Module 4'3: Repeaters, Bridges,

1
Module 4.3 Repeaters, Bridges, Switches

• Repeater
• Hub
• NIC
• Bridges
• Switches
• VLANs
• GbE

2
Hardware Components

• Repeater
• Layer 1 device that provides physical and
  electrical connections.
• It receives signals from one cable segment,
  regenerates, retimes, and amplifies them, and
  then transmits these revitalized signals to
  another cable segment.
• Transmits in both directions
• Joins two segments of cable
• No buffering
• No logical isolation of segments
• Hub
• Used to describe a repeater
• Can be repeater hub, switching hub, bridging
  hub.
• NIC
• Network Interface Card
• Performs layer-2 functions framing, error
  detection, and flow control.
• Performs layer-1 functions by converting the bits
  into electrical signals using appropriate coding
  scheme.

3
Bridges

• Layer 2 devices
• Interconnects two or more individual LANs or LAN
  segments.
• Desirable for separating traffic among segments.
  A segment is part of a LAN in which traffic is
  common to all nodes, i.e. it is a single
  continuous conductor, though it may include
  repeaters.
• Split the segment with bridges/switches, if link
  utilization is more than 30.
• Store-and-forward devices. They capture the
  entire frame before deciding whether to filter or
  forward it. Frames with bad CRC are not
  forwarded.
• Minimal buffering to meet peak demand

4
Bridges Standards

• Transparent Bridges
• Operate in promiscuous mode.
• Bridging is transparent to stations, as if they
  are on one single LAN.
• plug and play unit, learns addresses connecting
  to ports by examining source and destination
  addresses.
• examines the destination address to forward or
  filter frames.
• All broadcast and multicast frames are forwarded.
• Source Routing Bridges
• Sender provides routing information for frames.
• Routing information includes local or remote
  destination address, and list of intermediate
  route designators.
• A route designator contain 12-bit LAN number and
  4-bit bridge number.
• Every station has a map of the network (different
  routes to get to different destinations).
• Routes can be configured manually or by
  performing route discovery.

5
Bridges Standards (cont.)

• Network loops can happen. Can cause broadcast
  storms that can bring the LAN down.
• Spanning Tree algorithm resolves network loops.
• Algorithm is based on graph theory.
• Any connected graph, consisting of nodes and
  edges connecting pairs of nodes, there is a
  spanning tree of edges that maintains the
  connectivity of the graph but contains no closed
  loops.
• Each LAN is a node and each bridge is an edge.
• Specified in IEEE 802.1. It involves a brief
  exchange of messages among all bridges to
  discover the minimum-cost spanning tree.
  Whenever there is a change in topology, the
  bridges automatically recalculate the spanning
  tree.
• Disabling B3-LAN4 port will result in a spanning
  tree. If B4 fails, the algorithm should enable
  this port again.

Spanning Tree
6
Switches

• Switches can operate at different layers layer
  2, 3, 4, and 7..
• Basically a switch is hardware based, not
  software based.
• Three types of layer 2 switches
• Store-and-Forward Switch
• Similar to store-and-forward bridge. Store entire
  frame, check for errors, and then switch to the
  other ports, based on the destination MAC
  address.
• Cut-Through Switch
• The transmission of frame begins as soon as it
  reads the destination MAC address. Two switch
  fabric/matrix designs
• Crossbar
• Backplane with bus speed gt aggregate port speeds
• Hybrid Switch
• Reliability store-and-forward. Turn ON when
  errors are high.
• Low latency cut-through. Turn ON when errors are
  low.

7
VLANs

• VLAN is a logical grouping of nodes using
  Ethernet switches. Nodes dont need to be
  connected physically to the same switch. A
  broadcast frame will be heard by all nodes within
  VLAN.
• Benefits
• Isolates broadcasts
• Frees up network from physical locations
• Easily shares resources. A server can be part of
  multiple VLANs.
• Performance. Easily can be enhanced by creating
  new VLANs.
• Security. By containing who can listen to
  broadcast.
• VLAN Membership (implicit tagging)
• Port-based
• MAC-based
• Layer 3/IP
• Combination of the above

8
Gigabit Ethernet

• With GbE CSMA/CD
• Network diameter shrinks to 25 meter. This is
  not a good option.
• Therefore, minimum frame time was increased to
  512 bytes. This give network diameter of 200
  meter, but waste in bandwidth especially for
  small size data.
• Most common use is point-to-point fame switching.
  
• No CSMA/CD
• We have now 10GbE that can go more than 50 km
  over SMF.