsulated within
• Ex 0x0800 IPv4, 0806 ARP

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802.3 MAC Frame
46-1500
6
1
6
2
4
7
Destination Address
Source Address
Information
FCS
Pad
Preamble
Length
SD
Synch
Start frame
64 to 1518 bytes
Length lt 1518 Type gt 1536
Ethernet II Frame
6
1
6
2
4
7
Destination Address
Source Address
Information
FCS
Pad
Preamble
Type
SD
Start frame
Synch
64 to 1518 bytes
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802.2 -- LLC

• Logical Link Control
• Added more layer 2 header information that act as
  pointers to memory buffers.
• Provides information to layer 3.
• Allows multiplexing of upper layer protocols on
  the same network
• Service Access Points SAP
• Maybe either connection-oriented or connectionless

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Connectionless Services

• Type 1 LLC (unreliable)
• Unacknowledged Here ya go! Data accepted
  silently.
• Acknowledged Here ya go! Data accepted. Thanks!

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Connection-Oriented Service

• Type 2 LLC (reliable)
• Establish connection Hey are you listening to
  me? Do I have to I suppose. Good!
• How fast can we talk and can we talk at the same
  time?
• Transfer Data Same as before
• Close connection Ok, thats all I have to say.
  Oh good, hes done! Thanks for shutting up! Ok,
  he knows Im done.
• Adds sequence numbers

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LLC headers

• Adds Three fields between the address fields and
  the data field.
• Destination service access point (DSAP) a first
  bit1 indicates group
• Source Service Access Point (SSAP) a first
  bit1 indicates a command
• Control field identifies the type of PDU (first 3
  bits)
• DSAP SSAP used to identify sending and
  receiving protocol stacks

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1
1
1 byte
Source SAP Address
Destination SAP Address
Information
Control
Source SAP Address
Destination SAP Address
C/R
I/G
7 bits
1
7 bits
1
I/G Individual or group address
C/R Command or response frame
Figure 6.14
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Type 1 Commands

• Unnumbered Information Frame (UI) (no reply
  expected)
• Exchange Identification Request (XID) exchange
  supported LLC services and receive window size.
• XID reply
• Test Frame Request
• Test Frame Reply

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Type 2 Commands

• Information Frame (I-Frame) (blocks of
  sequentially number data also contains number
  of next expected block)
• Receiver Ready (RR) (used to ack received data
  when no data to send)
• Receiver Not Ready (RNR) (hold on a second)
• Reject (Rej) (Could you send that again?)

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Type 2 Commands (cont.)

• SABME (Sa-bim-ee) Set Async. Balanced Mode
  Extended used to establish link between two
  stations
• Disconnect (DISC) Used to terminate a connection
• Unnumbered Ack (UA)
• Disconnect Mode (DM) response to DISC
• Frame Reject (FRMR) catastrophic failure (cant
  be fixed by retrans.

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SNAP

• Sub-Network Access Protocol
• Adds 5 bytes of headers
• First 3 bytes are similar to an OUI often the
  same information. If not used, it is set to
  zero.
• Remaining two bytes contain information about the
  encapsulated protocol (similar to Ethernet II)

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Type
ORG
3
2
SNAP Header
SNAP PDU
Information
LLC PDU
AA AA 03
1
1
1
802.3 Frame
MAC Header
FCS
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Encapsulation Summary
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Snap SSAP and DSAP

• 00 Null LSAP02 Individual LLC Sublayer
  Management Function03 Group LLC Sublayer
  Management Function7E ISO 8208 (X.25 over IEEE
  802.2 Type 2 LLC)80 Xerox Network Systems
  (XNS)86 Nestar8E PROWAY (IEC 955) Active
  Station List Maintenance98 ARPANET Address
  Resolution Protocol (ARP)BC Banyan VINESAA
  SubNetwork Access Protocol (SNAP)E0 Novell
  NetWareF0 IBM NetBIOSF4 IBM LAN Management
  (individual)F5 IBM LAN Management (group)F8 IBM
  Remote Program Load (RPL)FA Ungermann-BassFE
  ISO Network Layer ProtocolFF Global LSAPOthers

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Ethernet II Type Field

• 0000-05DC IEEE 802.3 Length Fields (0x5DC1500d)
• 0101-01FF Experimental (for development) --
  Conflicts with 802.3 Length Fields
• 8038 DEC LANBridge
• 8039 DEC DSM
• 8137 Novell (old) NetWare IPX
• 8138 Novell
• 9000 Loopback (Conifguration Test Protocol)
• 9001Bridge Communications XNS Systems Management
• More info see http//www.wildpackets.com

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Layer 2 Addresses

• 12 hex digits (6 bytes)
• Unicast
• 6 hex digit vendor code (OUI)
• 6 hex digit serial number
• www.cavebear.com/CaveBear/Ethernet/vendor.html
• Broadcast
• All 1s (Valid for destination only)
• Multicast
• Reserved list of 12 digit addresses
• www.cavebear.com/CaveBear/Ethernet/multicast.html

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Frame Format Issues

• Notice the order of addressing information in the
  frame headers How is it significant? What
  impact does it have on network devices?
• How does the frame format relate to transmission
  order (both byte-wise and bit-wise)?

