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  1. Access networks lectures 2008/09 - winter term Part 2 : Access networks

  2. - Some terms from AN area – see file … -the older terms and functionalities: subscriber line, local loop (only twisted pair) MDF – main distribution frame CTE-central (or local) teleph.exchange junctions – betw.LE trunk- links among national switches and higher The Access Layer represents the access network that links the customers to the local switch. [1]

  3. - different posibilities of AN, according to customers number - not flexible DP – distribution point – 101 of customers CCP – cross connection point (cabinet) – 102 of customers SCP, PCP – for a lot of customers - flexibility

  4. 2.2 Architecture of AN 2.2.1General architecture Interfaces Switching network Primary netw. Secondary n. End devices netw. Fig. General architecture of AN LT – Line Termination – from the side of exchange DP – Distribution Point NT – Network Termination

  5. Architecture - continue 2.2.2 AN functionalities (functional architecture) • - Transport functions • - Functions of system ports (SNI - Service Network Interface) • Functions of subscribers ports - UNI (User-Network Interface) • Common functions (for all) – with support of TMN; interfaceQ3

  6. a/ c/ e/ d/ 2.2.3 Network Architectures - physical bus (line) b) Ring Tree Star Mesh (each node is connected to all other nodes) Fig. Physical topologyof LAN - interconnecting between switching centers

  7. 2.2.4 Logical topology: • Line topology (bus) • Token Ring – ring access – log. addressesdon’t must match with physical topology • see [2]

  8. 2.3 Methods of the access to shared communication media - communications protocol – its layerMAC (Medium Access Control) - 2 groups of the access methods: STOCHASTIC a DETERMINISTIC Stochastic access methods a – ideal network b – ideal n.with delay α=0.1 c – CDMA-CD d – synchronized ALOHA e – ALOHA (original) A – traffic offer Y – traffic power (normalized) Fig.2.3.1Comparing of access methods L–packet length, C-media inform.capacity, n-phys.media length,v-propagation velocity in the media for curve b): Yb= Ya (1-α)-1 ...packet delay ...cummulative delay

  9. Access methods -continue • notes: • The first version of the original ALOHA “protocol” (now called Pure ALOHA, and the one implemented in ALOHAnet) was quite simple: • If you have data to send, send the data • If the message collides with another transmission, try resending "later"

  10. Access methods -continue something between stochastic and deterministic methods (methods with minimalization of collisions) • CSMA: persistent (Carrier Sense Multiple Access) • inpersistent • p- persistent • CSMA / CD ( ... / collision detection) - IEEE 802.3 – signal „JAM“ • „Tree Walk Protocol“ – adaptive dynamical deviding of terminals according to collision expectation

  11. Access methods -continue Deterministic Access methods • they are preferred in AN • without collisions • -multiplexing methods = systems with accurate disposing of media capacity (TDMA, FDMA, WDMA, SCMA, CDMA) • - polling (e.g. Roll Call Polling) • - standardized token methods: Token Ring (for ring topology - IEEE 802.4), Token Bus (forbus topol.) – term ‘token’ = special data word . . . • -methods with transport capacity reservation (bit-mapprotocols) see fig. 2.3.2 in the following slide • advantages – negligible error rate, ideal (perfect)network behavior • - disadvantage - delay

  12. reservation --------data channels----reservation-------data channels Fig. 2.3.2An example of protocolwith reservation of media capacity – bitmap protocol

  13. Multiplex as methods of access to common transport media in AN • TDMA – the access in the reserved time frame • bit oriented • block oriented (see fig. 2.3.4) Fig. 2.3.3 Principle of TDMA

  14. Block oriented TDMA Fig. 2.3.4 - with fixedallocating of capacity (in the fixed frames) / with dynamicallocating of capacity (ATM) Obr. 2.3.5MAC encapsulation of a packet of data [5]

  15. FDMA . . . - frequency band is devided to segments for separate channels – between channels there are some gaps (guard intervals) - in applications with other access methods

  16. WDMA = Wave length Division Multiple Access - each terminal has its own channel, i.e.its own wavlengthλ – optical MUX or splitter Optical fiber - optical windows(850nm,1300nm,1550nm) Wave length Optical carriers - HDWDMA =... without EO conversions End station Customers with E/O conversions Fig.2.3.6 WDMA PON system

