Thin Ethernet (10B2 / IEEE 802.3a)

1
Thin Ethernet (10B2 / IEEE 802.3a)

• Segment length lt 185m and gt 0.5 m
• Up to 30 attached nodes
• Cable flexible and cheap
• Integrated or external transceiver connected via
  a BNC 'T' connector
• Used mainly for workgroups
• Difficult to manage (i.e. breaks in cable
  difficult to locate)

2
10BT/802.3i

• Connects the computer directly (i.e. using a
  point to point link) to a wiring hub.
• Segment length 100 m
• Cable flexible and very cheap
• Standard RJ-45 connector used (also can be used
  for telephone and other networks)
• Used mainly for work groups (requires a hub to
  connect to the LAN)
• Easy to manage (also can be used for telephone
  and other networks)

3
Ethernet Repeaters and Hub

• operate at the Physical Layer
• connect together one or more Ethernet cable
  segments to provide signal amplification and
  regeneration
• A network of repeaters and hubs is therefore
  called a "Shared Ethernet" or a "Collision
  Domain".
• Only one host is allowed to transmission within a
  Collision Domain

An 3Com Office Hub (8 ports)

• No (or little) processing (memory) unit
• all ports operate as one Ethernet LAN
• all connected NICs operate in the half-duplex
  (CSMA/CD) mode at the same transmission speed

Stack of several hubs to wire a building
4
Ethernet Bridges Switches

• bridge has to forward frames from one LAN to
  another
• Operates at the data link layer
• Bridges Must learn which addresses belong to the
  computers connected via each port.
• Separate Collision Domain

5
Ethernet Switches

• Solution to put 300 hosts in one Ethernet (in one
  building)
• Separate the Collision domain
• Contain a high-speed backplane
• Store-and-forward (inboard memory for each line)
• ranging from the simplest low cost devices
• to expensive high performance switches

3Com LinkSwitch 1000
Cisco Catalyst 5000 Switch
6
Frame Format

• Preamble
• Dest Addr 6-byte MAC address
• Source Addr 6-byte MAC address
• Length 1500 bytes maximum
• Data
• Pad minimum 64 bytes
• To distinguish valid fames and garbage
• Transmission should last longer than round-trip
  delay.
• Checksum

• A sent at t0
• B send around t0
• B realize collision and send noise burst
• A hear the noise burst at around t0 2
• A must not conclude transmission success before
  t02

7
Round-trip Delay and Transmission Time

• With maximum length 2500 meters
• Four repeaters
• Round-trip time 50 usec
• At 10 Mbps, one bit last 100 nsec
• Thus 500 bits are needed to last 50 usec
• Which is roughly 64 bytes

8
Binary Exponential Backoff

• How stations randomly wait after a collision
• One slot is 51.2 usec (round-trip delay)
• At first collision, each station waits 0 or 1
  slots
• After the second collision, each station waits
  either 0, 1, 2, or 3 slots randomly.
• If a third collision occurs, wait 0 to 7 slots.
• So on and so forth
• Maximum slots to wait is 1023 after 16 straight
  collisions

9
Fast Ethernet and Gigabit Ethernet

• 100 Base-T4
• 100 Base-TX

• All gigabit Ethernet is point-to-point
• Full-duplex mode
• Must use switch (not hub!)
• No contention
• No need for media sensing
• Half-Duplex mode
• Connected with Hub
• Collision is possible