Ten weeks of training at Sri Lanka Telecom
My sincere gratitude, to Mr. S.R.P Palliyaguru, senior assistance engineer, network — 1 (southern province) for giving me a complete understanding of all the theoretical concepts behind the works I engaged in the training period.
The training session was started on the 25th of November 2019 at Sri Lanka Telecom PLC, Tangalle Road, Matara and ended on the 31st of January 2020.
- Introduction to the Organization
Sri Lanka Telecom PLC(SLT) is one of the leading telecommunication service providers in Sri Lanka. The company provides telephony, internet access and IT services for both domestic and business purposes. 44.98% of shares in SLT owned by Global Telecommunications Holdings N.V. of Netherlands, 50.50% owned by the Sri Lanka government and balance shares remain among the general public. The vision of SLT is “connecting all Sri Lankans seamlessly with world-class information, communication and entertainment services.” And the mission is to be the “trusted and proven partner for innovative and exciting communication experiences delivered with passion, quality and commitment”.
2. Work done in 1st and 2nd weeks
In this period, I worked with the overhead team. Repairing faulty overhead wires, replacing faulty wires with new wires, and repairing some loose connections in MSANs were the main duties of the overhead team. In repairing faulty overhead wires finding the location of the fault in the wire was the most difficult task. Usually, faults occurred at the joints of long cables. Experienced technicians know which joint is more likely to have a fault. But it does not always work. Therefore, there is a piece of equipment called TDR, to find the location of the fault. TDR sends a low voltage pulse into the cable and at any impedance change, the pulse is reflected. Detecting the reflected pulse and measuring the duration, the distance to the fault can be measured.
Repairing faults in DPs is also a duty of the overhead team. DP is the nearest distribution point to a SLT user. Usually, a DP can offer 10 connections. DP is a passive device that works as a distribution board.
Usually, DPs are waterproof boxes. If water or moisture enters a DP, the connections would be corroded. Most of the faults in DP occurred due to this corrosion. As shown in figure 2 MSANs are the nearest active transmission device to a user. There are 2 main types of MSANs. The first one is the indoor second one is outdoor. Also, there are two subtypes of outdoor MSANs called pole mounted and ground-mounted. MSAN is a very complex device. But they are layer 2 devices. A MSAN has 3 main parts.
· Complex hardware parts (layer 2) called cards
· A cooling system
· A MDF
The most important part of a MSAN is its cards (shown in figure 3). There are many types of cards. Power cards (PRWGS), FTTH cards (GTGH), Alarm cards (CICG), Uplink cards (HUTG), Control cards (SCXN), Combo Cards (ACWCK), Data cards (SNWG) are some of these cards. They control all the functions of a MSAN. MSANs have an in-band maintenance system (both traffic and maintenance flow through a single line). Using this in-band maintenance system, MSANs can be remotely controlled by the NMS and NMS clients.
All the MSANs use in SLT are manufactured by ZTE and Huawei. Sometimes there are some errors in MSANs (not in hardware but software). In that kind of situation, SLT gets the assistance of MSAN manufacture (either ZTE or Huawei). If SLT gives them a packet capture of the fault they give a software patch, that can be installed by NMS, to resolve the problem. MSANs are in different VLANs. So two MSANs cannot have the same VLAN. When these hardware parts are working, they go to high temperatures. To control the temperature there is a cooling system with MSAN. In outdoor MSANs the cooling system is attached with it. But indoor MSANs are installed in air-conditioned rooms. MSAN controls each user connected to it. The place where users are connected to a MSAN is called MDF. As shown in figure 4 the connections of users are connected through MDF.
3. Work done in 3rd and 4th weeks
In the third week of the training period, I was assigned to the FTTH team. I worked with them for two weeks. Repairing broken fiber cables was the main task of the FTTH team. Except that they resolve some problems in home networks that have fiber connections. Single fiber connection can provide 3 IPTV (Peo TV) connections, two internet connections with one telephone connection. Fiber cables have more bandwidth than ordinary copper cables. And much faster than ordinary copper cables. They are flexible, transparent plastic cables thicker than a human hair.
