Chapter 5

Network Architecture

Physical Topologies

• Bus

• Ring

• Star

Bus Topology

• Single cable called the bus• Supports one channel (baseband)• The more nodes on a bus topology, the slower the

network will transmit and deliver data• Each node listens to the network to determine if it

can transmit• Nodes other that the destination node ignore the

transmission• A terminator (resistor) is needed at each end of the

bus otherwise signal bounce will occur

Properties of Bus Topology

• Inexpensive to set up• As you add more nodes, the performance

degrades (does not scale well)• Difficult to troubleshoot because it is

difficult to identify the location of the problem

• Not very fault tolerant – a break in the bus affects the entire network

Ring Topology

• The network forms a ring• Data is transmitted in one direction around the ring• Each workstation acts as a repeater to regenerate the

signal and sends it on to the next node• No terminator needed• Most rings use the token method to determine who

can transmit next.• A token is transmitted around the ring. A computer

wishing to transmit captures the free token, adds info to it and sends it to another node

Ring Topology – Continued

• The intermediate nodes recognizes that the token is not fee and simply passes it on around the ring to its destination.

• The destination node picks up the frame and returns an acknowledgement to the sender

• Upon receipt of the acknowledgement, the originating node releases the token and sends it down the ring.

• With this access method, there is no possibility of a collision

Properties of Ring Topology

• A malfunctioning workstation can disable the network since each node must receive and regenerate the signal

• The more workstations added, the slower the ring

• Access to ring topology is more fair than with bus topology access (Why?)

Star Topology

• Each node is connected through a central device such as a hub

• Each physical wire connects only two devices

Properties of Star Topology

• A break in a cable will only affect one computer• A hub failure can bring down the whole LAN• Because of the central connection point, star

topologies can be easily interconnected with other networks

• Star topology has become the most popular topology for use within LANs

Hybrid LAN Topologies

• Star-Wired Ring

• Star-Wired Bus

• Daisy-Chained

• Hierarchical

Star-Wired Ring

• Physical layout of a star is used, but logically it works like a ring

• Token passing data transmission method used• One physical connection from central location goes to

each workstation• The physical connection, however, has two channels

(receive and send)• The center unit is a specialized hub called a Multi-

Station Access Unit (MAU or MSAU)• Token ring network protocol, specified by IEEE 802.5,

uses this topology

Star-Wired Bus

• Physical layout of a star is used, but logically it works like a bus

• This arrangement is being used in CSC Building• Typically this topology is used with Ethernet and Fast

Ethernet• One physical connection from central hub to each

workstation• The workstation transmits a frame to the hub and then

the hub broadcasts this frame to all the workstations• Typical access method is CSMA/CD

Daisy-Chained Topology

• A daisy chain is a linked series of devices• Hubs can be daisy-chained together to form larger

LANs using the star-wired bus topology (See Figure 5-7, p. 158)

• MAUs can be daisy-chained together to form larger LANs using the star-wired ring topology (See Figure 4.7, p. 177 of handout)

• Daisy-chaining provides for easy expansion of LAN• There is a limit as to how many hubs (MAUs) may

be daisy-chained

Hierarchical Topology

• Attempt to distinguish workgroups by function or priority

• One way to divide groups is to divide them into layers• A hierarchical topology uses layers to separate devices

based upon their priority or function• You may have any number of layers and you may

connect different types of topologies• Figure 5-8 (p. 160) shows a hierarchical ring topology• Figure 5-9 shows a hierarchical star topology

Enterprise-Wide Topologies

• Strategies for connecting one organization to network

• Different from WAN in that only one organization’s resources are connected

• Backbone is cabling that connects hubs, switches and routers on a network

Types of Backbones

• Serial Backbone

• Distributed or Hierarchial Backbone

• Collapsed Backbone

• Parallel Backbone

Serial Backbone

• Simplest kind of backbone

• Consists of two or more hubs connected to each other by a single cable

• Identical to daisy-chained networks discussed earlier

• Not suited for large networks or long distances

Distributed (Hierarchial or tree) Backbone

• Consists of a series of hubs connected to a series of central hubs or routers in a hierarchy

• See Figure 5-9 for example using hubs• See Figure 5-10 for example using routers• Provides ability to segregate workgroups• Good for enterprise-wide network confined

to a single building

Collapsed Backbone

• Uses a router or switch as the single cerntal connection point for subnetworks

• A single router or switch is the highest layer of the backbone

• Failure of central router can bring down the entire network

• Does centralize all management and troubleshooting chores

• See Figure 5-11, p. 163

Parallel Backbone

• A variation of the collapsed backbone arrangement

• Consists of more than one connection from the central router or switch to each network segment

• See Figure 5-12, p. 164

Mesh Networks

• Most often used in enterprise-wide networks and WANs

• Routers are connected with other routers with at least 2 pathways between each two routers

• See Figure 5-13, p. 165

WAN Topologies

• Peer-To-Peer

• Ring

• Star

• Mesh

• Tiered

Peer-To-Peer WAN Topology

• Single Interconnection point for each location• Dedicated communication lines are used to connect the

sites together• Quite often T1 (1.544 Mbps throughput, 24 channels), T3

(44.736 Mbps throughput, 672 channels), or ISDN (Integrated Services Digital Network) lines are used

• See Webopedia for more information• This topology is suitable for only small WANs• All sites must participate in carrying network traffic• A single failure can cause communication between sites to

fail

Ring WAN Topology

• Each site is connected to two other sites

• A single cable problem does not affect communication as routers can redirect transmissions

• Again, T1 and ISDN lines are used to connect sites together

Star WAN Topology

• A single site acts as the central connection point

• A single cable problem only affect communication between the central point and the remote site

• Extending a Star WAN is easy• Failure a central point can bring down

whole network

Mesh WAN Topology

• Has each node connected directly to every other node

• If one line goes down, routers can redirect transmission around bad link

• Very costly to lease a large number of dedicated lines

• May implement a partial mesh

Tiered WAN Topology

• Similar to hierarchical LANs

• Interconnection sites are organized into layers

Switching

• A component of the network’s logical topology that determines how connections are created between nodes in the WAN

• Three types of switching– Circuit Switching– Message Switching– Packet Switching

Circuit Switching

• A connection is established between two network nodes before transmission of data begins

• Bandwidth is dedicated to this connection for the duration of the transmission

• All data follow the same path• Used in telephone system• Use in ATM, modems, ISDN, T1 and T3

Message Switching

• A connection is established between two intermediate nodes and this connection is maintained while all the information is transferred to this intermediate node.

• A new connection is established with the next intermediate node and the same thing happens here

• Referred to as “store and forward”• The connection between source and destination

nodes is not continuously maintained• Some E-mail systems use message switching

Packet Switching

• Data is broken down into packets before they are sent

• The packets associated with a message may travel different paths in getting to the destination

• The destination node is responsible for re-assembling the packets when they arrive

• Examples: Ethernet, FDDI, Internet

Ethernet

• Uses CSMA/CD

• Ethernet Versions– 10Base2– 10Base5– 10BaseT– 100BaseT Fast Ethernet

• 100BaseTX

• 100BaseT4

Switched Ethernet

• The network is divided into smaller segments that can operate simultaneously with the other segments

• Transmissions can take place at the same time on the different segments

Gigabit Ethernet

• IEEE 802.3z – currently finalizing specifications• Can run over UTP, but performs better over fiber• On UTP, distance limitation of 80 ft. currently but

IEEE is working to exceed this limitation• On fiber, distance of 3 kilometers can be achieved