Local Area Networks Topologies Transmission Media 635 412

Local Area Networks Topologies & Transmission Media 635. 412 Class #2 1

Topologies & Transmission Media Network Topology Overview n n n The topology of a network refers to the way in which end systems attached to the network are interconnected In many networks (especially today) there is a significant difference between the logical and physical topologies of the network (e. g. – virtual LANs or VLANs) Different types of topologies: n n n Bus and Tree Ring Star 635. 412 Spring 2005 Class 2: More Basics 2

LAN Topologies 635. 412 Spring 2005 Class 2: More Basics 3

Transmission on a Bus 635. 412 Spring 2005 Class 2: More Basics 4

Topologies & Transmission Media Bus and Tree Topologies n n n Requires the use of a multipoint medium The bus is a special case of the tree topology, with one trunk and no branches All stations attach to the transmission medium and can both send data on it and receive data from it Termination required at each end of the bus to ‘absorb’ the propagated signal, preventing reflections The tree topology consists of a connected set of branches with no closed loops – – The root of the tree is a device called the headend The tree structures can be very complex, with large numbers of nodes and branches 635. 412 Spring 2005 Class 2: More Basics 5

Topologies & Transmission Media Bus and Tree Topologies (continued) n Bus/Tree topologies present two significant problems: – – n n Since a transmission is received by all stations, an addressing scheme is necessary to denote who the intended recipient is There must be a mechanism for regulating access to the transmission medium To solve these problems, data to be transmitted is broken into frames for transmission – frames include addressing as well as control information If proper termination is present no special procedures are necessary to remove data from a bus/tree topology 635. 412 Spring 2005 Class 2: More Basics 6

Topologies & Transmission Media Ring Topology n n n A network consisting of a set of transmission nodes (repeaters) connected in a closed loop by a set of point-to-point links Data transmission around the ring is unidirectional Like the bus/tree topology data is broken into chunks and transmitted in frames Unless one station removes the frame from the ring, it will circulate around the ring indefinitely (usually the transmitting station removes the frame as it returns to the station) Requires a medium access control mechanism to control when stations transmit frames 635. 412 Spring 2005 Class 2: More Basics 7

635. 412 Spring 2005 Class 2: More Basics 8

Topologies & Transmission Media Star Topology n n Each station is connected by point-to-point links to a central node that regulates and controls transmission The star topology has two variants depending on the role the central controller plays: physical star/logical bus and physical star/logical star – – Physical star/logical bus: the central controller broadcasts incoming frames out to all stations Physical star/logical star: the central controller acts as a frame switching device; only the destination node sees and receives the frame 635. 412 Spring 2005 Class 2: More Basics 9

Topologies & Transmission Media Choice of Topology n Depends on a variety of factors: – – n n Reliability Expandability Performance Cost The choice is not independent of other factors such as transmission medium, wiring, & access control methods Although a bus topology can be used with different transmission media, only baseband coaxial cable has achieved widespread use Ring topologies can create very high speed links, but a single repeater or link failure can disable the entire network The star topology can take advantage of the natural layout of wiring within a building 635. 412 Spring 2005 Class 2: More Basics 10

Topologies & Transmission Media Choice of Transmission Medium n Besides topology, a number of other factors must be considered in choosing an appropriate transmission medium – – n Capacity Reliability Type of data supported Environmental scope General observations on the choice of transmission medium: – – Voice grade UTP (Category 3) cabling may already exist within a building, minimizing installation costs – however LANs it can support will have limited range and data rates Coaxial cable is more expensive than Cat 3 UTP but supports higher data rates 635. 412 Spring 2005 Class 2: More Basics 11

Topologies & Transmission Media Choice of Transmission Medium (continued) n The current trend is toward Category 5 or 5 e UTP, which supports high data rates but is less costly than coax Optical fiber can a number of attractive characteristics, but its use in LANs has been limited because of high cost & specialized maintenance requirements n Relationship between Medium & Topology n – – For several reasons the choice between transmission medium & network topology are not independent Table 4. 1: Medium versus Topology for LANs and MANs n n Ring may use (TP, Baseband/Broadband Coax, Optical Fiber, and Wireless) Bus may use (TP, Baseband/Broadband Coax, Optical Fiber, and Wireless) Tree may use (Broadband Coax) Star may use (TP, Optical Fiber, and Wireless). 635. 412 Spring 2005 Class 2: More Basics 12

Topologies & Transmission Media Bus & Tree Topologies n n While the bus & tree topologies are being replaced with star and ring topology LANs, they still enjoy a large installed base and are important in the CATV arena Characteristics of the Bus & Tree Topologies – Requires medium access control, since multiple stations could transmit simultaneously n n – Both centralized & distributed schemes can be used Distributed schemes most commonly implemented Also needs signal balancing across all possible transmitters and receivers n n Must be done between all pairs; scales quickly to a difficult problem A common solution is subdividing a network into smaller segments using repeaters 635. 412 Spring 2005 Class 2: More Basics 13