As shown in figure 5, first a digital signal converted into a light signal. Then the light signals transmitted through fiber cables by total internal refraction. Finally, at the destination, the light signal is converted back to a digital signal. The signal is attenuated while transmitting in a cable. There are special wavelengths at which the attenuation is low. As figure 6 shows attenuation is low at 850nm, 1350nm and 1550nm wavelengths. So these wavelengths are used in fiber optics telecommunication.
Since fiber cables are thin plastic cables, it is difficult to connect broken cables. There are special technics and special equipment to do that. First, the location of the fault should be identified. OTDR (figure 7) is used to identify that location. OTDR sends high power pulses through fiber cables and using reflected information the OTDR approximates the location of a fault.
Then the broken cable is spliced. The splicing happens only on plastic optical parts of the fiber cables. But it is in a rubber shield. So this rubber shield should be carefully removed before splicing. There is an opaque coat around the plastic optical part of a fiber cable. Also, this coat should be removed before splicing. To remove this coat optical fiber stripper can be used. It is a bit difficult to remove this coat using a mechanical stripper. But there are electrical strippers which are much easy to use. After the coat is removed, that part should be cleaned (surgical spirit was used). Then the ends fiber part should be cut using a cleaver. After all, broken fiber cables can be spliced using a fusion splicer. The splicer shows the loss of spicing. The loss should be less than 0.2dB. Finally, the spliced point is covered by a thick plastic tube.
FTTH team repair only homes fiber connections which have single optical fiber cable. Although, work involved with fiber in SLT is not only with FTTH. The core network of SLT totally made of fiber and there are very complex operations in this network. Usually, they are underground fiber cables. Instead of having a single optical fiber, there are a bunch of fibers. These types of fibers are called ribbon fibers. Since they are underground cables, they covered with a vax shield.
As shown in figure 9, fiber DP is the closest distribution point to a user. Like ordinary coper DPs, fiber DPs are passive devices. After the DP cable is connected to ONT. ONT can work as a router. A STB can be connected via RJ45 cable to ONT to use Peo TV and a PC can be connected to access the internet. Also, a telephone can be connected to the ONT. After the ONT the network is similar to an ordinary home network. For a better connection, the signal power of the connection at the fiber DP should be greater than -27dB.
In this period, I got a basic understanding of fiber technology, how fiber home networks install, maintain and resolve usual problems. And what are the theoretical concepts behind some of these works.
4. Work done in 5th and 6th weeks
In the fifth week of the training period, I was assigned to the ADSL team. Resolving faults in-home network was the main duty of the ADSL team. A simple diagram of a home network is shown in figure 10
Where, before the splitter, there is surge protection for the home network. So any high voltage coming through copper cables would be grounded without harm to the home network. Splitter is a low pass filter.
As shown in figure 12, up to 4Hz is dedicated to voice. 4kHz — 30kHz bandwidth is narrowband and dedicated for downloading. In ADSL very small bandwidth, greater than 30kHz is dedicated for uploading. Since the ADSL line carry all the above bandwidths (all the signals), there should be a mechanism to separate those signals. What the ADSL splitter does is separate the voice part. And the filtered-out signal can be directly fed into a telephone. If a customer uses Peo TV or the internet, a cable drawn from the splitter is connected to an ADSL router (there are only two ports in a splitter. One for telephone and another one for a router). ADSL router has four Ethernet ports and also it has a Wi-Fi facility. Among four Ethernet ports, one is dedicated to Peo TV and the other three are dedicated to the internet. Usually first port from the left is the Peo TV port. But it can be changed to any port. Devices that have Ethernet ports can be connected to those three ports and access the internet. Devices that have a Wi-Fi facility can be connected via Wi-Fi. Since the DHCP pool in the router has a limited number of IPs, only a limited number of devices can be connected to an ADSL router. Even though, the router issue IPs in the 192.168.1.0/24 range it cannot take 254 devices. Because IPs in the DHCP pool are limited to about 100 IPs. Even if the router can connect 100 devices to the internet, in real life it cannot give the same performance to 100 devices simultaneously. The range of IPs in the DHCP pool and available ports are shown in figure 12. The figure is a screen snip of a “router interface” section.
