LTE Attach Part 1: Goals of LTE Attach

Summary

This video script delves into the LTE attach process, crucial for mobile registration and communication. It clarifies that a mobile's identity in the network is not just the phone number. The script outlines the LTE architecture, detailing the roles of eNodeBs, Serving Gateway, and PDN Gateway. It explains the importance of IP address allocation, maintaining a constant address for uninterrupted communication, and the use of non-topologically correct addresses due to mobility. The video also covers the concept of tunnels for routing packets to mobile devices and the signaling process required to set up these connections, highlighting the significance of GTP and the S1-MME interface in LTE networks.

Takeaways

• 📱 The LTE attach process is akin to registration, allowing a mobile device to join a network, obtain an IP address, and initiate communication.
• 🆔 Your identity in the network is not just your phone number; there's more to it that the network uses to recognize you.
• 📊 Call flow is crucial as it illustrates the procedures within the network and is a primary method used in LTE specifications.
• 🔍 The concepts of Access Stratum and Non-Access Stratum are distinct and essential to understand their roles and differences in LTE.
• 🚀 Mobility Management and Session Management are key concepts in LTE, vital for understanding how the network operates.
• 🌐 Protocol stacks are important, with GTP being a main protocol in LTE networks, and the S1-MME interface being another critical component.
• 🏗️ The LTE architecture consists of eNodeBs, Serving Gateway, PDN Gateway, HSS for subscription info, and MME as the 'brains' of the system.
• 🌐 The IP address provided by the network is globally routable and is translated into a private address used within the network.
• 🔄 The IP address remains constant as a user moves within the network to maintain communication integrity.
• 🔄 The address provided by the P-Gateway is not topologically correct, necessitating the use of tunnels for routing packets to the mobile device.
• 🔄 Tunneling involves encapsulating IP packets within other IP packets to ensure they reach the intended destination, using protocols like GTP-U and UDP.

Q & A

What is the main goal of having a mobile phone according to the video script?

The main goal of having a mobile phone is to be able to communicate with other people.

What is the process of LTE attach and why is it important?

LTE attach is the process of registration by which a mobile device connects to a network, gets an IP address, and is able to communicate. It is important because it enables the mobile device to establish communication services.

What is the identity of a mobile device in the network and why might one's phone number not be the main identity?

The identity of a mobile device in the network is not necessarily the phone number. The network uses other identifiers for recognizing and managing devices, which can include SIM card details and unique device identifiers.

What is a call flow and why is it significant in the context of network procedures?

A call flow is a diagram or description of the path that a call takes through a telecommunications network. It is significant because it captures the various procedures in the network and helps in understanding how different components interact during a call or data session.

What are the differences between Access Stratum and Non-Access Stratum?

Access Stratum refers to the interface and protocols directly involved in the radio communication between a mobile device and a base station. Non-Access Stratum, on the other hand, involves the higher layers of the network that are not directly involved in the radio communication, such as the core network functions.

What are the two main concepts used in LTE for managing network operations?

The two main concepts used in LTE for managing network operations are Mobility Management (MM) and Session Management (SM).

What is the role of the Serving Gateway (S-GW) in the LTE architecture?

The Serving Gateway (S-GW) acts as an anchor for the user plane, managing the data flow between the mobile device and the Packet Data Network Gateway (P-GW), and handling the mobility aspect as the user moves between different base stations.

What is the function of the Packet Data Network Gateway (P-GW) in the LTE network?

The Packet Data Network Gateway (P-GW) serves as the connection point to the internet. It is responsible for routing all packets for the user equipment (UE) to and from the internet.

Why is the IP address provided by the P-GW not topologically correct?

The IP address provided by the P-GW is not topologically correct because it is a private address that does not correspond to the actual network segment of the base station the mobile device is connected to. This is due to the mobile nature of the devices, which frequently change their point of attachment in the network.

What is the purpose of GTP (GPRS Tunneling Protocol) in the LTE network?

GTP is used for tunneling user data through the LTE network. It enables the routing of packets to mobile devices that have non-topologically correct IP addresses by encapsulating the packets within another IP packet that can be routed to the correct network segment.

What are the signaling connections required to set up the necessary tunnels and connections for an LTE attach procedure?

