LAN topologies and access techniques (with a focus on Ethernet)

Transcription

1 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 Three base topologies 1. Bus 2. Star 3. Ring Hierarchical Star Mesh Wireless 1. Infrastructure 2. Ad-hoc Topics Covered Media access techniques Three common techniques 1. CSMA/CD 2. Token passing 3. CSMA/CA Network backbone design Four basic designs 1. Serial 2. Distributed 3. Collapsed 4. 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 1

2 Bus topology Each 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 topology Widely used today Can use different types of cable including various twisted pair and fiber Ring topology Ring topology typically uses the token passing access technique The 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 forwarded Packets flow in one direction 2

3 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 Mesh topology More theoretical than applied solution in terms of actual LAN / WAN topology Not used in practice, but is a valid topology Each device has dedicated physical wired connection to all other devices on network Bus portion of network (bolded) Star portion of network Mesh topology Not practical for most wired LANs Number of connections required: Mesh topology May be used as a model for WAN wiring (internetworking) Mesh internetwork has multiple paths between two destinations using redundant routers Number of NICs required Wireless topologies Term topology also used to describe wireless communication structural patterns 1) Ad hoc topology 2) Infrastructure topology Ad Hoc topology In wireless world, ad-hoc topology often synonymous with P2P architecture 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 3

4 Infrastructure topology Locality of Reference 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 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 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 typical toll road with many users 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 it is occupied Access control techniques The method or process for how different devices share the media Data 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/CD 2) CSMA/CA 3) Token passing 4

5 Ethernet (IEEE 802.3) Ethernet (IEEE 802.3) Most commonly used data link standard / protocols 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 Does NOT have a central control structure controlling when devices can transmit Encapsulates data received from Network Layer (Layer 3) into Ethernet frame (Layer 2) where standard IEEE specifies: MAC address Size and format of frame Physical layer specifications (Layer 1) Ethernet (IEEE 802.3) All standards use the Carrier Sense Multiple Access (CSMA) access technique Uses activity on cable to determine status Device checks for activity before transmitting If activity, the device waits Checking 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 5

6 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 ) Generally considered to be slower and less reliable than wired technologies IEEE 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 ) IEEE also provides Data Link (2) Layer specifications Frame - use standard 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 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 IEEE provides NO Physical Layer (1) specifications Original IBM Token Passing employed a variety of cable specifications Modern generally use Cat 5/5e/6 and RJ45 connectors IEEE 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 6

7 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 Logical operation of MAU 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 Public versus Private? Public users pay fees to access 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 7

8 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 service Shared / public service Topology and Access methods Three basic topologies Bus Star Ring Three basic access methods CSMA/CD CSMA/CA Token passing Mapping Topologies do not map to access methods on a one-to-one basis CSMA/CD CSMA/CA Token Passing Topology Star, bus Star, bus Ad hoc infrastructure Any Architecture Any Any Any LAN technology Ethernet Wireless, Localtalk HSTR 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 8

9 Distributed Backbone Hierarchy consisting of multiple intermediate devices connected to one or more central devices Distributed Backbone Provides the ability to segregate specific workgroups or functional areas Collapsed Backbone Single router or switch is central connection point for multiple LANs or subnetworks Parallel Backbone Most robust backbone consisting of redundant connections from all central routers to all switches and network segments Lecture Summary LAN topologies Media access techniques Network backbone 9