As figure 12 shows, port number 1,2 and 3 are internet ports and port number 4 is Peo TV. And IPs in the DHCP pool are starting from 192.168.1.100. As DHCP table shows, router has issued 3 IPs 192.168.1.100 (iPhone), 192.168.1.102 (android), 192.168.1.103 (desktop). If the IP of the desktop computer is checked using the ‘ipconfig’ command on the command prompt (terminal in Linux) it would give the result that shown in figure 13. The result says the desktop has the IP 192.168.1.103.
If a customer uses Peo TV, an Ethernet cable should be connected to port number 4. Then the other end of the cable is connected to STB. After that, a TV (or a display) can be connected to STB via an HDMI cable or video cable. The MAC address of the STB is entered into the Peo TV customer account. If the STB is replaced by a new one, the MAC address of the new address should be entered into the customer account. Otherwise, the customer has no authentication to use Peo TV.
As mentioned at the beginning of the article the main duty of the ADSL team is to resolve the problems in home networks. Most of the faults recorded were due to faults in routers. By resetting the router most of the faults were solved. To reset the router there is a small button in the router. By pressing and holding this button for about 20 seconds the router can be reset. When a router is reset, it loses its configurations. So the router should be reconfigured before using it. Each router has an account in the BRAS server. What the router loses in resetting is the information need to access that account. In reconfiguration, this information should be re-entered. To do that first we should log into the router. The logging page of any router can be accessed by searching the IP of the router (gateway IP) in the address bar of any web browser.
Usually, the Gateway IP is 192.168.1.1 but there may be other private IP ranges. Figure 10 can be seen after logged into the router. But to log into a router the username and password should be provided. These usernames and passwords are not the ones in the BRAS server. These accounts are made to protect router configurations from unobvious changes. These are identical to a particular router type. Table 2.1 shows the few router types currently used by SLT and their logging information.
After logged into the router, the user name and password can be entered. The router cannot make any account or destroy any account in the BRAS server. It can only use already made accounts. What usually happens, after resetting a router is, also resetting the BRAS account and re-enter the username and passwords to the BRAS account. So technicians have to request to network operations center to reset the account and enter logging information. There is a systematic way to assign usernames and passwords. But it is possible to use any username and password. All that need to fulfill is the router should have the same username and password that is in the BRAS server. The BRAS account information in a TP-Link router is shown in figure 15.
By resetting the router most of the problems in the home network can be resolved. If the fault is not solved by resetting the router, the network profile of the connection is changed by the network operation center. If both methods did not work, especially for a fault in the internet connection, the router has to be replaced by a new router. If an STB has replaced with a new one, the MAC address of the new STB should be entered into the system. Other faults in home networks are very obvious for example corrosion of cables. They can simply resolve by changing the cable with a new cable.
In these two weeks, I understood the home network and the operation of each device. But more theoretical details provided in chapter 4 I got to know during the last two weeks when I was in the network and transmission section.
