Computer Networks

 

College of Information Science and Engineering Ritsumeikan University

Week  05

Architectures  &  Topologies  

The Three Basic Types of Network Architecture

l  Peer-to-peer networks are decentralized networks for interactions between similar software and devices, such as personal computers

l  In client-server networks, a central computer, such as a workstation or server, serves as a source and destination for many different computers

l  Most networks are a combination of these two types of architecture and are called hybrids.

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Peer-to-Peer Basics l  Allow resources (files, printers, etc.) to be shared l  Best for smaller groups l  Allow easy, decentralized communications

l  Each peer-to-peer network is called a workgroup l  Multiple workgroups can be implemented on the

same network l  The workgroup name allows users to know

which peer-to-peer network is being used

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Client Server Basics l  Centralized architecture for data storage,

security, applications, network administration and other services

l  Also called server-based networks, the client-server network architecture is highly scalable

l  Servers provide central points from which all services can be provided

l  Management, administration, and security can all be controlled centrally by administrators

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Hybrid Network Architectures

l  Many networks employ some characteristics of both client-server and peer-to-peer architectures

l  Central management allows control and security of client-server architectures

l  Local resources can be allocated on a peer-to-peer basis through workgroups for flexibility

l  Network access and security measures, such as password control, can be troublesome for both administrators and users

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Topologies: Main Points

l  Topologies show how the network (usually a LAN) is connected

l  Physical topologies describe how the lines of a network are connected and there are several types

l  Logical topologies describe how the information moves through a network and there are only a couple of types

Physical vs. Logical Topologies Physical

l  How the wires are connected

l  How different devices are physically connected

l  Several types: bus, ring, star, mesh, and several hybrids

Logical l  How the data is

transferred l  How media is

accessed and packets are transmitted

l  Two types: bus or ring

Physical Topologies l  Three most common types are bus,

ring, and star l  Best physical topology is determined

by cost, scalability, bandwidth capacity, ease of installation and troubleshooting

l  Most networks use star topology

Physical Bus Topology l  Single central cable (usually coaxial cable) is

the axis of the network l  Advantages: inexpensive, require less cable,

easy to add on, good for small networks l  Disadvantages: depends on the backbone,

number of devices limited, difficult to isolate problems, can have slow access times

l  This legacy topology is important because of the history of Ethernet

Physical Bus Topology

terminator device physical cable

Bus Components l  Terminators were

placed at either end of the bus

l  BNC (Bayonet Neill-Concelman) allowed each device to be attached to the network

Bayonet mount

Ethernet as a Physical Bus l  Metcalfe’s original conception of the “Ethernet”

for connecting a desktop computer to a printer

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Physical Ring Topology l  Stations are connected in a circle and

data moves in one direction (single-ring topology) or in both directions (dual-ring topology)

l  Advantages: high speeds, no collisions, easy to find problems, no terminators

l  Disadvantages: more cable, less fault tolerance, adding requires shutdown, less common so equipment is scarce

Single Physical Ring

device

direction

cable

Dual Physical Ring

device

direction

physical cable

Physical Star Topology l  All devices are connected by a central device l  Most Ethernet LANs are star or extended

star systems connected by hubs or switches l  Advantages: easy to add or remove devices,

good fault tolerance, central management, easy to troubleshoot, most common topology, easy to upgrade

l  Disadvantages: requires a lot of cable and devices, may cost more, depends on the central hub

Physical Star

device

cable

hub, switch, etc.

Physical Extended Star l  Similar to (or the same as) the

topology known as the tree topology l  Hubs of more than one star topology

are connected

Physical Mesh and Fully-Connected Topologies

l  All devices may be connected to all other devices

l  Disadvantages: extremely expensive and difficult to install, maintain, troubleshoot, low scalability

l  Advantages: extremely fault tolerant for reliable connection

Fully Interconnected

Logical Topologies l  Only two basic types need to be studied:

the logical bus and the logical ring l  Recall the physical bus and the physical

ring l  The logical topologies describe how the

data moves from device to device: like a bus or like a ring

Logical Bus Topology l  Ethernet hubs act as buses l  Data moves out from the source in

both directions on the “bus” l  Advantages: fault-tolerance, common,

easy to change or add l  Disadvantages: collisions can occur,

access is one-at-a-time

Logical Ring Topology l  Token rings pass special data frames called

“tokens” around l  When a device has data to transmit, it waits

for an empty token and inserts its data l  Multistation Access Units (MAU) act as the

ring in physical star topologies l  Advantages: amount of data that can be

transmitted is greater l  Disadvantages: not fault tolerant, devices

must wait for empty token

WLAN Topologies l  Wireless LANs (WLAN) are not bound by

ordinary topology conventions l  Transmission is omnidirectional l  The logical topology is usually a logical

bus, where radio is the medium l  Wireless peer-to-peer may be a logical

mesh or fully interconnected l  Wireless access points (WAP) may be

connected to wired topologies