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Next Generation Networks > Fixed Access Networks > Flashcards

Fixed Access Networks Flashcards

1
Q

Explain the difference between access, metro and core network.

A

Access; link between end-users and wider telco network

Metro; Connects multiple access networks within a city to the core network

Core; forms the backbone of the entire telco infastructure, providing high-speed, long-distance data transport between different metro areas.

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2
Q

Talk about access network speed

A

Since metro are built on progressive aggregation of traffic, the capacity of access connections are typically lower than those in the metro and core.

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3
Q

Why are access networks such a big issue to upgrade:

A. The technology from the core
does not work in the access
B. It is harder to lay fibre in the
access than in the core
C. Each access link only feeds one
user so the cost per user is high
compared to the core, which
instead aggregates many users
D. None of the above, there is no
issue with access network and
we all have 10Gb/s at home
(including rural areas)

A

C

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4
Q

Explain issues with access networks

A

The issue is cost per connection upgrade, which is on a per-user basis

Explains why many connections, especially in the rural areas are based on copper.

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5
Q

How can we reduce the distance of
the copper link?
A. We move the houses
closer to the central
office
B. We invest in new
technology hoping it
will deliver higher
capacity over the same
copper distance
C. We shorten the copper
length by start laying
fibre in the side close
to the central office.

A

C

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6
Q

Explain fibre-to-the-cabinet

A

Replace the first part of the copper (that’s shared by most of the users) with fibre

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7
Q

Where does the significant portion of the noise in the line is due?

A

Due to the cross-talk between adjacent copper pairs.

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8
Q

What is G.fast?

A

Increases the rate by using much larger bandwidth.

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9
Q

What is FTTDP

A

Fibre to the distribution point

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10
Q

FTTH

A

Fibre to the home

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11
Q

What’s the problem with point-to-point fibre?

A
  1. Requires one individual fibre per user.
  2. At the network end it requires one termination port per user.
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12
Q

What are passive optical networks?

A
  1. Share the optical fibre into a tree structure using passive optical splitters.
  2. Allows one network termination or Optical Line Terminal (OLT) to serve many Optical Network Units (ONU’s) at the user side.
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13
Q

When talking about time division multiplexing PON (TDM-PON), what is the name of the protocol?

A

Time-Divison Multiplexing: a method of transmitting and receiving independent signals over a common signal path by dividing the signal into time splots.

TDMA is also used, and is a specific application of TDM -> share different time slots in the same frequency band. == Share frequency band without overlapping transmissions.

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14
Q

Explain the difference between upstream and downstream channel

A

Upstream: OLT -> ONU

  1. Implement a MAC to avoid collision of signals from different ONU’s.
  2. Access to the upstream channel is TDMA

downstream: user-end -> source

  1. OLT -> ONU
  2. ONUs filter out the packets destined for them
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15
Q

What does the OLT receiver operate in?

A

It operated in ‘burst-mode’ -> receives short burst of data. This is for downstream

A burst-mode receiver is much more complex and expensive than a continuous-mode one.

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16
Q

Explain how the downstream frame work

A
  1. Each frame is 125 microseconds long
  2. Frame is passed through a function that transforms it in a different sequence of bits.
  • Used to avoid too many 1s or 0s in sequence
  1. Continuous flow
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17
Q

Explain the upstream burst structure

A
  1. Arranged in bursts rather than continuous frames.
  2. Sequence of bursts delimited within a frame of 125 microseconds, to symmetry the downstream frame.
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18
Q

Explain dynamic bandwidth assignment

A

DBA is the process by which the OLT decides how to assign upstream transmission opportunities (e.g. BW) to the ONUs

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19
Q

Explain how dynamic bandwidth assignment works

A
  1. Status reporting: the ONU informs the OLT on how much BW it needs
  2. traffic monitoring: OLT decides based on the observed traffic pattern
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20
Q

How bandwidth assigned in dynamic bandwidth allocation

A
  1. A small fixed bandwidth is typically assigned to each ONU
  2. A variable bandwidth is assigned based on demand -> guaranteed bandwidth and additional (if there is enough spare bandwidth)
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21
Q

Explain how status reporting DBA works?

A
  1. ONUs send their DBRu (current queue size)
  2. DBA calculates how much capacity each ONU should get
  3. Sends the BW map which informs all ONUs when to transmit and how many bytes for the next upstream frame.
22
Q

Give a brief explanation on ranging operations

A
  1. ONUs have all different distance from the OLT
  2. Their different propagation time needs to be considered.
  3. Otherwise two burst might overlap at the OLT input.
23
Q

Explain the ranging phase

A
24
Q

What does GPON and EPON stand for, and explain what they are.

