Chapter 3
Understanding Network Architecture
Lesson 1
Access Methods – set of rules which govern how network traffic is controlled. Defines
how a computer puts data on and takes data off the network cable.
Collision – data packets collide with packets from other computers destroying the packet
Methods to access the wire—
Contention
CSMA/CD
CSMA/CA
Token Passing
Demand Priority
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Carrier Sense Multiple Access Collision Detect (CSMA/CD)
o Each computer listens for traffic on the wire ( carrier sense)
o If a computer senses the cable is free it sends frame
o No other computer can send until the cable is free again
o If a collision occurs the sending computers wait a random time and resend
 CSMA/CD considerations ( used in Ethernet)
o Due to attenuation of signal, collision detection is not effective beyond
2500 meters
 Attenuation—loss of signal strength over distance
o More computers or network traffic the longer the wait
Carrier Sense Multiple Access Collision Avoidance (CSMA/CA) (used in
LocalTalk)
o Like CSMA/CD but transmits an intent to send first
o Considerations—
 More network traffic
 Slower
Token Passing ( used in Token Ring and ArcNet)
o Special packet called a token circulates the ring
o Data is transmitted in frames with additional information
o No contention, no collisions
Demand Priority ( used in 100VG-AnyLAN)
o Designed for 100 Mbps Ethernet standard
o Repeaters manage network access by doing round robin searches for
requests to send from all end nodes, and verify functionality of all end
nodes
 End nodes – computer, bridge, router or switch
o Hub can prioritize requests or alternate between the same priority requests
o Computers user 4 pairs of wire allowing them to transmit and receive at
the same time
Lesson 2
How Networks Send Data
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Packet – a unit of information transmitted as a whole from one device to another on a
network. Large data is broken into manageable packets which are the basic unit of
network data communication.
o Data is broken into packets to
 Avoid flooding the cable speeding up transmissions
 Lower the impact of retransmitting large units
Common Packet Components (* common to all protocol packets)
o *header
 alert signal and/or clock information
 *Source address
 *Destination address
 *Instructions for reassembling
o *Data
 varies from 512bytes-4KB depending on the network
o *Trailer
 *Error checking
 CRC (cyclical redundancy checko Mathematical calculation preformed on the packet
at the source and again at the destination
Protocol—set of rules or standards designed to enable computers to connect with one
another and to exchange information with as little error as possible.
Network Architectures
Lesson 3
ISO (International Standards Organization) developed the OSI (Open Systems
Interconnection) model standards --specifications for connecting dissimilar devices.
IEEE developed the 802 standards for design and compatibility for hardware
components operating in the data-link and physical layer of the OSI.
Common 802 standards
802.3 Ethernet
802.12 Demand Priority Access
802.5 Token Ring
802.4 token passing on a bus Arc-net loosely maps to this standard
 Ethernet— ( follows 802.2 and 802.3 standards)
o Most popular architecture ( Win95/98/2000/NT,MS LAN
manager,Netware,IBM LAN server, AppleShare and UNIX)
o bus or a star-bus topology
o baseband transmission 10Mbps or 100Mbps speed
o CSMA/CD access method
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o Thicknet, thinnet or UTP cable
o Frame –packet of information transmitted as a single unit, between 64 and
1518 bytes in length with at least 18 bytes used by the Ethernet frame.Data
in an Ethernet frame can be 46 to 1500 bytes in length.
 Ethernet II frame is used for TCP/IP
Ethernet alternatives
o 10BaseT
 10Mbps Baseband over Twisted pair wire (cat 3,4 or 5or 6)
 star pattern, internal bus signal
 hub is a multiport repeater
 maximum segment length is 100meters
 maximum computers is 1024
 minimum distance between computers is 2.5 meters
 RJ-45 connections, transceiver on the NIC
o 10Base2
 10Mbps Baseband over thinwire coaxial
 bus pattern
 maximum segment 185 meters, max network length 925 meters
 .5 meter minimum distance between computers
 maximum of 30 nodes per segment, 90 nodes in all
 BNC connectors and 50 ohm terminators
 5-4-3 rule applies
 5 segments 4 repeaters 3 populated segments
o 10Base5
 10Mbps Baseband over thickwire ( backbone)
 maximum segment length 500 meters, max network length 2500
meters
 bus topology
 minimum of 2.5 meters between nodes
 maximum of 100 nodes per segment, 300 nodes per network
 components include
 vampire tap
 transceiver
 transceiver cable ( max length 50 meters)
 AUI (attachment unit interface) connectors
 50 ohm terminators N series
 5-4-3 rule applies
o 10baseFL
 standard for Ethernet over fiber optic cable
 maximum distance per segment 2000 meters
o 100Mbps Standards
 100BaseVG AnyLAN
 802.12 standard
 transmits 802.3 frames and 802.5 packets
 100Mbps minimum transmission
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cascaded hubs star topology over cat 3,4,5 TP and fiberoptic
 demand priority access
 maximum of 250 meter cable segment
 specialized hubs and cards
100BaseX Ethernet
 extensions of standard Ethernet
 100BaseT4- 4 pair cat 3,4,5 UTP
 100 BaseTX – 2 pair cat 5 STP
 100 BaseFX – two strand fiber optic
Lesson 4
 Token Ring (802.5 standard)
o Developed by IBM to facilitate TP wiring in the wall
o ANSI/IEEE standard ( ANSI is the American representative of ISO)
o Star-wired ring
o Computers connect to a central hub containing the logical ring
o Token passing access method one direction of travel one token
o STP and UTP IBM type cables 1, 2, 3
o 4 to 16 Mbps Baseband transmission
o used with Fiber Optic and switches for high speed and increased distance
o summary of functionality:
 1st computer online generates token and monitors the network
(checks for frames that have circulated more than once and
multiple tokens)
 active monitor sends beacon announcement every 7 seconds
 if the downstream computer does not receive the beacon it
puts a message on the ring including the source address,
and the address of the upstream computer
 ring attempts to reconfigure
 initializes new computers
 checks for duplicate addresses
 notifies other computers of existence
o hardware components:
 Hub- Mutistation Access Unit (MAU, MSAU or SMAU)
 8 computers per hub, 33 hubs per ring connected via Ring in and
Ring out
 72 computers UTP or 260 computers with STP
 MSAU will detect NIC failures and disconnect from the failing
system
 Type 3 UTP –45 meters, Type2 STP 100 meters,Type 1STP
maximum cable segments
Lesson 5
 AppleTalk (Used by Apple MacIntosh environment)
o Proprietary network for small groups
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o Commonly referred to as LocalTalk (also refers to the physical
connections)
o CSMA/CA access method
o Bus or tree topology
o STP cable or Fiber optic and UTP
o 230.4 Kbps transmission rate
o max 32 devices with LocalTalk, others vendor devices can be used for
greater connections
o AppleShare is the file server
o Zones – logical groupings to join LocalTalk networks
o EtherTalk – allows AppleTalk to run on Ethernet coaxial cable
o TokenTalk- allows connection to 802.5 network
ArcNet (Plus) –( maps closely to 802.4 standards)
o token passing access method moves according to the order in which the computer connected to
the hub
o star-bus topologyo 20Mbps transmission
o 4096 bytes of data per frame
o 93 ohm RG-62 coaxial cable, TP or fiber optic
o maximum cable length from hub 610 meters, or 305 meters on a bus
segment, or 244 meters using TP