Layer 2 LAN Technologies

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Layer 2 LAN Technologies Media Access
Methods(II)
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Minimum Ethernet Frame Size

• To ensure that no node may completely receive a
  frame before the transmitting node has finished
  sending it, Ethernet defines a minimum frame size
  (i.e. no frame may have less than 46 bytes of
  payload).
• The minimum frame size is related to the
  distance which the network spans, the type of
  media being used and the number of repeaters
  which the signal may have to pass through to
  reach the furthest part of the LAN.
• Together these define a value known as the
  Ethernet Slot Time, corresponding to 512 bit
  times at 10 Mbps.

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Minimum Ethernet Frame Size

• The longest time between starting to transmit a
  frame and receiving the first bit of a jam
  sequence is twice the propagation delay from one
  end of the cable to the other.
• This means that a frame must have enough bits to
  last twice the propagation delay.
• The 802.3 CSMA/CD Bus LAN transmits data at the
  standard rate of r 10Mbps.
• The speed of signal propagation is about v
  2?108m/s.

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IEEE 802.3 Minimum Frame Length
In order to calculate the minimum frame length,
we must first work out the propagation delay from
one end of the cable to the other.
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IEEE 802.3 Minimum Frame Length
Example 1 Cable 400m, transm. speed 10
Mbit/sec, propagation speed 2108 m/sec
Propagation delay time
The round-trip propagation delay is, of course,
twice this. Thus the round trip delay is
With a data rate of
each bit has duration
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IEEE 802.3 Minimum Frame Length

• Example 1 cont.
• The number of bits we can fit into a round-trip
  propagation delay is

The minimum frame length is thus 40 bits (5
bytes). A margin of error is usually added to
this (often to make it a power of 2) so we might
use 64 bits (8 bytes).
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IEEE 802.3 Minimum Frame Length
Example 2
Two nodes are communicating using CSMA/CD
protocol. Speed transmission is 100 Mbits/sec and
frame size is 1500 bytes. The propagation speed
is 3108 m/sec. Calculate the distance between
the nodes such that the time to transmit the
frame time to recognize that the collision have
occurred.
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IEEE 802.3 Minimum Frame Length

• The standard frame length is at least 512 bits
  (64 bytes) long, which is much longer than our
  minimum requirement of 64 bits (8 bytes).
• We only have to start worrying when the LAN
  reaches lengths of more than 2.5km.
• 802.3 CSMA/CD bus LANs longer than 500m are
  usually composed of multiple segments joined by
  in-line passive repeaters, which output on one
  cable the signals received on another cable.
• When we work out the minimum frame length for
  these longer LANs, we also have to take the
  delays caused by the passive repeaters (about
  2.5?sec each) into account as well.

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Shortest Ethernet Frame
Why specify a shortest frame of 64byte?
64 bytes sent at 10Mbps ? 51.2?sec
500m/segment, 4 repeaters between nodes ?2500m
?25 ?sec propagation delay The frame should be
longer enough for sender to detect the
collision(2x25 or about 50 ?sec )
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IEEE 802.3 Non-Deterministic

• The 802.3 CSMA/CD bus LAN is said to be a
  non-deterministic network. This means that no
  host is guaranteed to be able to send its frame
  within a reasonable time (just a good probability
  of doing so).
• When the network is busy, the number of
  collisions rises dramatically and it may become
  very difficult for any hosts to transmit their
  frames.
• A real-time computing application (such as an
  assembly line) will demand that data is
  transmitted within a specified time period.
• Since the 802.3 bus LAN cannot guarantee this,
  its use for real-time applications may not only
  be undesirable but potentially dangerous in some
  situations.

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Ethernet evolution through four generations
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100 Mbps IEEE Standards

• The most widely accepted Ethernet standard today
  is 100BaseT, which is also called fast Ethernet
• The current IEEE standard for 100BaseT is 802.3u
• Subcategories
• 100BaseTX Two-pair Category 5 or higher UTP
• 100BaseT4 Four-pair Category 3 or higher UTP
• 100BaseFX Two-strand fiber-optic cable
• Because of its widespread use, the cable and
  equipment in fast Ethernet are inexpensive
• Architecture of choice for all but heavily used
  servers and multimedia applications

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100BaseTX

• 100BaseTX is the standard thats usually in mind
  when discussing 100 Mbps Ethernet
• Requires two of the four pairs bundled in a
  Category 5 twisted-pair cable
• Although three cable types are available for
  100BaseT, 100BaseTX is the most widely accepted
• Generally called fast Ethernet