5. Work done in 7th and 8th weeks
In the seventh week of the training period, I was assigned to the network operation center of the SLT, Matara. The NOC uses a network management system to control the devices attached to the SLT network. NMS depends on MSAN manufacture. MSAN manufacture provides NMS with MSANs. Since SLT uses Huawei MSANs and ZTE MSANs there are two NMS in SLT. Since NMS can remotely control devices attached to the SLT network it is very important to protect the NMS from unauthorized parties. So the PCs that run NMS automatically (by the network) change their username and password after some time (about once per two weeks). NMS used in SLT, Matara (or any other district except Colombo head office) is not actual NMS. They are NMS clients. NMS called soft switch, is installed in Colombo head office. NMS can control all the devices attached to the SLT network. But NMS clients have the authority to do a fewer number of functionalities. Among those limited number of functionalities NMS clients in different sections permitted to do few specific functionalities relevant to these sections. As an example, the NMS client used in NOC cannot install software patches in particular MSAN. It is only permitted to do the NMS used in switching transmission units. So the NMS in NOC can only perform the functionalities that are relevant to NOC. Those are mostly rest router password and usernames, entering new account details, helping technicians who are working in the field by checking the line conditions and changing the line profiles. Therefore, the duty of NOC is fulfilling the above tasks. As mentioned at the beginning of this chapter there are two NMS, ZTE and Huawei, in SLT. Therefore, there are two types of NMS clients in NOC, one for ZTE and the other for Huawei. If ZTE one is considered, to enter into a user account first the username of the account should be entered. As an example, if the customer phone number is 041 22d1d2d3d4d5 then the username is xx22d1d2d3d4d5. Where ‘xx’ represents the closet town to the customer. For example, ‘mh’ for Matara, ‘wj’ for Waligama, ‘kkn’ for Kakanadura. That is a shortcut method to access an account. The actual method to find the account is finding in MSAN cards. There is a visual representation of MSAN cards in NMS. That is a bit difficult and consumes considerable time to do if the worker is not experienced enough in this task. After entering into the account, the line profile, username password (in BRAS) of routers can be changed. Usual password is xx d3d4d5. It is not permitted to publish the SLT NMS.
In this period, I get a basic understanding of how the network management system of SLT works.
6. Work done in Last two weeks
In the last two weeks, I was assigned to the switching and transmission section of SLT. During the last 8 weeks, I was working with different teams in OPMC. But the switching and transmission section is totally different section. Like in the OPMC section, there is an engineer to maintain the work carry out by the section. The switching and transmission section takes care of the entire network of SLT. Also, they are responsible for the power distribution and maintenance of SLT. Even more, they maintain the connection from the CEA node to MSAN, OLT, and signal towers. Almost all the connections of this network are optical fiber. A single fault in this network may cause to loss the connection of a part of the district.
In the first two days, I studied basic concepts of networking, like layered architecture, TCP IP, CSMA/CD, Ethernet, DHCP servers and basic command important for networking in command prompt. And I was able to check DHCP functions in the ADSL router.
DHCP is a process of giving IP addresses to TCP/IP networks. And it is a function at the application layer of the TCP/IP task. When a device is connected to an ADSL router, it assigns an IP address to the device. The DHCP pool in the ADSL router supplies this IP. DHCP section in a router shows every detail about the DHCP server. Figure 16 shows the DHCP section in the TP-Link router.
As the DHCP table shows the DHCP pool has assigned three IP addresses to three devices. And the pool has 101 IPs starting from 192.168.1.100. This range of IP addresses is called the scope of the DHCP server. These IP addresses will be changed after some time and a new address will be assigned. The time that a given IP address remains unchanged is called lease time. It is shown in figure 16.
During the time in the switching transmission unit, I was able to get a basic understanding of how the entire SLT network is distributed in the country and how this network provides services to customers. For example, how to access the internet using this network, how Peo TV service is provided, how VPN services are provided to companies and voice transmission on this network. Currently, SLT use packet switching in their network. This network totally based on routers. But an old network of SLT used circuit switching and it was not a router-based network.
As figure 17 shows the core network is built with nine provider routers (called P-router) that are connected with each other as a ring. But not like in a typical ring network, the traffic can flow in either direction of the ring. So a fault at a point in the core network does not break down the network. The ring can carry traffic 100GBs-1 speed. And it is totally made of optical fibers. There are two P-routers in Colombo head office. Others are installed in Kaluthara, Kandy, Chilaw, Wattala, Maradana, and Havelock Town. P-router cannot provide services to customers, Provider edge routers (PE-router) are required to provide services to customers. If the P-router in Kaluthara is considered, as shown in figure Galle PE-router is connected to Kaluthara P-router. And Matara, Hambanthota, Rathnapura and Awissavella PE-routers are connected with each other. Awissavella PE-router is connected with P-router at Colombo head office. This network is called a transport network and carries traffic at 10GBs-1 speed. The PE-router can provide services to customers. But there is less number of ports, in PE-router. PE-routers are very expensive. So it is not practical to provide services directly via PE-router.