Topologies & Transmission Media Coaxial Cable for Bus & Tree LANs n For bus & tree topology LANs coaxial cable is a very popular choice for the transmission medium – n Two choices of transmission techniques using different types of coaxial cable are generally available: baseband broadband Baseband Coaxial Cable – – – The most popular transmission medium for bus & tree topologies; uses baseband digital signaling (usually Manchester or differential Manchester) Baseband signaling prevents multiple channels and the attenuation limits network size to around 1 -2 kilometers Transmission is bi-directional necessitating termination resisters at each end a bus topology (Cannot have trees/branches) 635. 412 Spring 2005 Class 2: More Basics 14

635. 412 Spring 2005 Class 2: More Basics 15

Topologies & Transmission Media Baseband Coaxial Cable n A special 50 ohm coaxial cable is used instead of 75 ohm CATV coax to minimize noise and reflections from tap insertions – – n Two choices for baseband coaxial cable are defined in the 802. 3 standard: 10 BASE-5 and 10 BASE-2 [Table 4. 3] The thinner 10 BASE-2 cable allows for easier and cheaper installations at the expense of total LAN size Repeaters may be used with either cable type to extend the length of the LAN and allow more stations to connect – – – Repeaters must connect together LANs of the same type Repeaters do no buffering of data, transparently passing digital signals in both directions To prevent interference, only one path of segments and repeaters are allowed between any two station 635. 412 Spring 2005 Class 2: More Basics 16

Topologies & Transmission Media Broadband Coaxial Cable n n Uses analog signaling – RF signaling and FDM is possible allowing multiple data and/or video channels Most components can be used in either bus or tree topologies Comparison of baseband broadband [Table 4. 2] Dual and split broadband configurations – – – Instead of using expensive bi-directional amplifiers, systems use two different cables: upstream & downstream These two data paths are connected together at a point called the headend: all stations transmit upstream to the headend and signals from the headend are transmitted downstream to all stations Stations usually send & receive on the same frequency but on different cables 635. 412 Spring 2005 Class 2: More Basics 17

Topologies & Transmission Media Broadband Coaxial Cable n The split system uses different frequency portions of one cable for the upstream & downstream paths – – – Stations send and receive on different frequencies Bi-directional amplifiers are necessary to amplify both the upstream and downstream data paths The headend provides frequency conversion to change upstream to downstream signals; may also include remodulation to clean up the downstream signals 635. 412 Spring 2005 Class 2: More Basics 18

Topologies & Transmission Media Ring Topologies n Description – – – A ring consists of a number of repeaters each connected to two others by unidirectional transmission links to form a single closed loop Repeaters are the attachment point for stations on the ring Proper ring operation requires repeaters to perform three functions: data insertion, data reception, and data removal n As a frame passes by the station will examine the address of the frame, if it matches it will copy the frame into its buffer for the station n A frame must be removed from the ring or it will continue to circulate around the ring indefinitely – – 635. 412 Spring 2005 The frame could be removed by the addressed repeater A better alternative is to have the transmitting repeater remove the frame; allowing for automatic acknowledgements & multicasting Class 2: More Basics 19

Topologies & Transmission Media Ring Operation n n The Ring consists of a number of repeaters connected by unidirectional point-to-point links in a closed loop To accomplish its primary functions repeaters are designed to operate using three different states: listen, transmit, and bypass 635. 412 Spring 2005 Class 2: More Basics 20

Topologies & Transmission Media Ring Operation – Listen State n n n In the listen state the repeater is examining frame addresses and control bits while retransmitting the frame to the next station In addition the repeater copies bits of ‘interesting’ frames to the attached station The repeater may modify a bit in the frame as it passes (e. g. - acknowledgements) 635. 412 Spring 2005 Class 2: More Basics 21

Topologies & Transmission Media Ring Operation – Transmit State n n Here the repeater ‘breaks’ the ring and begins transmitting a frame from the attached station onto the ring Meanwhile, bits may be received -- depending on the bit length of the ring two things may happen: – – The bit length of the ring < the frame transmitted: the frame has circulated around the ring and is sent back to the station to check it for successful reception The bit length of the ring > the frame transmitted: other frames may be received during transmission and must be buffered for retransmission 635. 412 Spring 2005 Class 2: More Basics 22

Topologies & Transmission Media Ring Operation – Bypass State n This state completely isolates the repeater from the ring, allowing signals to propagate past the repeater with no delay – – – Not absolutely required for ring operation Helps improve the reliability of the ring Improves performance by reducing repeater delay; a result of bypassing stations not active on the network 635. 412 Spring 2005 Class 2: More Basics 23

