Transcript Hub

Physical and Logical Topologies
Chapter 5
Four standard physical topologies
• Bus
• Star
• Ring
• Hybrid
Bus Topology
• Single cable connecting all computers
– Coaxial cable
• Easy to set up
Bus topology network
• Signal is heard by all computers, but only the
destination computer accepts the data
• Passive technology--no active electronics to
amplify or regenerate the signal
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Bus topology network
• Only one computer at a time can send data
– Data collisions occurs if two computers
attempt to send data at the same time
• Network must be
terminated at
both ends to
prevent signal
bounce-back
Terminator
Advantages of a Bus
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Easy to set up
Inexpensive
Easy to add new users
Repeaters can be added to amplify signal
Disadvantages of a Bus
• Data collisions may result in slower network
traffic
• Each connection weakens the electrical signal
• Can be hard to troubleshoot
• An unplugged cable
is not terminated and
will take down the network
Ring network
• Each device is
connected to the next
device in the network
• Data is passed from one
device to the next
• Data flows around the
ring in one direction
Ring network
• A token is passed around the
ring
• To send data, a computer
intercepts the token
• Packet of data is attached to
the token and addressed to a
specific computer
• When data is received,
message is sent to sending
computer and token is placed
back on the network
Sending
Computer
Data
Destination
Computer
Data
Advantages of a Ring
• Avoids data collision
• Equal access by all computers on the network
• Each station acts as a repeater
– Regenerates the signal
Disadvantages of a Ring
• Additional cost of network adapters and
software
• Entire network can be affected by a single
break in the cable or failure of one computer
• As more computers are added to network,
response time slows
• It is more difficult to troubleshoot a ring
network
Star network
• Each computer is connected to a central hub
– Active hubs—amplify or regenerate signal
– Passive hubs—pass signal, no amplification or
regeneration
Hub
Star network
• All data is sent to the hub.
– Broadcast star--hub sends data out to all
attached devices (passive or active hubs)
– Switched star--hub sends data to specific device
Hub
Advantages of a Star
• A break or unplugged
cable takes down only
the unplugged computer
• Easy to add new
computer to network
• Hub provides centralized
location for diagnosing problems
Disadvantages of a Star
• If hub is down, entire network is down
• Additional cost for hub and cabling
• If passive or active hubs are used, data
collisions can occur slowing network traffic
Hybrid Networks
• Star-bus
• Star-ring
Star-ring network
• Computers are attached to a hub (MSAU) but
within the hub, a ring is implemented to pass
data
Hub (MSAU) showing the
internal ring and clockwise
token path
Star-bus network
• Computers are attached to a hub (star). Hubs are
attached to other hubs using bus topology.
Hub
Hub
Hub
Enterprise-wide Topologies
• Expand on the simple star and star-bus
network
• Connects hubs, switches, and routers on a
network
• Fiber-optic cable often used as backbone of
enterprise-wide network
• Many different ways to implement depending
upon needs of organization
Enterprise-wide Topologies
• Simplest implementation—series of hubs
connect to each other
– Creates a single collision domain
• All devices on the network have to look at all
packets
• Hubs to Bridges/Switches
– Segments the network
• Controls flow of traffic
• Routers—create subnetworks
Mesh Network
• Provides redundant physical links between
network devices
• Used in small installations or where it is
essential that there always be a path to
critical devices
• Very fault tolerant
• Difficult and expensive to install and
reconfigure
Mesh Network
• True mesh–Each device
is connected to every
other device
• Hybrid mesh–Redundant
paths are created to
critical devices
WAN Topology
• WAN—Wide Area Network
– Connects geographically distinct locations
• Topologies
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Peer-to-Peer (Bus)
Ring
Star
Mesh
Tiered
Logical Topologies
• The way data will be transmitted between
nodes
• Most popular logical topologies is:
– Ethernet
• Other logical topologies include:
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Token Ring
FDDI
ATM
LocalTalk
Switching
• Technique that determines how connections
are made and how data is moved
• 3 types of switching
– Circuit switching
– Message switching
– Packet switching
Circuit Switching
• Connects sender and receiver by a single
physical path for the duration for the
conversation
• Telephone call is an example of circuit
switching
• Route selected from sender to receiver varies
with each connection but once the connection
is made the path stays the same
Advantages of Circuit Switching
• Guaranteed data rate
– Once circuit is established there are no delays
waiting for an available channel
• Can be used for time-critical transmission
such as audio and video
Message Switching
• Messages are stored and forwarded from one
intermediate device to another until the
message reaches the destination
• E-mail messages are an example of message
switching
– The message is sent from server to server
• Sometimes referred to as “store-and-forward”
Message Switching Advantages
• Provides effective traffic management
– Priority can be assigned to messages so that
you can assure that high priority messages are
handled promptly
• Reduces network traffic
– Message can be stored at intermediate devices
until a communications channel is available
• Provides asynchronous communications
across time zones
Packet Switching
• Messages are broken into packets
– Each packet contains header information including
source, destination, sequence number etc.