  17. Standards • ITU-TG.983 • APON (ATM Passive Optical Network)-the first Passive optical network standard - primarily for business applications, and was based on ATM. • BPON (Broadband PON) - standard based on APON. It adds support for WDM, dynamic and higher upstream bandwidth allocation, and survivability. It also created a standard management interface, called OMCI, between the OLT and ONU/ONT, enabling mixed-vendor networks. • ITU-TG.984 • GPON (Gigabit PON) - an evolution of the BPON standard - supports higher rates, enhanced security, and choice of Layer 2 protocol (ATM, GEM, Ethernet). In early 2008, Verizon began installing GPON equipment, having installed over 800 thousand lines by mid year. British Telecom, and AT&T are in advanced trials. • IEEE802.3ah • EPON or GEPON (Ethernet PON) is an IEEE/EFM standard for using Ethernet for packet data -is now part of the IEEE 802.3 standard. • IEEE802.3av • 10G-EPON (10 Gigabit Ethernet PON) is an IEEE Task Force for 10Gbit/s backwards compatible with 802.3ah EPON. 10GigEPON will use separate wavelengths for 10G and 1G downstream. 802.3av will continue to use a single wavelength for both 10G and 1G upstream with ATDMA separation. It will also be WDM-PON compatible (depending on the definition of WDM-PON). It is capable of using multiple wavelengths in both directions. • SCTEIPS910 • RFoG (RFoverGlass) is an SCTE Interface Practices Subcomittee standard in development for Point to Multipoint (P2MP) operations that MAY have a wavelength plan compatible with data PON solutions such as EPON,GEPON or 10GigEPON. RFoG offers an FTTH PON like architecture for MSOs without having to select or deploy a PON technology.

  18. SCMA . . . = SubCarrier Multiple Access Fig.2.3.7Principle of 1-channel SCMA for direction multipoint-to-point • 1-channel SCMA : el. subcarriersare modulated by users signals – modulation of λi–composition of λiby means of optical coupler one complex (color) signal (composite signal) • multichannel SCMA –first, user signals are compounded in el. area  broadband FDM signal – this one modulates optical carrierλiλi transports information from lots of channels !

  19. CDMA . . . • = Code Division Multiple Access • Spread Spectrum (SS) Method - application in transport systems with SS • pseudorandom sequence(PNS = PseudoNoise Sequence) (nChips) – similar properties as noise – it containes all components of spectrum and probabilities of „0“ a „1“ areequal – (But: this PNS is deterministic actually – because of its originating – it is created by generator as in fig. 2.3.9. • PNS is multiplyed binary information signal (fig.2.3.8) – as the new low power pseudorandom signale arises as noise šumu (obr. 2.3.10) – one can detect it only if the PNS is known, what is the process on the receiver side actually (correlation method – multiplying of received spectrum by known PNS etc….. more can be presented in the subject of prof.Kocur– Spread Spectrum Communications Systems - SSCS)

  20. 64 kbps P /S A / D DP 1 user data......data bit product of them results in spectrum spreading by means of PNS 13 μs PNG 1 PNS generator, 1 chip PNG 2 uncorrelated generators of several different users ... PNG N Obr. 2.3.8 Generating of spread spectrum signal

  21. 1. 2. 3. N. modulo-2 sum (mod 2) shift register with N memory places Obr.2.3.9Linear PNS generator

  22. Simple model of SSCS Obr. 2.3.10 Spectrum of information signal in baseband and after spreading

  23. Frequency-hopping spread spectrum (FHSS) • direct-sequence spread spectrum (DSSS) • time-hopping spread spectrum (THSS) • chirp spread spectrum (CSS) • and combinations of these techniques are forms of spread spectrum • (more for examaple on http://en.wikipedia.org/wiki/Spread_spectrum)

  24. The basic properties of CDMA: advantages disadvantages

  25. References: [1] http://www.angelfire.com/nt/access1/web2001chap1.htm [2] Chen & Comp.: 3 - Communication Technologies, CRC Press LLC, 2000.