Therefore, to increase the number of ports a CEA switch is connected to PE-router. If the Matara PE-router is considered, all the CEA switches are connected with each other as shown in figure 17. MSANs, Signal towers and OLTs are connected to the CEA switch. After the MSANs it is a typical home access network.
Most of the P-routers used in SLT are juniper M320 and PE-routers are Cisco 12406. Figure 18 shows the PE-router in Matara SLT. The yellow color cables connected with the router are fiber cables called patch codes. There are two types of patch codes, Yellow color patch codes and orange color patch codes. Yellow color patch codes are single-mode fiber and orange ones are multimode fiber.
SLT provides all the services using this network. Different kinds of devices have attached to provide those services. If internet service is considered, it required the BRAS server to access the internet using this network. BRAS server provides global IP addresses to access the internet. It is installed in Colombo head office. Usually, PPP is used by devices to access the internet. Normally PPP client is in the router. If needed PPP client can be made on the computer. Where the BRAS server works as the PPP server. If the PPP client is on the computer, the process can be represented as in figure 19.
First, the PC broadcasts a PPP active discovery initiations (PAID). If there is a PPP server, the server unicasts a PPP active discovery offer (PADO) with a global IP address to the PPP client. Then PPP client unicasts a PPP active discovery request (PADR) with the username and password of the client to the AAA server (triple-A server). Triple-A server check,
· Authentication: — check whether request form a correct domain.
· Authorization: — check whether username and passwords are correct.
· Accounting: — check bill payments.
If the PADR is from the @sltbb domain, it is considered an authenticated request. Then triple-A server checks whether there is an account for the details in the PADR request. If account details are valid, then the triple-A server checks whether bill payments are clear. If all the conditions are satisfied triple-A server unicasts a PPP active discovery session confirmation (PADS). After all computers (PPP clients) can access the internet using the global IP address given by the BRAS server. There is a global IP pool in the BRAS server. Since global IPs are not free, there is a limited number of IPs in the IP pool. Therefore, only a limited number of users can access to the internet at a time. So increase the number of users BRAS server uses the network address translation (NAT) function. NAT distributes one global IP address into few local IP addresses.
One of the main services of SLT is ‘voice calls’. Currently, SLT uses their router-based network shown in figure 20 for voice transmission. As an example, voice transmission between customer 041 22 20000 (sender) and customer 026 22 40000 (receiver) is considered. When the sender lifts the phone receiver, the telephone sends a message to the soft switch informing that the customer 041 22 20000 has lifted the receiver. Then the number is dialed, the telephone sends a message again to the soft switch with the dialed number. After that, the soft switch sends a message to the relevant MSAN that the dialed number belongs to. Then the MSAN rings the telephone (026 22 40000). When the receiver is lifted the soft switch is no longer used. Then the telephones are connected via the shortest path available. If the call is for a non-SLT customer or IDD, the soft switch hand over the task to the Other User switch or IDD switch respectively. Where the main controller is a soft switch. The soft switch should know the MSAN IP and TID of a user. Otherwise, it is not possible to take a phone call using that router-based network. These details are manually entered into the soft switch while giving a new connection to a customer.
As figure 21 shows TID consist of four blocks. These are four numbers. Using these numbers, the location where the particular customer is connected in the MSAN can be found.
