review of topology and access techniques / switching concepts
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Review of Topology and Access Techniques / Switching Concepts. BSAD 141 Dave Novak Sources: Network + Guide to Networks, Dean 2013. Overview. Three “base” wired topologies Bus, star, ring Two wireless topologies Ad-hoc, infrastructure Three basic access techniques - PowerPoint PPT PresentationTRANSCRIPT
LAN topologies and access techniques (with a focus on Ethernet)
BSAD 141Dave Novak
Chap 5 Network+ Guide to Networks, Dean
Topics Covered LAN Topologies
Three base topologies1. Bus2. Star 3. Ring
Hierarchical Star Mesh Wireless
1. Infrastructure2. Ad-hoc
Topics Covered Media access techniques
Three common techniques1. CSMA/CD2. Token passing3. CSMA/CA
Network backbone design Four basic designs
1. Serial2. Distributed3. Collapsed4. Parallel
Network Topologies
Topology – physical or logical arrangement or design used to connect devices to network medium – addresses logic of how packets move from one device to another
How is topology different from architecture?
Network Topologies The topology is related to the
networking technology (defined at Layer 2) and the specific standards that are being followed
Cannot adopt any standard or any networking technology to every topology
Can create separate networks using different standards and different topologies and connect them using bridges, switches, or routers
Network Topologies
Three base network topologies 1) Bus 2) Star 3) Ring
Bus topologyEach device is cabled directly to the device
next to it“Legacy” Ethernet standards support this
topology• Coaxial cable: Thicknet (10Base5) and Thinnet
(10Base2)
Bus topology Signal is sent along connecting media and all
attached devices receive the signal Signal travels in both directions along the bus
Ends of bus medium must be terminated Devices coordinate to ensure that only one
device transmits a signal at a time This is not a common used topology today –
the Ethernet standards that employ the bus topology are antiquated
Star topology All devices are attached to central access
or connection point (hub or switch) Separate cable connection from each
device to connection point
Star topology Most modern Ethernet standards (as well as
standards using other access technologies and layer 2 protocols) employ some variation of star topologyWidely used today
Can use different types of cable including various twisted pair and fiber
Ring topology Ring topology typically uses the token
passing access techniqueThe ring topology is a LOGICAL loop,
although it is implemented in a physical ring layout
Ring topology Ring topology physically looks like a star
topology The “ring” is a logical function of the
connection point (the MAU or hub) Packets passed to each computer
sequentially in round-robin fashion Each computer acts as a repeater Signal regenerated and forwardedPackets flow in one direction
Hierarchical Star / Star Bus Variation of the base star
Once all ports on switch are used, new switch is added to the network• Switches are connected to each other - one end
of cable plugged into the uplink port of one switch
Bus portion of network(bolded)
Star portion of network
Mesh topology
More theoretical than applied solution in terms of actual LAN / WAN topology
Not used in practice, but is a valid topologyEach device has dedicated physical wired
connection to all other devices on network
Mesh topology Not practical for most wired LANs
Number of connections required:
Number of NICs required
Mesh topology May be used as a model for WAN wiring
(internetworking)Mesh internetwork has multiple paths
between two destinations using redundant routers
Wireless topologies Term “topology” also used to describe
wireless communication structural patterns1) Ad hoc topology2) Infrastructure topology
Ad Hoc topology In wireless world, ad-hoc topology often
synonymous with P2P architectureGroup of wireless devices communicate
directly with one anotherInside the communication range of the
wireless technology• Free to communicate • Free to roam
Works for small number of devices in a small geographical setting
Infrastructure topology Wireless network is joined to a wired
network via a wireless access point (AP)Wireless devices communicate directly with
the AP – NOT directly with each other (as in ad hoc mode)• All communication between both wired and
wireless devices runs through the AP
Most wireless networks utilize the infrastructure topology
Locality of Reference LAN technology is inexpensive and widely
available Locality of reference principle:
• 1) Communicate most frequently with devices nearby
• 2) Communicate repeatedly and most frequently with the same subset of devices
Shared media LANs typically rely on shared media
Devices attached to network must coordinate use of the network• Coordination requires communication• Communication requires time• Time increases as distance between computers
growsShared networks with long delay are
inefficient• Spend more time coordinating actions than
sending data
Shared versus Dedicated? Shared – media / channel capacity is used
jointly by multiple users or applications as neededEnvision a typical toll road with many users
Dedicated – media / channel capacity is used exclusively by a single user or applicationEnvision a toll road where users pay to
reserve their own lane and no one else can use that lane while it is occupied
Access control techniques The method or process for how different
devices share the mediaData Link (2) Layer has 2 sublayers
• 1) LLC• 2) MAC
Access control defined by MAC mechanism Different networking technologies have
different MAC mechanisms
Access control techniques Access control defined by Data Link Layer
protocols (Layer 2)1) CSMA/CD2) CSMA/CA3) Token passing
Ethernet (IEEE 802.3) Most commonly used data link standard /
protocolsProvides many Physical Layer (1)
Specifications including (see Table 5.1 on page 121):• 100BaseTX – Cat 5 UTP, Star, 100 Mbps, 100 meters
• 1000BaseSX (160 MHz) – 62.5/125 multimode fiber, Star, 1000 Mbps, 220 meters
