EDWIN NGARUIYA

TRANSMISSIONS BACK OFFICE REPORT

Date: 22nd April 2011.

-Optical backhaul network

-Microwave backhaul network

OPTICAL BACKHAUL NETWORK

For the optical network, the vendor equipment in use is mostly Alcatel.

There are two modes of transmission over the optical fibre. They are as follows

 PDH – Plesiochronous Digital Hierarchy

 SDH – Synchronous Digital Hierarchy

Plesiochronous Digital Hierarchy

The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunications networks to

transport large quantities of data over digital transport equipment. E1/T1 is based on this
technology. In many places it is now being replaced with SDH(explained later). Plesiochronous,
means “almost synchronous,” and relates to the inputs that can be of slightly varying speeds relative
to each other and the system’s ability to cope with the differences.Signals are called plesiochronous
if they have the same nominal frequency. The frequencies vary but within certain specified limits.
Two signals with two different clocking rhythms are plesiochronous.

The PDH system effectively develops the idea of primary multiplexing using time division
multiplexing (TDM) to generate faster signals. This is done in stages by first combining (multiplexing)
E1 or T1 links into what are known as E2 or T2 links, and if required, going even further by combining
(multiplexing) E2 or T2 links, etc.

These groups of signals can be transmitted as an electrical signal over a coaxial cable, as radio
signals, or optically via fiber-optic systems. As such, PDH formed the backbone of early optical
networks.

The aggregate signal can be sent to line at any stage of the hierarchy, using the appropriate
transmission medium and modulation techniques.

PDH Network Operation — PDH network equipment is now quite physically small, allowing for its
deployment in locations other than a telephone exchange.

PDH systems are generally used only for point-to-point communications systems because the signals
must be fully demultiplexed to access a single information channel. In addition, proprietary alarm
configuration and management means that equipment at either end of a PDH system must be from
the same manufacturer restricting its interaction with other vendor equipment.
PDH advantages include:

i. Equipment small enough for use in street cabinets

ii. Good for point-to-point connections
iii. Cost-effective support for access networks

PDH disadvantages include:

i. Manufacturer-specific systems
ii. Multiplexer mountains
iii. No integrated network management
iv. Limited management available
v. Loss of quality

The diagram below describes the PDH add-drop multiplexing mechanism

- 2 Mbit/s service signals are multiplexed to 140 Mbit/s for transmission over optical fiber or
radio.
- Multiplexing of 2 Mbit/s to 140 Mbit/s requires two intermediate multiplexing stages of 8
Mbit/s and 34 Mbit/s.
- Multiplexing of 2 Mbit/s to 140 Mbit/s requires multiplex equipment known as 2, 3 and 4
DME.

Synchronous Digital Hierarchy

SDH (Synchronous Digital Hierarchy) is an international standard for high speed telecommunication 
over optical/electical networks which can transport digital signals in variable capacities. It is a
synchronous system which intend to provide a more flexible , yet simple network infrastructure.
In SDH, transmission is synchronized through one master clock; its main difference with PDH. This
makes it possible to assemble higher order frames by byte interleaving instead of bit interleaving as
used in PDH.

Preceding SDH was SONET (Synchronous Optical NETwork). This technology is still in use in the USA
and Canada.

The Synchronous Digital Hierarchy (SDH) was developed from the American SONET (Synchronous
Optical Network) and is designed to provide an effective, well-managed, reliable, and efficient
system for use with optical-fiber (high-bandwidth) links. It was developed to be compatible with
existing systems and can therefore carry PDH channels as well as other formats.
Although seen as an expensive option compared to the tried and trusted PDH alternative, the
advantages of SDH are well recognized, and SDH is now the accepted standard for digital
transmission around the world. SDH has manyadvantages over PDH, most notably:

 It is designed to get the best out of high-capacity fiber-optic cables.

 It is compatible with many other accepted standards such as E1 and T1.
 It has built-in network performance monitoring and management facilities.
 It is compatible with both European and American standards.

SDH can multiplex together a variety of different digital signal types, including those that are already
multiplexed using PDH, or even SDH. These signals are arranged by the system onto a standard
frame, called a synchronous transport module (STM), ready for transmission. The smallest of these is
STM-1, which operates at 155 Mbps. There are larger frames, denoted STM-x. The x merely implies
the number of STM-1 equivalents transmitted (systems can employ STM-4, STM-16, STM-64, or even
higher). The inputs are known as tributaries.

STM-1 is equivalent to 63 × E1 links, or 1890 telephone channels. The common implementation

throughout Europe is a 155.52-Mbps link (carrying many multiplexed channels) in STM-1
(synchronous transfer module) format, which can itself be multiplexed into higher capacity levels
(mainly STM-4, STM-16, STM-64). These signals are typically transmitted over optical fiber, although
it is possible to send STM-1 over modest distances using coaxial cable or radio.

Every voice or data channel is identifiable in the STM-x and allows selective demultiplexing. This has
the advantage of eliminating the multiplexer mountains of PDH and allows new network structures
beyond simple point-to-point connections. This also allows some or all of the channels to be
effectively protected in case of a network failure. The ability to automatically protect traffic is an
inherent feature of SDH.

SDH has inherent management capabilities built into its structure. It is possible to control and
configure an entire network remotely. This has given rise to large NOCs (network operation centers)
where an operator can monitor, identify, and react to any fault in a network within minutes.

