EPS NETWORK CRITICAL CONCEPTS

Let us kick off this discussion with the First Critical Concepts i.e.Mobility Management and Session Management.

In 2G/3G/4G  Packet Core, this IP network is called a "Packet Data Network" (PDN). 

In EPS (4G), the IP connection to this PDN is called a "PDN connection" & A terminal may access a single PDN at a time or it may have multiple PDN connections open simultaneously, for example one PDN connection to internet and another PDN connection to IMS network for voice services.

For example, below are 2 PDN Connections:

1). IP Address 1 : (X.X.X.X), PDN : (INTERNET).

2). IP Address 2 : (Y.Y.Y.Y), PDN : (INTERNET - IMS).

In simple terms, the PDN connection is a logical connection setup between a specfic IP address assigned to a UE and a particular Packet Data Network.

TOPOLOGY SHOWING UE WITH MULTIPLE PDN CONNECTIONS:

Critical Components in a PDN connection:

1). APN: Usually configured in the HSS & defined in UE & PDN GW to identify the destination PDN.

2). PDN: This is the destnation N/W which the UE accesses.

3). IP Address: This is either IPv4/IPv6 address that is assigned to the UE.

4). PDN GW:  This is the anchor point which provides service for the UE to a given PDN.

IP Address & Allocation:

Ways In Which IP's (IPv4 or IPv6) are allocated:

1). During Attach Process.

2). Allocated by DHCP after attach process.

3). Allocated by AAA during Authentication

EPS Bearers:

1). A Bearer is one of the key components in EPS. More than one bearer can  exist in the same PDN connection at the same time.

2). The EPS bearer provides a logical transport channel between the UE and the PDN for transporting IP traffic.

3). Each EPS bearer is associated with a set of QoS parameters that describe the properties of the transport channel,e.g bit rate, delay and bit error rate, scheduling policy in the radio base station, etc.

There are two types of bearers:

1). Default bearer: is created in the attach procedure, the default bearer will remain established for the lifetime of the PDN connection. In most cases, the QoS associated with the default bearer is for user traffic that does not need special treatment, like web browsing.

2). Dedicated bearer: is  created for an already existent PDN connection, these bearers are activated on demand.e.g: some VoIP applications  might need some specific guaranteed bit rate that the default bearer cannot satisfy, in this case, it will try to establish a dedicated bearer. They will be deactivated after they are no longer needed. For example a Video Conference call.

EPS Bearers Traffic Flow Template:

In the case where more than one EPS bearer is created for the same PDN connection,The UE, S- GW &  PDN GW must be able to distinguish IP traffic and the related EPS bearer to provide differentiated QoS treatment. 

For example, web browsing traffic will use default bearer which has 2Mbps bandwidth, VOIP 
traffic  will use a dedicated bearer which has 4Mbps. Each EPS bearer is associated with a 
Traffic Flow 

Template (TFT) that includes the packet filters for the bearer.

A packet filter may include the following attributes:

1). Remote IP address and subnet mask

2). Protocol ID for IPv4 

3). Local port range

4). Remote port range

5). Type of Service (ToS) for IPv4 etc 

Each packet filter in a TFT is associated with a precedence value that determines in which 
order the filters shall be tested for a match.

How to identify different EPS bearer context?

TFT is also used to identify different EPS bearer contexts corresponding to one PDP address. 
The difference lies in that TFT is used by a PGW or  MS to match a received downlink Packet 
with a corresponding EPS bearer context so that the PGW or MS can use the relevant tunnel 
for transmission.

EPS bearer ID is used by an MME or a SGW/PGW to match different EPS bearer contexts.

In which situation TFT is used?

TFT is used in dedicated bearer activation procedure.Dedicated bearer activation procedure is 
triggered in the case when the same IP address is used to initiate an activation request with the 
same APN but QoS in this activation is different from existing bearer.

Comparison of GPRS/UMTS/LTE (GUL) Session Management Concepts:

Next we will discuss Mobility Management Concept.....Stay Tuned

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