LAN topologies and access techniques (with a focus on Ethernet) BSAD 141 Dave Novak Chap 5 Network+ Guide to Networks, Dean

Topics Covered LAN Topologies Bus Star Ring Hierarchical Star Mesh Wireless

Topics Covered Media access techniques (4) Network backbone Ethernet – CSMA/CD Priority access Token passing Wireless/Localtalk – CSMA/CA Network backbone

Network Topologies Topology – physical / spatial arrangement or design used to connect devices to network medium How is topology different from architecture?

Network Topologies Topology is typically related to the networking technology and the specific standards used Cannot adopt every standard and technology to any topology Can create separate LANs using different standards and topologies and connect them using bridges, switches, or routers

Network Topologies Three basic network topologies 1) Bus 2) Star 3) Ring

Bus “Legacy” Ethernet standards support this topology Each device is cabled directly to the device next to it Early Ethernet used bus topology with coaxial cable: Thicknet and Thinnet

Bus 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 must coordinate to ensure that only one device transmits a signal at a time

Star All devices attached to central point (hub or switch) Separate cable from each device to hub

Star Most Ethernet LANs (as well as LANs using other access technologies and layer 2 protocols) employ some variation of star topology Can use different types of cable including twisted pair and fiber Functionally, star-wired Ethernet LAN with hub uses shared medium Basic hub propagates signals it receives out all other ports

Ring In terms of signal transmission, ring is like bus – each device logically connected to the next Two ends connected forming an endless loop

Ring The ring is a LOGICAL loop, not necessarily a physical ring layout Ring topology LAN physically look like a star topology LAN The “ring” is a logical function of the MAU to which all PCs are connected Each device acts as a repeater Signal regenerated and forwarded Packets passed to devices one at a time (sequentially) Packets flow in one direction

Hierarchical Star / Star Bus Variation of the base star Once all ports on hub are used, new hub is added to the network Hubs connected using standard cable - one end of cable plugged into the uplink port of one of the hubs

Mesh In terms of many LAN / WAN connections, more theoretical than actual solution Not used in practice, but is a valid topology Each device has dedicated connection to all other devices on network

Mesh Not practical for most wired LANs Number of connections required: Number of NICs required

Mesh May be used as a model for WAN wiring (internetworking) Mesh internetwork has multiple paths between two destinations using redundant routers

Wireless Term “topology” also used to describe wireless communication patterns 1) Ad hoc topology 2) Infrastructure network

Ad Hoc Topology Group of wireless devices communicate directly with one another Inside 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 devices communicate with wired network using wireless transceivers Wireless devices communicate directly with transceivers (physical access points to the wired network) Allows communication between both wired and wireless devices through transceivers Typical in larger network with mix of wireless and wired devices

Compare / Contrast Base Topologies

Sharing medium on a LAN 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 grows Shared 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 needed Envision a toll road Dedicated – media / channel capacity is used exclusively by a single user or application Envision a toll road where users pay to reserve their own lane and no one else can use that lane while occupied

Public versus Private? Public – users pay fees to use a shared network Often a “pay-as-you-go” approach Parts 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” approach Parts of the physical infrastructure network are dedicated just for the private user

Point-to-Point and PSTN In many cases we are talking about “classification” or a stratification of the same physical infrastructure network… For example, AT&T infrastructure networks can provide users with both: Dedicated / private service Shared / public service

Locality of Reference LAN technology is inexpensive and widely available Locality of reference principle: 1) Communicate most frequently with computers nearby 2) Communicate repeatedly and most frequently with the same subset of computers

Access control techniques Describe how devices share media The process or method devices follow for sharing media Data Link (2) Layer has 2 sublayers 1) LLC 2) MAC Access control defined by MAC mechanism used

Access control techniques Access control defined by Data Link Layer protocols (Layer 2) 1) CSMA/CD 2) CSMA/CA 3) Token passing 4) Demand priority

Ethernet (IEEE 802.3) Most commonly used data link standard / protocols Does NOT have a central control structure controlling when devices can transmit Provides 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) Provides Data Link Layer (2) Specifications: Encapsulates data received from Network Layer into Ethernet frame which specifies: MAC address Size and format of frame

Ethernet (IEEE 802.3) Carrier Sense Multiple Access (CSMA) Uses activity on cable to determine status Cable is free, no activity – used, activity Device checks for activity before transmitting If activity, the device waits Checking for activity is called Carrier Sensing

CSMA/CD 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 backoff In the event of a collision how do devices using CSMA/CD know when to attempt to transmit again?

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 utilized Install switch, bridge, router

Wireless (IEEE 802.11) Generally considered to be slower and less reliable than wired technologies Provides Physical Layer (1) specifications: 802.11 supports different modulation techniques at Physical Layer 1) Direct Sequence Spread Spectrum (DSSS) 2) Frequency Hopping Spread Spectrum (FHSS)

Wireless (IEEE 802.11) 1) DSSS 2) FHSS

Wireless (IEEE 802.11) Provides Data Link (2) Layer specifications Frame - use standard 802.3 frame CSMA/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 Logical ring, physical star Passive MAU – packets are forwarded to single device at a time in order

Token Passing (IEEE 802.5) Provides NO Physical Layer (1) specifications Original IBM Token Passing employed a variety of cable specifications Modern 802.5 generally use Cat 5/5e/6 and RJ45 connectors

Token Passing (IEEE 802.5) Provides Data Link Layer (2) specifications Token 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 transmission Provides 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

FDDI Provides unique Layer 1 and 2 specifications - frame format different from token ring, although access method (token passing) the same

Demand priority Access method REQUIRES use of star topology Each device sends request to active hub Hub processes requests and assigns a communication order – one at a time (FCFS) Fair access Central hub controls traffic Devices do not compete for use of media

Comparison of Access Methods CSMA/CD CSMA/CA Token passing Demand priority

Topology and Access methods Three basic topologies Bus Star Ring Four basic access methods CSMA/CD CSMA/CA Token passing Demand priority

Mapping Do topologies map to the access methods on a one-to-one basis? CSMA/CD Demand Priority Token Passing CSMA/CA Topology Star, bus Ad hoc infrastructure Any Star Architecture LAN technology 802.3 Ethernet 802.11 Wireless, Localtalk 802.5 HSTR 802.4 Token bus 802.12 Demand priority

Backbone Networks 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