week05 - 立命館大学 - beyond borders -piumarta/networks/slides/net-05-ar… · ·...
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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