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100BaseT4

• 100BaseT4 Ethernet uses all four pairs of wires
  bundled in a UTP cable
• Advantage capability to run over Category 3
  cable
• One of the biggest expenses of building a network
  is cable installation, so many organizations with
  Category 3 cabling chose to get the higher speed
  with the existing cable plant by using 100BaseT4
  instead of 100BaseTX

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100BaseFX

• 100BaseFX uses two strands of fiber-optic cable
• Advantages
• Impervious to electrical noise and electronic
  eavesdropping
• Can span much greater distances between devices
• Disadvantage far more expensive than
  twisted-pair
• Rarely used as a complete 100BaseTX replacement
• Used as backbone cabling between hubs or switches
  and to connect wiring closets between floors or
  buildings
• Connect client or server computers to the network
  when immunity to noise and eavesdropping is
  required

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100BaseT Design Considerations
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100BaseT Design Considerations
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10 Mbps IEEE Standards

• Four major implementations of 10 Mbps Ethernet
• 10Base5 Ethernet using thicknet coaxial cable
• 10Base2 Ethernet using thinnet coaxial cable
• 10BaseT Ethernet over UTP cable
• 10BaseF Ethernet over fiber-optic cable
• Of these 10 Mbps standards, only 10BaseT and
  10BaseF are seen today
• 10Base2 and 10Base5 are essentially obsolete

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10BaseT
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10BaseF
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Gigabit Ethernet IEEE 802.3ab and 802.3z
Standards

• Gigabit Ethernet implementations
• 802.3z-1998 covers 1000BaseX specifications,
  including the L (long wavelength
  laser/fiber-optic), S (short wavelength
  laser/fiber-optic), and C (copper jumper cables)
• 802.3ab-1999 covers 1000BaseT specifications,
  which require four pairs of 100 ohm Category 5 or
  higher cable

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1000BaseT
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1000BaseLX
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1000BaseSX
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1000BaseCX
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10 Gigabit Ethernet 10 Gbps IEEE 802.3ae Standard

• Defined to run only on fiber-optic cabling, both
  SMF and MMF, on a maximum distance of 40 km
• Provides bandwidth that can transform how WAN
  speeds are thought of
• Runs in full-duplex mode only
• CSMA/CD is not necessary
• Primary use as network backbone
• It also has its place in storage area networks
  (SANs)
• Will be the interface for enterprise-level servers

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10 Gigabit Ethernet 10 Gbps IEEE 802.3ae Standard

• Standards
• 10GBASE-SR Runs over short lengths (between 26
  and 82 meters) over MMF
• For high-speed servers, SANs, etc.
• 10GBASE-LR Runs up to 10 km on SMF
• For campus backbones and MANs
• 10GBASE-ER Runs up to 40 km over SMF
• Primary applications are for MANs
• 10GBASE-SW Uses MMF for distances up to 300 m
• 10GBASE-LW Uses SMF for distances up to 10 km
• 10GBASE-EW Uses SMF for distances up to 40 km

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Wireless Ethernet IEEE 802.11b, a, and g

• AP serves as the center of a star topology
  network
• Stations cant send and receive at the same time
• CSMA/CA is used instead of CSMA/CD
• 802.11b/a/g use handshaking before transmission
• Station sends AP an RTS and it responds with CTS
• Standards define a maximum transmission rate, but
  speeds might be dropped to increase reliability
• No fixed segment length
• Maximum of 300 feet without obstructions
• Can be extended with large, high-quality antennas

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Broadband Technologies

• Baseband systems use a digital encoding scheme at
  a single fixed frequency
• Broadband systems use analog techniques to encode
  information across a continuous range of values
• Signals move across the medium in the form of
  continuous electromagnetic or optical waves
• Data flows one way only, so two channels are
  necessary for computers to send and receive data
• E.g., cable TV

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Cable Modem Technology
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Digital Subscriber Line (DSL)

• Competes with cable modem for Internet access
• Broadband technology that uses existing phone
  lines to carry voice and data simultaneously
• Most prominent variation for home Internet access
  is Asymmetric DSL (ADSL)
• Splits phone line in two ranges Frequencies
  below 4 KHz are used for voice transmission, and
  frequencies above 4 KHz are used to transmit data
• Typical connection speeds for downloading data
  range from 256 Kbps to 8 Mbps upload speeds are
  in the range of 16 Kbps to 640 Kbps