Drive Test

Drive Test will ensure the network performance and it is directly related to the QoS of the network. Normally drive tests are conducted by Technical Officers (TO) under the guidance of an Optimization Engineer. In Drive Test we drive on a vehicle which is well equipped to measure network performance. Gathered data are recorded using a drive test tool and we can analyze them later. Reasons for doing a Drive Test:

• For Network Optimization: changing antenna azimuths and tilts
• To identify network performance of area
• To troubleshoot customer complaints
• To check network coverage
• After a major change in the network E.g. After a parameter change in site 
• To check signal levels After commissioning a new site

In addition to these occasions we sometimes conduct indoor drive tests in situations when there are issues inside a building. In such situations a person wearing drive test tool and other required equipment will walk inside the building where we need to collect data. Here we face the problem of receiving GPS location. At such scenarios first a reference GPS point is taken at a location of the building. Then a sketch of the floor plan of the building is drawn on the map of the drive test tool. Then the drive test tool is taken on a path inside the building area in a fixed speed. We should travel on a fixed and steady speed because otherwise recording may not have correct details of the recorded data.

Equipment and Tools used in a Drive Test

I got to know about two Drive Test Tools. They are Ericson ® TEMS and JDSU (Agilent). TEMS is for 2G and Agilent is for 3G. Tool will record the details to a “.log” file.  Equipments used:

• A laptop computer with drive test tool installed in it
• Two mobile phones which supports the tool (one in Dedicated mode and other one in Idle mode)
• GPS module to track location
• The license dongle for the authentication of the drive test tool

Two mobile phones, GPS module and license dongle are connected to the laptop. We can upload the existing site file to the tool and according to the location of GPS, data will be recorded. When the record is done we can analyze the recorded data later.

Why MPLS

For describing purposes lets divide Internet Service Providers into three categories. They are:

• Layer 1 (e.g.: SLT)
• Layer 2 (e.g.: Etisalat, Mobitel, Dialog)
• Layer 3 (not present in Sri Lanka)

In IP routing, Router maintains two tables for routing purposes. Namely RIB (Router Information Base) and RIFB (Router Information Forwarding Base). First router looks at all the networks connected to it. Then build up the RIB including all the IPs of externally connected networks. Using RIB router builds another table called RIFB. That table includes shortest/best path of a particular source and destination. When a packet arrives to a Router, router will first check the RIFB for the destination IP. Then it will do another router lookup to find out next hop. So for each packet two router lookups needed. If this packet goes through thousands of routers in the internet it will take lot of time. Additionally at each hop source and destination MAC (Media Access Control) addresses changed. To speed up the process of packet traversing in the internet we use MPLS. 

Using Label Distribution Protocol (LDP) routers distribute their labels. LDP is not a routing protocol as OSPF (Open Shortest Path First) or IS-IS (Intermediate System to Intermediate System). As in previous case (in IP routing) in MPLS routers build two tables namely LIB (Label Information Base) and LIFB (Label Information Forwarding Base). LIB consists of labels of connected networks. And LIFB consist of label mapping of the networks. No data loss occurs as in previous case, only label switching happens. Now a large table with labels build up. To overcome, we can use a protocol like BGP (Border Gateway Protocol). Following figure will give you an idea of how a LFIB - MPLS table looks like.

 

MPLS (Multi-Protocol Label Switching)

MPLS is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links (paths) between distant nodes rather than endpoints. MPLS can encapsulate packets of various network protocols. MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL.

MPLS header consists of following.

• Label (20 bits)

•  QoS (3 bits)

• Bottom of Stack (1 bit)

• Time To Live (8 bits)

And a MPLS network consist of two types of routers namely Label Edge Router (LER) and Label Switch Router (LSR).

LSR: Routers that perform routing based on labels.

LER: Routers that are in the entry and exit points of the MPLS network. They push a MPLS label into the packets which are coming into the MPLS network and pop off the MPLS labels from the packets leaving the MPLS network.  Refer Figure: MPLS. 

An advantage of IP over TDM

If we are using E1/TDM for any interconnection, for each additional interconnection we have to lay E1s. But if we are using IP technology we just need to add a configuration to the network. For example, in TDM world if we wanted to connect to another operator (especially international) we need to lay E1s. If we are doing it in IP we can fulfill our requirement by adding an additional configuration through an existing connectivity.

Why we can’t use a normal Firewall instead of a SBC

Normal internet browsing traffic consist of comparably large in packet size and lesser number of packets (i.e. lesser number of headers). A typical firewall will inspect and check the headers. VoIP traffic is different from normal internet browsing traffic. VoIP traffic consists of large number of smaller (smaller in length of the packet) packets which comes in bursts.  IP packet headers are more or less the same. If we are using a Firewall in VoIP, it has to process large amount of headers. Hence need more processing in a VoIP (That's why we have to introduce a SBC instead of a normal Firewall). On the other hand in a normal firewall, provided security is low as well. Therefore we use an SBC instead of Firewall. Security level of a SBC is very high.