• 10Base2 – RG-58 coaxial, Bus, 10 Mbps, 185 meters
Ethernet (IEEE 802.3) Does NOT have a central control structure
controlling when devices can transmitEncapsulates data received from Network
Layer (Layer 3) into Ethernet frame (Layer 2) where standard IEEE 802.3 specifies:• MAC address• Size and format of frame• Physical layer specifications (Layer 1)
Ethernet (IEEE 802.3)
All 802.3 standards use the Carrier Sense Multiple Access (CSMA) access techniqueUses activity on cable to determine statusDevice checks for activity before transmittingIf activity, the device waitsChecking for activity (in use or idle?) is
called Carrier Sensing
CSMA/CD Demonstrate sharing on legacy Ethernet
(think about a bus topology for conceptual reasons)
One device has exclusive use of cable during transmission
After one is done, another can transmit
CSMA/CD Collision detection (CD) – most important
phase of transmission Electrical and fiber Collisions do not harm hardware
• Result in data being destroyed or corrupted
Devices detect collisions using collision detection Once a collision is detected, any device currently
transmitting stops and sends jamming signal After collision occurs, devices wanting to transmit
must wait for the wire to become idle again
Binary exponential back off In the event of a collision devices using
CSMA/CD use the binary exponential back off algorithm to retransmit
CSMA/CD Collisions are normal part of Ethernet
operation More devices and more activity result in
more collisions Collisions result in retransmission and delay Ethernet does not perform well when
heavily utilizedInstall switch, bridge, router
Wireless (IEEE 802.11) Generally considered to be slower and less
reliable than wired technologies IEEE 802.11 provides Physical Layer (1)
specifications that support different modulation techniques at Physical Layer• 1) Direct Sequence Spread Spectrum (DSSS) • 2) Frequency Hopping Spread Spectrum (FHSS)
Wireless (IEEE 802.11) IEEE 802.11 also provides Data Link (2)
Layer specificationsFrame - use standard 802.3 frameCSMA/CA (collision avoidance) as opposed
to CD (collision detection)• CD requires full duplex – not available in wireless• Performs layer 2 error detection on incoming
packets – if no errors, sends ACK indicating no collisions
• If sender does not receive ACK, assumes a collision and retransmits
Token Passing (IEEE 802.5)
Far less common than Ethernet Hardware is generally more expensive Single shared media Historically uses a ring topology Passive MAU – packets are forwarded to
single device at a time in order
Token Passing (IEEE 802.5)
IEEE 802.5 provides NO Physical Layer (1) specificationsOriginal IBM Token Passing employed a
variety of cable specificationsModern 802.5 generally use Cat 5/5e/6 and
RJ45 connectors
Token Passing (IEEE 802.5)
IEEE 802.5 provides Data Link Layer (2) specificationsToken Ring Frames
• 4 different frame formats used in communication (Ethernet employs only 1): 1) Data Frame, 2) Token Frame, 3) Command Frame, and 4) Abort Delimiter Frame
Token Passing (IEEE 802.5)
Devices must wait for the token before transmitting
Device can only transmit if it has token When sending device is finished, the token
is passed to the next device on the ring
Token Passing (IEEE 802.5)
Logical operation of MAU
Token Passing (IEEE 802.5)
Considered more efficient than CSMA/CD because access technique works well even under heavy load
Provides each device with equal opportunity to transmit
Collision free environment
FDDI / CDDI Prior to Fast Ethernet, FDDI was only data
link protocol to offer 100 Mbps transmissionProvides redundancy to avoid failure
• Two rings• Only one used when network is functioning properly
• Counter rotating• Data flow in opposite directions on the two rings
• Self healing• Hardware can detect failure and recover automatically• Failure is bypassed
FDDIProvides unique Layer 1 and 2 specifications - frame format different from token ring, although access method (token passing) the same
Public versus Private? Public – users pay fees to access a shared
network Often a “pay-as-you-go” approachParts of the physical infrastructure network
are available to the general public for sharing Private – users pay fees to obtain a
dedicated portion of the network Often a “flat fee” approachParts of the physical infrastructure network
are dedicated just for the private user
Public versus Private? Concepts of differentiated service /
differentiated pricing
Point-to-Point and PSTN In many cases we are talking about a
“classification” or a stratification of the same physical infrastructure network…
For example, AT&T infrastructure networks can provide users with both:Dedicated / private serviceShared / public service
Topology and Access methods Three basic topologies
Bus Star Ring
Three basic access methods CSMA/CD CSMA/CA Token passing
http://www.youtube.com/watch?v=Cl6PerDT_ew
Mapping Topologies do not map to access methods
on a one-to-one basis
CSMA/CD
Token Passing
CSMA/CA
Topology
Star, bus
Star, busAd hocinfrastructure
Any
Architecture
Any
Any
Any
LAN technology
802.3 Ethernet
802.11 Wireless,Localtalk
802.5 HSTR802.4 Token bus
Backbone Network Design Serial Distributed Collapsed Parallel
Serial Backbone Simplest backbone consisting of two or more devices
connected directly to each other via a single medium – daisy chain
Dean, Network+ Guide to Networks (2013)
Distributed Backbone Hierarchy consisting of multiple intermediate devices
connected to one or more central devices
Dean, Network+ Guide to Networks (2013)
Distributed Backbone Provides the ability to segregate specific workgroups or
functional areas
Dean, Network+ Guide to Networks (2013)
Collapsed Backbone Single router or switch is central connection point for
multiple LANs or subnetworks
Dean, Network+ Guide to Networks (2013)
Parallel Backbone Most robust backbone consisting of redundant
connections from all central routers to all switches and network segments
Dean, Network+ Guide to Networks (2013)
Lecture Summary
LAN topologies
Media access techniques
Network backbone