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Sources of Frame Corruption

• Collisions
• Jabber
• Runt Frames / Oversized Frames
• Reflections (mismatched imped., no term., sharp
  bends)
• Electrical Noise
• Crosstalk
• NEXT

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10 Mbps Ethernet

• 10Base2 50 ohm coax, 185 meters
• 10Base5 50 ohm thick coax, 500 meters
• 10BaseFB Multimode Fiber, star topology, 2000
  meters
• 10BaseFP Multimode Fiber, star topology, 500
  meters, 33 computers
• 10BaseFL Multimode Fiber, node to node 2000
  meters
• 10BaseFX collective name for the fiber variants
• 10BaseT UTP, star topology, 100 meters

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10BaseT

• UTP with max segment length of 100m
• Manchester Line Coding, No Block Coding
• 5-4-3 Rule Between any two nodes on a network,
  there can only be a maximum of five segments,
  connected through four hubs and only three of the
  five segments may contain user connections
• 10/100 UTP cabling 90m main run,6m patch,3m
  workstation

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EIA/TIA Cabling Standards

• 568-a
• 568-b

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Other cabling issues

• MDF, IDF
• Where do you locate them
• POP (not meaning email here)
• How many?
• Max 2 Hubs 2 100m Segments (plus patch cables
  up to 10m)
• 7 switches between devices (14 hops)

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100 Mbps

• 100BaseTX 802.3, gt cat 5 UTP, 100 meters, 2
  pairs (4 wires) EIA/TIA 568/569
• 100BaseT4 100BaseT4 uses four pairs of wiring
  one for transmission, one for reception, and two
  that can do either one. It basically has three
  pairs of wiring to either transmit or receive
  data. It divides the 100Mbit/sec data signal
  between the three pairs of wiring, thus reducing
  the average frequency of signals on the cable.
  This lets you work with cat 3 and up UTP and cat
  1 STP.
• 100BaseFX 62.5/125 fiber, 400 meters
• 100VG.

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100BaseTX

• Cat 5 UTP, Uses 2 pairs (1 for RX 1 TX)
• Uses MLT-3 Line Coding
• Uses 8B10B Block coding
• One hub per collision domain as a general rule
  (10BaseT allowed them to be cascaded 4 deep)
• 100BaseTX switches will generally operate at
  either 10 or 100 Mbps and can be used to connect
  10 and 100 Mbps segments

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Ethernet Switch
Server
100 Mbps links
Ethernet Switch
10 Mbps links
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MLT-3
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Auto-negotiation

• Uses a series of link pulses to transmit a 16 bit
  word (Fast Link Pulse Burst)
• 17-33 pulses similar to Normal Link Pulses used
  by 10baseT to test link
• Burst lasts about 2ms
• Interleaves clock pulses with data pulses. No
  pulse within time frame after clock pulse
  indicates a 0.

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Auto-negotiation

• 5 Selector bits (0001802.3)
• 8 Tech. Ability Bits
• Priority 1000Base-T FD, 1000Base-T HD,
  100Base-TX FD, 100Base-T4 (HD), 100Base-TX HD,
  10Base-T FD, 10Base-T
• Hubs have to be half duplex same speed why?
• Causes problems if improper cables used.
• Other black magic

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802.3 Full Duplex Operation

• Only done by switches hubs claiming to be full
  duplex capable are actually switches?
• Why? What do switches and hubs look like
  logically?
• What impact does this have over the maximum
  (unrepeated segment length)? Maximum overall
  segment length? Why?
• Impact on frame size? (min or max)?

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100BaseT4

• Designed to run over older voice grade cat 3 UTP
• Uses 4 pair (8 wires) with each wire running at
  25 Mbaud (33.3 Mbps).
• One pair is dedicated TX, One pair is dedicated
  RX, and Two pair are bidirectional

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100BaseT4

• Uses 8B6T encoding
• 00(hex)-00-
• 01(hex)0--0
• Maps 28 (256) ? 36 (729) values
• Combination of line coding and block coding
• 3 wire pairs x 25 M symbols/sec x 8 bits/6
  symbols 100 Mbps

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Gigabit

• 1000BASE-LX 1000-Mbps gigabit Ethernet
  specification using two strands of multimode or
  single mode fiber-optic cable per link. To
  guarantee proper signal recovery, a 1000BASE-LX
  link cannot exceed 550 meters over multimode
  fiber or 10 km over single mode fiber.
• 1000BASE-SX 1000-Mbps gigabit Ethernet
  specification using two strands of multimode
  fiber-optic cable per link. 1000BASE-SX links
  cannot exceed 550 meters.