Topologies & Transmission Media Ring Benefits & Problems n Ring Benefits – – The ring is a multi-access network like the bus/tree topology allowing multicast & incremental cost growth Uses point-to-point links, which have several benefits: – – – n Allows the ring to span greater distances with cheaper electronics Fiber optic links can be used allowing very high data rates Maintenance is easier than multipoint links Potential Ring Problems – – Cable vulnerability: can be a single point of failure if the ring isn’t designed for redundancy Repeater failure: again a single point of failure 635. 412 Spring 2005 Class 2: More Basics 24

Topologies & Transmission Media Potential Ring Problems n Perambulation: locating a failure requires logically working around the ring to find the fault; can take a long time with large rings n Installation headaches: installation of a new repeater can present difficulties including locating adjacent nodes and installing cabling n Initialization and recovery: all stations must cooperate smoothly when initialization & recovery procedures are necessary (e. g. – loss of the token) n Size limitations: the ring’s size is constrained by the aggregate delay of the ring and accumulated timing jitter 635. 412 Spring 2005 Class 2: More Basics 25

Topologies & Transmission Media Potential Ring Problems n Timing jitter – – An effect caused by the fact that the stations pass clocking via the data encoding scheme to maintain the synchronization necessary to properly recover data The delay distortion in the transmitted bits causes a deviation in the recovered clocking The recovered clocking is used to retransmit the signal, meaning that clocking errors are passed on (shows up as the gain or loss of bits on the ring) Ways to minimize timing jitter: n n Use a PLL in each repeater to allow more accurate reception of clocking Use a jitter buffer & master clock at to provide clean clocking at each repeater 635. 412 Spring 2005 Class 2: More Basics 26

Example: Ring Network Throughput n n n n A ring network with N segments, each has a link of length L (meters) and a repeater. The average packet size is P (bits) and the data rate is R (bps). The token size is TK (bits). Find the Ring Throughput and Efficiency. The generation time is T 1= P/R (transmisstion) + N/R (N repeaters). Average time for token arrival (assuming network is free) is T 2=(TK +N/2)/R. Overall packet time is T 1+T 2 The throughput is P/(T 1+T 2) (bps) The efficiency is E= P/R(T 1+T 2), where E is between 0 and 1. 635. 412 Spring 2005 Class 2: More Basics 27

Topologies & Transmission Media Star-Ring Topologies n n Most of the benefits derived from the ring topology are related to its logical characteristics, while several of its major drawbacks have to do with the physical topology This realization led to the development of the Star-Ring architecture; a physical star topology overlaid by a logical ring [Figure 4. 8] A central concentrator is located in a central closet and incorporates the ring bypass function This architecture eliminates several problems with ring architectures: – – – Easier fault isolation and installation of new stations. Equalizes the distance between repeaters minimizing signal level reception issues Allows the development of a switched-ring architecture 635. 412 Spring 2005 Class 2: More Basics 28

Topologies & Transmission Media Multiple Rings & Bridges n Bridges will allow multiple rings to be used to overcome the limitations of a single ring or star-ring architecture – – – n Bridges provide connectivity between rings plus: – – – n More stations can be attached to the network The network can span larger distances & provide higher throughput A failure will not disable the whole network Input filtering & buffering Switching (a bridge may connect more than two rings) Output buffering & transmission Challenges to using multiple rings and bridges – – Inter-ring traffic loses automatic acknowledgement feature Performance suffers if the network is not designed to minimize inter-ring traffic 635. 412 Spring 2005 Class 2: More Basics 29

Topologies & Transmission Media Star Topologies n Twisted Pair & Fiber Optic Star-based LANs – – – Twisted pair does not currently enjoy any performance benefits over coaxial cable, but it has a huge installed base and spare pairs can be used for a LAN (minimizing cost) Such twisted pair cabling is typically in a star topology homed out of a central closet; the hub is then located in the closet to create the central station Star-wired fiber optic cable is used to provide higher speeds and longer links n n While physically a star, it is a logical bus Many star network topologies use a mixed configuration of fiber and UTP; fiber is usually used for backbone links 635. 412 Spring 2005 Class 2: More Basics 30

635. 412 Spring 2005 Class 2: More Basics 31

635. 412 Spring 2005 Class 2: More Basics 32

Topologies & Transmission Media Twisted Pair & Fiber Optic Star-based LANs n n Multiple levels of hubs can be employed as long as there are no closed loops and full connectivity; both data frames & a collision presence signal must be propagated throughout the LAN in order to preserve the logical bus Twisted pair LANs commonly operate at a 10 or 100 -Mbps speed though faster networks up to 1 Gbps have been developed requiring refinements to the cabling systems – – Higher performance cable (Cat 5 or better) must be used Installations require more rigorous design and installation standards 635. 412 Spring 2005 Class 2: More Basics 33

