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Access networks

lectures 2008/09 - winter term

Part 2 : Access networks

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- Some terms from AN area – see file …-the older terms and functionalities: subscriber line, local loop (only twisted pair)

MDF – main distribution frameCTE-central (or local) teleph.exchange

junctions – betw.LEtrunk- links among national switches and higher

The Access Layer represents the access network that links the customers to the local switch.

[1]

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- different posibilities of AN, according to customers number

DP – distribution point – 101 of customers

CCP – cross connection point (cabinet) – 102 of customers

SCP, PCP – for a lot of customers

- not flexible

- flexibility

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Fig. General architecture of AN

LT – Line Termination – from the side of exchange

DP – Distribution Point

NT – Network Termination

2.2 Architecture of AN

2.2.1 General architecture

Interfaces

Switching network

Primary netw. Secondary n. End devices netw.

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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; interface Q3

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2.2.3 Network Architectures - physical

a/

c/

e/

d/

Fig. Physical topology of LAN - interconnecting between switching centers

bus (line)

b) Ring

Star

Mesh (each node is connected to all other nodes)

Tree

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2.2.4 Logical topology:

• Line topology (bus)

• Token Ring – ring access – log. addresses don’t must match with physical topology

see [2]

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Fig.2.3.1 Comparing of access methods

A – traffic offer

Y – traffic power (normalized)

2.3 Methods of the access to shared communication media

- communications protocol – its layer MAC (Medium Access Control)

- 2 groups of the access methods: STOCHASTIC a DETERMINISTIC

PR TT /]b.sb,[s, / -1CLTR ...packet delay

]m.sm,[s, / -1vnTP ...cummulative delay

for curve b):

Yb= Ya (1-α)-1

Stochastic access methods

L–packet length, C-media inform.capacity, n-phys.media length,v-propagation velocity in the media

a – ideal network

b – ideal n.with delay α=0.1

c – CDMA-CD

d – synchronized ALOHA

e – ALOHA (original)

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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"

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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

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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 (for bus topol.) – term ‘token’ = special data word . . .

-- methods with transport capacity reservation (bit-map protocols) see fig. 2.3.2 in the following slide

- advantages – negligible error rate, ideal (perfect)network behavior

-- disadvantage - delay

Access methods - continue

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reservation --------data channels----reservation-------data channels

Fig. 2.3.2 An example of protocol with reservation of media capacity – bitmap protocol

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Multiplex as methods of access to common transport media in AN

TDMA – the access in the reserved time frame

Fig. 2.3.3 Principle of TDMA

- bit oriented

- block oriented (see fig. 2.3.4)

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Block oriented TDMA

Obr. 2.3.5 MAC encapsulation of a packet of data [5]

Fig. 2.3.4

- with fixed allocating of capacity (in the fixed frames) / with dynamic allocating of capacity (ATM)

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FDMA . . .

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

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WDMA = Wave length Division Multiple Access

without EO conversions

with E/O conversions

- optical windows (850nm,1300nm,1550nm)

- HDWDMA =...

- each terminal has its own channel, i.e.its own wavlength λ – optical MUX or splitter

Fig.2.3.6 WDMA PON system

Optical fiber

Wave length

Optical carriers

End station

Customers

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StandardsITU-T G.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-T G.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.

IEEE 802.3ah 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. IEEE 802.3av

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. SCTE IPS910

RFoG (RF over Glass) 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.

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SCMA . . .

Fig.2.3.7 Principle of 1-channel SCMA for direction multipoint-to-point

= SubCarrier Multiple Access

-1-channel SCMA : el. subcarriers are modulated by users signals – modulation of λi – composition of λi by 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 carrier λi λi transports information from lots of channels !

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CDMA . . .

= Code Division Multiple Access

- Spread Spectrum (SS) Method - application in transport systems with SS

- pseudorandom sequence (PNS = Pseudo Noise Sequence) (n Chips) – similar properties as noise – it containes all components of spectrum and probabilities of „0“ a „1“ are equal – (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)

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DP A / D P /S

Ss

1310.64

13

PNG 1

PNG 2

PNG N

13 μs

uncorrelated generators of several different users

PNS generator, 1 chip

...

product of them results in spectrum spreading by means of PNS

64 kbps

1 user

Obr. 2.3.8 Generating of spread spectrum signal

data......data bit

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1 2 3( ) ( 1) ( 2) ( 3) ( )Na k C a k C a k C a k C a k N

1

( ) ( )N

ii

a k C a k i

( )a k N( )a k

1.( 1)a k

2.( 2)a k

3.( 3)a k

N.( )a k N

1C

2C3C

NC

modulo-2 sum (mod 2)

0,1iC

shift register with N memory places

Obr.2.3.9 Linear PNS generator

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Narrow band transmitted signal ....

Operator of spreading ....

Spread spectrum signal (wide-band) ....

Operator of “dispreading” of signal spectra .....

Decoded signal: ....

Simple model of SSCS

ns

[.]

[.]

w ns s

[ ]n ws s

Obr. 2.3.10 Spectrum of information signal in baseband and after spreading

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• 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)

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1. Narrow band interference and jamming robustness

2. Low sensitivity to multi-path propagation

3. Some level of security and protection of communications channels.

4. Relatively low level of power spectrum density of transmitted signalls.

Complexity of signal – low chance to detect it by unauthorized person

5. Accurately measurement of distance, unique and exact time providing integrated switching, navigate and control systems

The basic properties of CDMA: advantages

1. Complex decoding circuits, transmitters and receivers circuits.

disadvantages

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References:

[1] http://www.angelfire.com/nt/access1/web2001chap1.htm

[2] Chen & Comp.: 3 - Communication Technologies, CRC Press LLC, 2000.