A
  1. Gigabit Passive Optical Network:
  2. Ethernet Passive Optical Network
25
Q

Explain what the most economically convenient duplex scheme is and what do we need in terms of wavelength in order to achieve this?

A

The wavelength division duplex is the most economically convenient wavelength division duplex over single fibre, although two-fibre systems are allowed by the standard.

Different wavelengths are required to avoid reflections from the transmitted signal adds to the received signal.

26
Q

What dispersion has a single-mode fibre?

A

1.3 micrometres

27
Q

What is the split ratio?

A

The split ratio indicates the total number of ONUs that can be served by one OLT

28
Q

Give the advantages and disadvantages of higher splitting ratios

A

Pos: Allow better cost sharing

Disadv: Lower capacity per user

Increases the optical loss: 3*log2(N)

Each doubling of the split increases the loss by 3dB

29
Q

What are the security issues with GPON?

A

Messages are physically broadcast by the ONUs and filtered by the ONUs.

  • A user could tamper with an ONU to eavesdrop data destined to other ONUs
29
Q
A
30
Q

What is WDM-PON?

A

Each user is served by a separate wavelength channel and a WDM splitter is placed in the cabinet to separate the wavelengths into different fibres.

31
Q

Draw the difference between WDM-PON and TWDM-PON?

A

ipad

32
Q

In TWDM-PON, the ONUs have tunable lasers and tunable filters. What advantage does this have?

A
  1. If one OLT is congested, ONUs can be moved to a different OLT
  2. Or one ONU can be linked to a dedicated OLT for the time required to complete a service
33
Q

Passive splitters and tunable end points is the most flexible solution - offer different capacity, on demand, to different types of users.

What’s the issue with wavelength tuning?

A
  1. Wavelength tuning is expensive -> tunable lasers require individual characterisation.
34
Q

What is the current service offering the highest speed to most users?

A

Virgin media -> copper service over coaxial cable, higher bandwidth with less attenuation and interference

35
Q

Explain DOCSIS

A
  1. The standard for cable broadband
  2. Copper technology, but utilises a coaxial cable instead of a twisted copper pair.
36
Q

Name the socio-economic issues with access broadband

A
  1. Broadband infastructure is essential for our information-based economy
  2. Study have shown that doubling broadband speed brings 0.3% GDP growth
37
Q

What is the problem with rural broadband?

A

The cost per user is too high and might not be covered ina reasonable time

38
Q

What is the local loop?

A

The local loop is the transmission medium from the central Office to the end user, hence the name

39
Q

What is the local loop unbundling?

A

Gives new operators (Other Licensed Operators - OLO) access to the network of the ex national operator (incumbent operator)

40
Q

What type of connection is the local loop?

A

A physical connection as the OLO physically connected the end user to its own equipment in the central office.

41
Q

Explain the why Sub Loop Unbundling is needed and draw the structure

A

Technology like VDSL2 that use FTTCabinet need to share copper form the cabinet to the end user.

ipad

42
Q

Explain bitstream access

A

Bitstream access involves the creation of virtual circuits so that an OLO can offer broadband to an end user through a virtual circuit.

  1. VC is created through VLANs
  2. The OLO does not need to physically terminate the user at the MDF
43
Q

What is the advantage of bitstream access?

A

The new operators do not need to provide the physical infastructure to terminate the copper lines

44
Q

What is the problem with Network Sharing?

A. Neither LLU nor bitstream
can deliver high capacity
B. LLU is expensive and
bitstream doesn’t allow
operators to fully control
their service
C. Bitstream cannot work with
vectoring because
operators’ traffic flows are
divided
D. Regulators are against
access network sharing

A
45
Q

What is the problem with bitstream?

A
  1. All OLOs can only offer to the end user the same type of service.
  2. An OLO cannot differentiate their product from other OLOs or the incumbent.
  3. Hard for competition
46
Q

Explain how Virtual Unbundling Line Access allows OLOs to differentiate their product

A
  1. Able to decide QoS parameters
  2. Being in control of the Customer Premises Equipment
47
Q

Explain Next Generation Access (NGA) Bitstream?

A

It provides a way for OLOs to avoid the need to interconnect at every Local Exchange, while offering better service than legacy bitstream

48
Q

How do you tackle PON unbundling?

A
  1. PtP is easy as it allows for LLU
  2. PON depends on technology:

i. WDM-PON -> OLOs could access different wavelengths

ii. TDM-PON, signal from multiple ONUs goes to the same OLT, cannot physically split, but needs to be accessed after the OLT.

iii. TWDM-PON offers the best options, on an infrastructure with power splitters

49
Q

Draw the difference between wavelength-routed and power-split PON

A

Next Generation Networks (25 decks)

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