Network Design

1
Network Design

• Topologies

2
Introduction

• Topology - structure of the network
• Physical topology - actual layout of the wire
  (media)
• Logical topology - how the media is accessed by
  hosts

3
Physical Topology

• Bus
• Ring
• Star
• Extended Star
• Hierarchical
• Mesh
• Hybrid

4
Bus Topology

• Single cable connecting all nodes on a network
  without intervening connectivity devices
• The single cable is called the bus and can
  support only one channel for communication
• Most bus networks use coaxial cable
• At the ends of each bus network are terminators

5
Ring Topology

• Each node is connected to the two nearest nodes
  so that the entire network forms a circle
• Data is transmitted clockwise, in one direction
  (unidirectional), around the ring
• A ring topology differs in that it has no ends
  and data stops at its destination

6
Star Topology

• Every node on the network is connected through a
  central device (e.g. hub or switch)
• Requires more cabling than ring or bus networks
• Each node is separately connected to a central
  connectivity device, they are more fault-tolerant

7
Extended Star Topology

• What do you think an extended star topology is
  and looks like?

8
Hierarchical Topology

• Reasons for separating devices in a hierarchy
• Security
• Cost
• Scalability
• Network addressing
• Bandwidth
• Reliability

9
Hierarchical Topology

• Similar to an organisational chart in a company,
  where groups are divided by function
• One way to group devices on a network is to
  divide them into layers
• In the context of topologies, think of a layer as
  a logical division between devices on a network
• A hierarchical hybrid topology uses layers to
  separate devices based on their priority or
  function

10
Mesh Topology

• More expensive because it requires more cabling
  and potentially more connectivity device
• Fault tolerant

11
Hybrid Topology

• What do you think a hybrid topology is and looks
  like?

12
Backbone

• A network backbone is the cabling that connects
  the hubs, switches and routers on a network
• Backbones are usually capable of more throughput
  than the cabling that connects workstations to
  hubs
• The backbone is the most significant building
  block of enterprise-wide networks

13
Logical Topology

• Logical topology refers to the way in which data
  is transmitted between nodes
• Two most common types
• Broadcast
• Token-passing  

14
CSMA/CD

• Carrier Sense Multiple Access with Collision
  Detection
• The access method used in Ethernet
• The term Carrier Sense refers to the fact that
  Ethernet NICs listen on the network and wait
  until they detect (or sense) that no other nodes
  are transmitting data over the signal (or
  carrier) on the communications channel before
  they begin to transmit

15
CSMA/CD

• The term Multiple Access refers to the fact
  that several Ethernet nodes can be connected to a
  network and can monitor traffic, or access the
  media, simultaneously
• Collision detection refers to the way nodes
  respond to a collision
• When two transmissions interfere with each other
  this is known as a collision

16
CSMA/CD

• The NIC issues a special 32bit sequence that
  indicates to the rest of the nodes that its
  previous transmission was faulty and that those
  data frames are invalid (called jamming)
• A collision domain is the portion of a network in
  which collisions occur if two nodes transmit data
  at the same time
• A data propagation delay is the length of time
  data takes to travel from one point on the
  segment to another point

17
CSMA/CD
18
Logical Topology

• Broadcast topology
• Means each host sends its data to all other hosts
  on network medium
• No order to use network - first come, first serve
• Ethernet

19
Token Ring

• Traditionally more expensive to implement than
  Ethernet
• A 3-byte packet, called a token, is transmitted
  from one node to another in a circular fashion
  around the ring
• The active monitor maintains the timing for ring
  passing

20
FDDI

• Fiber Distributed Data Interface
• FDDI (pronounced fiddy) uses a double ring of
  multimode or single mode fiber
• More reliable and more secure than transmission
  methods that depend on copper wiring
• Has a high cost

21
Wireless Networks

• Each wireless technology is defined by a standard
  that describes unique functions at both the
  Physical and Data Link layers of the OSI model
• These standards differ in their specified
  signaling methods, geographic ranges and
  frequency usages
• The most popular wireless standards used on
  contemporary LANs are those developed by IEEEs
  802.11 committee

22
Wireless Networks

• 802.11 - another name for Wireless Local Area
  Networks (WLAN)
• 802.11 standards specify the use of Carrier Sense
  Multiple Access with Collision Avoidance
  (CSMA/CA) to access a shared medium

23
Wireless Networks

• 802.11a
• High throughput is attributable to its use of
  higher frequencies, its unique method of encoding
  data and more available bandwidth
• Higher frequency signals require more power to
  transmit and travel shorter distances than lower
  frequency signals
• The additional access points, as well as the
  nature of 802.11a equipment, make this standard
  more expensive than either 802.11b or 802.11g

24
Wireless Networks

• 802.11b
• Also known as Wi-Fi, for Wireless Fidelity
• Was the first to take hold and remains the most
  popular
• It is also the least expensive of all the 802.11
  WLAN technologies

25
Wireless Networks

• 802.11g
• Benefits from being compatible with 802.11b
• Has high throughput
• Its compatibility with the more established
  802.11b has caused many network managers to
  choose it over 802.11a, despite 802.11as
  comparative advantages
• Laptops could roam between the ranges of the
  802.11b and 802.11g access points without an
  interruption in service

26
Wireless Networks

• Bluetooth
• A mobile wireless networking standard which
  achieves a maximum theoretical throughput of 1
  Mbps
• Designed to be used on small networks composed of
  personal communications devices, also known as
  personal area networks (PANs)
• Its low throughput and short range makes it
  impractical for business LANs

27
Summary

• Logical topology
• Physical topology
• Broadcast
• Token passing
• Wireless