Protection and management systems work best where the fiber optic (or other medium on which
SDH is running) is organized in ring structures to provide alternative reconfigurable routes, and
therefore more reliable connections for the user.
The main difference between SDH and SONET is in the size of its basic frame. SDH has a basic frame
size of 155.52 Mbps whereas for SONET, 51.84 Mbps. Also, SDH can be used over an electrical
interface (coaxial cable) whereas SONET is restricted to the optical fibre.

The bitrate of an SDH channel is given by

Bitrate=n x 155.52 Mbps

Where n is the order of the STM i.e STM-n.

The block diagram below represents the ETSI multiplexing scheme used for SDH

This form of multiplexing paves way for the KLM(3.7.3) numbering scheme used in SDH frames
where

K= The Number channel of VC4

L= The Number channel of TUG-3
M=The Number channel of TUG-2
Softwares

The Alcatel NMS has a lifecycle described below

i. Define
ii. Allocate
iii. Implement
iv. Commission – This stepis mandatory for Ethernet paths but not with SDH paths

To create a path the following steps are taken

1. Create physical connection – this involves coming up with virtual stations

2. Name the connection – Give the connection a unique name
3. Select the port
4. Name the port
5. Configure, implement the refresh the page
To create the SDH channel
1) Choose end points
2) Create path, choose type and port
3) Allocate
4) Implement
5) Commision
MICROWAVE BACKHAUL

There are two types of microwave systems

 Point-to-point
 Point-to-multipoint

For the microwave network, the vendor equipment in use is Ericsson. The equipment mostly used
for this is the Mini-link.

The diagram above shows the Mini-link equipment. It consists of

 Radio Access Unit – connected to the microwave antenna. It is an outdoor unit.

 IF cable – Connects the magazine to the RAU.
 Access Module Magazine – It is the core of the mini-link system. It is an indoor unit. From
here the equipment is linked to other transmission equipment in the network.

AMMs come in three types differentiated by the number of slots it has

 AMM 2P
– Contains edge and repeater nodes
– 2 full-width/ 2 half-width plug-in units
 – Maximum of 2 x 1+0 radio terminals
 AMM 6P
– Medium sized aggregation nodes
– 6 full-width plug-in units
– Maximum of 5 x 1+0 radio terminals
– Has redundant DC power
 AMM 20P
– Large size aggregation nodes
– 20 full height plug-in units
– Maximum of 18 x 1+0 radio terminals
– Has redundant DC power

The AMM contains numerous slots in which cards with specific functions are put in. The cards and
their functions are described below:

1. Power Filter unit

Responsible for power distribution in the magazine. The PFU performs filter functions to protect
from external noise and power surges. It also offers protection from short circuits and under-
voltage occurrences.

2. Fan Unit

This is a mandatory equipment in any magazine. It is responsible for keeping the temperature in
the magazine at the recommended value and thus prevents any instances of overheating of the
cards.

3. Line Termination Unit

LTUs can contain an electrical interface(for coaxial cables) or an optical interface (for fiber) and
an electrical interface as well.
They differ according to the type of transmission multiplexing the traffic has i.e PDH or SDH.
PDH
This is done via soffix connectors. Each connector is 4 x E1.
SDH
STM-1 termination is done at the front.
The LTU 155e has an electrical interface only and carries 63 E1s.
The LTU 155e/o has both electrical and optical interfaces.

4. Node processor Unit

It is the ‘brain’ of the magazine. It holds the configurations making it a mandatory plug-in unit in
the AMM. It provides the interface for the management and local interfaces. The NPU provisions
for the DCN IP router and SNMP agent.

An NPU has an E1 line termination interface. Some also have an Ethernet termination unit on it.

5. Modem Unit (MMU)

 They provide the interface between the Radio Access Units and the AMM. The radio cable is
terminated onto an MMU. Only one radio per MMU is required.

Like the LTUs they differ based on the multiplexing on the traffic, PDH and SDH.

A new feature on the MMU is the XPIC, stands for Cross Polarization Interchange Cancelling. It
refers to transmitting waves on the same frequency but on two planes, horizontal and vertical.

6. Ethernet Termination Unit

It is a board with an Ethernet output.

7. ATM aggregation Unit

There are of three types

VBR (Class B) – Allows users to specify a throughput capacity and a sustained rate but data is not
sent evenly. This is used for transmitting packetized voice and video data.

CBR(Class A) – User determines a fixed bandwidth at the time the connection is set upso that
data can be sent in a steady stream. It is used for transmitting fixed rate uncompressed stuff.

UBR(Class C) – Does not guarantee any throughput and uses only available bandwidth. It is used
for transmitting data that can tolerate delays.

The diagram above is the AMM is seen from the mini-link craft remote login. To be noted is the
absence on the NPU. This is because the craft we are using is of a lower version than the NPU
installed.

Replacing a card

When a card is faulty, the red LED goes on. When this happens, the card should be replaced.
To replace a card, one should press the BRS for controlled removal. Once pressed, one should wait
until the yellow LED comes on. Once it comes on, the card can be removed. If it is to be replaced, the
new card can be slotted right in.

Once a new card is installed, the following is configured:

– Transmission interface parameters

– Traffic routing
– Traffic protection
– DCN configuration
– User I/O configuration
– Security parameters

Softwares
The AMMs can be configured remotely by using the Service-on Element Manager. Below is a
screenshot of it.