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Gigabit (cont)

• 1000BASE-X General Term for gigabit over fiber.
• Use 8B10B block coding and NRZ Line coding
• 1000BASE-T Gigabit over copper, IEEE 802.3ab,
  Cat 5 UTP, 4 pair, 100 meters

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Gigabit Over Copper

• 802.3ab
• Full Duplex only
• 8B1Q4 converts 8 bits to 4 quinary symbols
• 4D-PAM5 encoding
• 8 bits are mapped to 4 symbols, each symbol has a
  value of 2,1,0,-1, or 2
• Maps 28 (256) bits into 54 (625) symbols
• 125 Mbaud per pair x 4 pair x 8 bits / 4 symbols
  1 Gbps

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Gigabit over Cu (Cont)

• Sends code groups of 2,0-2 when idle
• Start of Stream delimiter sent when new frame
  arrives
• Use of hybrids and cancellers enables full
  duplex transmission by allowing symbols to be
  transmitted and received on the same wire pair at
  the the same time IEEE 802.3ab supplement to
  802.3 std.

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10 Gigabit Networks

• IEEE 802.3ae
• Frame length 40 bytes to 16,380 bytes
• No autonegotiation
• Full Duplex Only
• What impact does this have on frame
  encapsulation? I.e. length field? What does it
  to to the minimum length?

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10gae

• Somewhat being defined by an organization of
  manufacturers
• Why has Ethernet not been a WAN standard
• Why might 10gae be WAN capable? How?
• SONET a layer 2 WAN technology can be used to
  carry Ethernet Frames.
• Use features of both technologies to overcome
  problems

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10G Ethernet

• Both parallel and serial standards exist
• 10GBase-(S,L, or E)(R,W, or X)
• S short wavelength (850nm), L long wavelength
  (1300nm), and E ext. wavelength (1550 nm)
• R uses 64B/66B, W uses 64B/66B encap. In STS-192
  for SONET, and X uses 8B/10B

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Parallel LAN 10G Ethernet

• WWDM LAN PHY uses 4 parallel channels of 8B/10B
  coded data.
• Each channel is called a lane and sends at 2.5G
  symbols/sec 3.125 Gbs
• 10Base-LX4
• Added IPG (not the same as the padding done at
  a lower layer)

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Serial 10G Ethernet

• LAN PHYs 10GBase-SR (2-300m), 10GBase-LR
  (2-10km), 10GBase-ER (2-40km)
• WAN PHYs 10GBase-SW (2-300m), 10GBase-LW
  (2-10km), 10GBase-EW (2-40km)

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Block Line Coding Review
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Additional Special Frames

• VLAN Frames (802.1Q)
• Pause Frames

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VLANs

• Break switches up into multiple virtual switches
• Can be either tagged or untagged
• Untagged VLANs use normal ethernet frames
• Tagged VLANs add additional header information

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802.1Q VLAN Frame Tags
Tag 0x8100 (always -- at least for
Ethernet) Tag Ctrl Info 1st 3 bits indicate
priority, then a 1 bit Canonical Format Indicator
(CFI) set to zero for Ethernet switches,
remaining 12 bits indicate which VLAN the frame
is from/for
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VLANs

• Useful for breaking up large switches so that
  LANs can be segmented for performance or
  security reasons
• How does this affect performance?
• How does this affect security?

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Pause Frames

• Sent by full duplex DTEs only
• Requests that transmission to the sending DTE be
  inhibited for a brief period
• Uses multicast address 0180C2000001
• Not forwarded by 802.1D capable bridges/switches
• Pause time is a 2 byte field specifying how many
  512 bit times transmission should be inhibited

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Pause Frames
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FCS

• Used to check frame integrity
• Sent MSB first!
• CRC-32
• Usually done with a shift register and XOR gates

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Polynomial Codes

• Relatively easy to implement with shift
  registers.
• Cyclic Redundancy Checks (CRC)
• Use polynomials with binary coefficients to
  represent information symbols, codewords, and
  error vectors

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CRC Codes
i k information bits
r(x) is the remainder of the polynomial division
of the shifted form of i(x)/g(x) g(x) has
dimension (n-k) Gives n codeword bits First part
shows k info bits shifted n-k bits to the
left Second part is the (n-k) bit remainder
polynomial (n codeword bits, k info bits, and n-k
check bits)
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CRC Codes
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CRC Encoding Steps

• Multiply i(x) by x(n-k) to shift i(x) n-k bits
  to the left, inserting 0s on the right
• Divide x(n-k)i(x) by g(x) to get q(x) r(x).
  (polynomial division uses modulo-2 addition
  i.e. xjxj(11)xj0

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Division Example

• k4, n6
• i(x)x3x21
• g(x)x21
• How many check bits will there be?
• Find q(x) and r(x)
• How long would b(x) be? What is the value of
  b(x)?

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Binary Example

• i(x) x2x 110
• g(x) x3x11011
• k3, n6, how many check bits?
• What is q(x), r(x)
• What would b(x) be in a.) binary form b.) poly
  form

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CRC and Ethernet

• Ethernet uses CRC-32 which uses
  G(x)x32x26x23x22x16x12x11x10x8x
  7x5x4x2x1 (degree 32 polynomial)
• How could the receiver use this? (What two ways
  could it calculate to see if the FCS was
  correct?)
• A bit more to it