Topologies & Transmission Media Multi-level Star Topologies n n It is very common to build hierarchical star topologies as long as one ‘header’ hub acts as the central point of the star Three-level Star Topology 635. 412 Spring 2005 Class 2: More Basics 34

Topologies & Transmission Media Hubs and Switches in Star Topologies n n To boost LAN performance it has become popular to replace hubs in star topologies with switches Instead of repeating an incoming frame out to all other stations a switch only forwards a frame out the line attached to the appropriate destination Scales well & allows ‘in-place’ upgrades Switches all but eliminate collisions and allow each device to have dedicated capacity equal to the original LAN 635. 412 Spring 2005 Class 2: More Basics 35

Topologies & Transmission Media LAN Switch Modes n LAN switches can operate in two modes: store-andforward and cut-through – – Store-and-forward switches buffer the entire frame before transmitting it on the outbound line Cut-through switches read the MAC address in the frame and as soon as it determines the outbound line it begins to transmit the frame Cut-through yields the highest possible throughput (with the lowest latency) but will transmit bad frames while store-and-forward switches will verify the CRC on frames before forwarding The most intelligent switches have an adaptive algorithm for switching between the two modes based on the actual error rate seen on incoming frames 635. 412 Spring 2005 Class 2: More Basics 36

Topologies & Transmission Media LAN Switch Modes n Explanation of half-duplex & full-duplex modes – – – In half-duplex operation a station is connected to the switch in a two-node Ethernet With full-duplex switching each station is effectively connected to the switch with two Ethernet LANs, theoretically allowing total transfer speeds of 20 -Mbps More about LAN Switching & Ethernet in upcoming classes! 635. 412 Spring 2005 Class 2: More Basics 37

Topologies & Transmission Media Structured Cabling Systems n Overview – – To aid in the design and installation of LANs and associated infrastructure, standards have been issued specifying the cabling types and layout for commercial and residential buildings Such standards are referred to as Structured Cabling Systems and have the following characteristics: n n Refers to all data, voice, & video wiring within a building The cabling layout and cable specs are independent of vendor and active end-user equipment The cabling layout is designed to reach all areas of the building minimizing the need for future cable installation Should ‘future-proof’ the building against the need to rewire to accommodate new technologies 635. 412 Spring 2005 Class 2: More Basics 38

Topologies & Transmission Media Structured Cabling Systems Overview n n n The core standards for commercial building structured cabling systems are the EIA/TIA-568 (United States) and the ISO 11801 (International) BICSI is another important SCS standards development organization General layout in these standards are based on a hierarchical star-wired cable layout [Figure 4. 16] – – – Interbuilding cables terminate in a main closet, usually in the basement Backbone cabling connects together the main closet with other telecommunications closets and equipment rooms Horizontal cabling connects end-user equipment to the nearest closet 635. 412 Spring 2005 Class 2: More Basics 39

Topologies & Transmission Media Structured Cabling Systems Overview n n Maximum distances for each type of cabling are specified in the standards Many vendors have implemented proprietary or standards-compliant structured cabling systems: – – – Lucent/Avaya Systimax Ortronics Hubble 635. 412 Spring 2005 Class 2: More Basics 40

Topologies & Transmission Media Structured Cabling Systems Hierarchy ER: equipment room MC: Main cross-connect IC: Intermediate cross-connect X: cross-connect HC: Horizontal cross-connect WA: working area 635. 412 Spring 2005 Class 2: More Basics 41

635. 412 Spring 2005 Class 2: More Basics 42

Topologies & Transmission Media Appendix: Characteristic Impedance n One of the most important parameters associated with communications cable is its characteristic impedance – – n Any transmission line has a distributed level of capacitance & inductance which can be modeled by an equivalent circuit The characteristic impedance is a function of the line construction and is independent of whatever signal is transmitted over the line! Characteristic impedance is most important for power matching – – – If a transmission line is terminated with a load or circuit matching its characteristic impedance no power is reflected back from the junction between the two When matched all power is transferred across the junction Reflections are to be avoided because they will cause interference and errors 635. 412 Spring 2005 Class 2: More Basics 43

KNOCKOUT SWITCH 635. 412 Spring 2005 Class 2: More Basics 44

PILED SWITCH 635. 412 Spring 2005 Class 2: More Basics 45

TENDOM BANYAN SWITCH 635. 412 Spring 2005 Class 2: More Basics 46

TREE SWITCH 635. 412 Spring 2005 Class 2: More Basics 47

DILATED SWITCH 635. 412 Spring 2005 Class 2: More Basics 48

PIPELINED SWITCH 635. 412 Spring 2005 Class 2: More Basics 49