• Individual packets may not follow the same path
from the sender to the receiver
• The Internet is an example of a packet-switched
WAN
Advantages of Packet Switching
• If a particular path goes down during
transmission, packets can be rerouted
• Better use of bandwidth because packets can
be sent via different routes
Ethernet
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Most popular architecture in use today
Bus, star, or star-bus based technology
Uses baseband signaling
Uses CSMA/CD access method
10Mbps or 100Mbps transfer rate
– Higher rates are now possible
• Cable: Thinnet, Thicknet, UTP/STP, Fiber
CSMA/CD
• Carrier sense multiple access with collision
detection media access method
• Contention-based system
• Only one workstation can use network at a time
• When a collision is detected, workstations wait a
random amount of time and retransmit packet
• More nodes on the network—the more collisions
– Users complain of slow network response
Ethernet Networks
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10Base2 (Thinnet)
10Base5 (Thicknet)
10BaseT (Twisted-pair)
10BaseF (Fiber)
100BaseT (twisted-pair)
100BaseFX (Fiber)
Gigabit Ethernet
(Fiber or twisted-pair)
10Mbps
bandwidth
100Mbps
bandwidth
1000Mbps
bandwidth
10Base2 Ethernet (Thinnet)
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10Mbps transmission speed; baseband
Maximum segment length--185 meters
Maximum segments--5
Maximum repeaters--4
– Repeater--device that regenerates network signal
to extend the distance signal can travel
• Maximum segments with nodes--3
• Maximum nodes per segment--30
• Maximum overall length with repeaters--925
meters
10Base2 Ethernet
• Specification is 50-ohm RG-58A/U or RG58C/U coaxial cable
– TV cable is RG-59
• Uses BNC connectors
• Transceiver is built into NIC cards--no
separate vampire tap transceiver
– Connected to NIC by BNC T connector
• Terminator must be installed at both ends of
network
10Base5 Ethernet (Thicknet)
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10Mbps transmission speed; baseband
Maximum segment length--500 meters
Minimum distance between taps--2.5 meters
Maximum number of taps/segment--100
Maximum number of segments--5
Maximum number of repeaters--4
Maximum number of segments with nodes--3
10Base5 Ethernet
• Maximum 300 nodes
• Maximum overall length with repeaters-2.5 kilometers (2500 meters)
• Maximum AUI cable length--50 meters
– Distance from transceiver (vampire tap)
to the NIC)
10Base2 & 10Base5 Rule
• The 5-4-3 rule; 5
segments, 4 repeaters,
and 3 populated
Repeater 1
segments
Repeater 2
Trunk Segment 2
Trunk Segment 3
Repeater 3
Trunk Segment 1
Repeater 4
Trunk Segment 5
Trunk Segment 4
10BaseT Ethernet (Twisted-Pair)
• 10Mbps transmission; baseband
• Physically wired as a star
– each device connected directly to a hub
– hub acts a repeater
• Cable: Category 3, 4, or 5 UTP
• Maximum segments--1024
10BaseT Ethernet
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Maximum segment length--100 meters
Maximum nodes per segment--2
Maximum nodes per network--1024
Maximum hubs in a chain—4
RJ-45 connectors
100BaseT Ethernet (Fast Ethernet)
• 100BaseTX
– Uses 2 pairs cable
– Category 5 UTP, Cat 5E or higher
• 100BaseT4
– Uses 4 pairs cable
– categories 3, 4, & 5 twisted pair
100BaseT Ethernet
• 100Mbps
• Specification for maximum segment length, #
of segments, #of nodes per segment, etc. are
the same as 10BaseT
10BaseF and 100BaseFX
• 10Mbps or 100Mbps throughput
• Cable: Fiber—multimode
• Maximum segment length
– 10BaseF—1000-2000meters (1-2 kilometers)
– 100BaseF—400meters
• Star-bus topology
Gigabit Ethernet
• Typically used for backbone
– Hub to hub; hub to switch; hub to router
• Fiber-optic cable (typically)
– Can be implemented for UTP
• Speeds in excess of 1000Mbps (1Gbps)
Token Ring
• Robust, reliable network architecture
developed by IBM
• Addressed by 802.5 standard
• Physically may look like a star but logically
works like a ring
– Star-ring topology
• Workstations connect to a MAU, MSAU or
SMAU
– MSAU--MultiStation Access Unit
Token Ring Specifications
• Bandwidth--4Mbps or 16Mbps
– All cards on the network must be configured
the same
– 100Mbps token ring is available but not widely
implemented
• Called HSTR (High-Speed Token Ring)
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Cable--UTP, STP, fiber
Maximum MSAU--33
Maximum nodes per MSAU--8 (IBM)
Maximum nodes—255 (STP) 72 (UTP)
Adding hubs while maintaining the logical ring
Ring out
Rin
g
in
Ring
out
Ring in
Ring out
Ring in
Ring in
Ring in
Ring out
Ring
out
FDDI
(Fiber Distributed Data Interface)
• Networking standard
originally specified by ANSI in
the 1980s and later refined by
ISO
• Uses a dual fiber-optic ring to
transmit data at speeds of
100 Mbps
– Dual counter-rotating rings
provide fault tolerance
• Used to connect LANs located
in multiple buildings, e.g., a
large campus
FDDI (Fiber Distributed Data Interface)
• Cable lengths can extend 100 km (62 miles) with
repeaters every 2 km
• Access to network is controlled using a token passing
scheme
• Can attach up to 500 nodes
• Computers can be attached to only the primary ring or to
both rings
– Computers attached to both rings are called Class A stations
(dual-attached stations)
– Computers attached to only the primary ring are called Class
B stations (single-attached stations)
ATM
(Asynchronous Transfer Mode)
• WAN transmission that relies on a fixed packet size to
achieve data transfer rates from 25 to 622 Mbps
• Cell—the fixed-length packet in ATM
– Exactly 53 bytes
– 48 bytes of data and a 5 byte header
1000 bytes of data
5 byte header
48 bytes of data
53 bytes
LocalTalk
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Specially for Macintosh networks
Maximum throughput—230Kbps
Cannot support non-Mac equipment
Use CSMA/CA media access
– Carrier sense, multiple access with collision
avoidance
• NIC signals its intent to send data before it
actually sends the data in order to avoid
collisions