SLT provides IPTV service to their customers with a TSTV facility. Peo TV is the trade name used by SLT for IPTV. Multicast server (live TV server) provides live TV facility and unicast server (TSTV server) provides time-shifted TV. When a customer uses lives the multicast server sends 3M stream to the MSAN and MSAN distributes it to the customer. So the bandwidth required for live TV is relatively low. Because no matter how many customers use live TV, they no need to enter the core network or transportation network, not even into the aggregation network. So the traffic flow on this network is low hence required less bandwidth. When customers use TSTV, the unicast server has to handle each and every user and send streams. Then every user has to access to core network therefore, it requires more bandwidth. Then there is high traffic flow in the network and it is not practical to use. So, to reduce the traffic in the core network, every SLT main exchange has an image of the TSTV server. Then users do not want to access into core network for TSTV.
A company that has many branches are usually interconnected their branches so that they can communicate very effectively with each branch. Connecting several branches together in different locations is not easy. It consumes a lot of money to make that kind of network. If a company has such a network, it should be carefully maintained. Therefore, making such a network for a company is not practical. So SLT provides VPN services for companies to interconnect their branches together. These connections are called lease lines. Leases are wired using blue and white twisted pair cables in MDF. As shown in figure 24, VPN service is provided by the SLT router base network.
Currently, SLT provided all the services using their router-based network that was described in section 6. And SLT uses packet switching on this network. Before this router-based network established they used circuit switching on their old network. In circuit switching, hardware is reserved for a user, who is using the network. So it is very secure. But in a practical situation, the user never sends a continuous stream of traffic. There are some traffic-free intervals. But hardware is dedicated to the user. Therefore, circuit switching is wasting of resources. In packet switching, this waste is reduced. Although, packet switching is less secure with respect to circuit switching. On the other hand, circuit switching is basically designed for voice transmissions. And currently, SLT provides data and video that are hard to provide by circuit switching.
On the last day of training, I was able to take a basic understanding of the power distribution of the SLT Matara branch. Power distribution is managed under the switching transmission section. They maintain a power supply for all the devices in SLT including outdoor MSANs. CEB provides the main supply. Except that SLT Matara has a battery capacity of 4000Ah that is enough for eight operating hours.
7. List of Abbreviations
ADSL : — Asymmetric Digital Subscriber Line
ATM : — Asynchronous Transfer Mode
BRAS : — Broadband Remote Access Server
CEA : — Carrier Ethernet Aggregation
CEB : — Ceylon Electricity Board
CEO : — Chief Executive Officer
COO : — Chief Operational Officer
CRO : — Chief Regional Officer
CSMA/CD : — Carrier Sense Multiple Access with Collision Detection
DGM : — Deputy General Manager
DHCP : — Dynamic Host Configuration Protocol
DP : — Distribution Point
FTTH : — Fiber To The Home
GM : — General Manager
HDMI : — High Definition Multimedia Interface
IDD : — International Direct Dialing
IPTV : — Internet Protocol Television
MAC : — Media Access Control
MDF : — Main Distribution Frame
MOD : — Music on Demand
MSAN : — Multi-Services Access Node
NAT : — Network Address Translation
NMS : — Network Management System
NOC : — Network Operational Center
OLT : — Optical Line Terminal
ONT : — Optical Network Terminal
OPMC : — Outside Plant Maintenance Center
OTDR : — Optical Time Domain Reflectometer
PADI : — PPP Active Discovery Initiations
PADO : — PPP Active Discovery Offer
PADR : — PPP Active Discovery Request
PADS : — PPP Active Discovery Session Confirmation
PE : — Provider Edge
PLC : — Privet Limited Company
PPP : — Point to Point Protocol
ROT : — Regional Telecom Office
SLT : — Sri Lanka Telecom
SNR : — Signal to Noise Ratio
STB : — Set-Top Box
TCP : — Transfer Control Protocol
TDR : — Time Domain Reflectometer
TID : — Terminal Identification
TSTV : — Time Shifted Television
VLAN : — Virtual Local Area Network
VOD : — Video on Demand
VPI : — Virtual Path Identifier
VPN : — Virtual Privet Network
