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.

Next Generation Network (NGN)

In the NGN Architecture, it has two major parts namely WCS (Wireless Call Server) and MGW (Media Gateway). The real intelligent or call processing part is WCS. Different types of 

media and technologies are connected to MGW. All the processing part of connected technologies and media is handled in WCS. WCS and MGW communicate using a protocol named as EGCP (Standard protocol =H.248. EGCP is an Alcatel proprietary protocol). WCS and MGW is connected through an IP/MPLS network. And that IP/MPLS (Multi-Protocol Label Switching) backbone consists of Routers and Switches. When we connect two nodes we will always add a redundant path too. Because especially in telecommunication networks Redundancy is considered as one of the most important concepts. We never put one, we always put at least two. Because if one link fails, there is always another link there to carry out the service. We need to keep the utilization of a link below 50% to make sure that redundant path can take over the traffic of the failed link.

In a NGN network we need to provide various services. Such as SIGTRAN, Operation & Maintenance of the NGN, VoIP services, RNC traffic (IuB, IuPs, etc), etc. These services/networks should independent from each other. That’s why we use MPLS network. Using MPLS concepts like VLANs (Virtual Local Area Network) and VPRNs (Virtual Private Routed Network) we can configure these networks to function independently. Following Figure: A typical LAN connection will give an idea about this concept.

 

Technical Floor/Engineering Section LAN is connected to the LAN of IT. Technical floor is also divided into few sub sections. I.e. SWITCH, IN. if a SWITCH engineer wants to do some operation & maintenance in the telecom network, he can easily do it by logging into the LAN of SWITCH. Another important thing is how CDRs were received by the Billing department. As shown in the Figure Billing Server receives CDRs through IT network. 

If we compare Legacy and NGN architectures, MSCs are replaced by WCSs and MGWs. There were two types (conceptually) of MSCs namely VMSC and GMSC. The following Figure: WCS & MGW will provide an example on how GMSC functions were replaced by WCS and how MGWs connected to BSCs.

Consider a company where majority of IDD calls are VoIP. For VoIP we use SIP (Session Initiation Protocol). Now companies are using a device  called SBC (Session Border Controller) which is regularly deployed in VoIP networks all over the world. SBC will protect the network and devices from malicious attacks and at the same time will allow different parts of the network to communicate through the use of variety of techniques such as NAT (Network Address Translation), SIP, IPV4/6, etc.
 Following figure will give you a clear idea on how the previous system was there and how it is replaced now using a SBC.