The signaling connections required include the S1 Application Protocol (S1-AP) between the eNodeB and the MME, GTP Control Plane (GTP-C) between the MME and the Serving Gateway, and another signaling connection between the Serving Gateway and the P-GW for setting up user plane tunnels.

Outlines

00:00

📱 LTE Attach Process Overview

This paragraph introduces the LTE attach process, which is essential for a mobile device to register with a network, obtain an IP address, and initiate communication. It clarifies that the primary identity of a device in the network is not the phone number but another identifier. The paragraph outlines the topics to be covered, including call flow, access and non-access stratum concepts, and the distinction between mobility and session management in LTE. It also mentions additional resources on protocol stacks, such as GTP primer and S1 MME interface, to provide a deeper understanding of LTE network operations.

05:00

🌐 Understanding LTE Network Elements and IP Addressing

This section delves into the LTE architecture, describing the roles of eNodeBs, the Serving Gateway, and the PDN Gateway. It explains how the PDN Gateway connects to the internet and routes packets to and from the user equipment (UE). The paragraph highlights the importance of a constant IP address for maintaining communication as the UE moves within the network. It also discusses the concept of topologically incorrect IP addresses provided by the P-Gateway and the necessity of tunnels for routing packets to the mobile device, introducing the GTP-U protocol and its role in the process.

10:03

🔄 Signaling and Tunneling in LTE Networks

The final paragraph focuses on the signaling process required to set up the necessary connections for data transmission in an LTE network. It explains the three logical signaling connections between the UE and eNodeB, the S1-MME interface for communication between eNodeB and MME, and the GTP-C protocol for signaling between MME and the Serving Gateway. The paragraph also details the process of tunneling packets from the P-Gateway to the Serving Gateway and then to the eNodeB, emphasizing the use of UDP and tunnel identifiers for routing packets to the correct destination. It concludes by discussing the signaling connections required for the LTE attach procedure to establish the network 'plumbing' for data flow.

Keywords

💡LTE Attach

LTE Attach refers to the process by which a mobile device registers with a network, obtains an IP address, and becomes capable of communication. It is fundamental to the operation of mobile phones, as it allows them to connect to the internet and communicate with other devices. In the script, this process is the main focus, illustrating how a mobile device like Mehmet's smartphone becomes part of the LTE network.

💡Identity in the Network

The script mentions that one might assume their phone number is their identity in the network, but this is not the case. The network uses a different set of identifiers to recognize and manage connections. This concept is crucial for understanding how networks maintain security and individuality for each user, even though it may not be immediately apparent to the user.

💡Call Flow

A Call Flow is a diagram or description that traces the path a call or data packet takes through a telecommunications system. In the context of the video, it is used to illustrate the various procedures that occur within the network, showing how a mobile device like Mehmet's goes through the LTE attach process.

💡Access Stratum

The Access Stratum (AS) is the part of the LTE network architecture that deals with the radio interface between the mobile device and the base station. It is distinct from the Non-Access Stratum and is responsible for the physical connection and communication over the air. The script explains the differentiation between AS and NAS, which is essential for understanding the layered structure of LTE.

💡Non-Access Stratum

The Non-Access Stratum (NAS) is the part of the LTE network that handles the signaling and data delivery functions that are independent of the radio access technology. It is responsible for higher-level functions such as mobility management and session management. In the script, NAS is contrasted with AS to highlight the division of responsibilities within the LTE network.

💡Mobility Management

Mobility Management (MM) is a concept in LTE that deals with the tracking and management of mobile devices as they move around the network. It ensures that the network can maintain communication with the device even as it changes location. The script mentions MM as a key concept, emphasizing its importance in the LTE network's ability to handle mobile users like Mehmet.

💡Session Management

Session Management (SM) in LTE is responsible for the establishment, maintenance, and release of user sessions. It ensures that data can be sent and received during a communication session. The script identifies SM as an important concept, which is essential for understanding how LTE networks manage communication sessions for users.

💡GTP Primer

GTP Primer refers to a basic introduction to the GPRS Tunneling Protocol (GTP), which is a key protocol used in LTE networks for tunneling user data through the network. The script suggests that viewers interested in the technical details of LTE may want to look at a primer on GTP to better understand the protocol's role in the network.

💡S1-MME Interface

The S1-MME Interface is the logical connection between the eNodeB (base station) and the MME (Mobility Management Entity) in an LTE network. It is used for the exchange of control plane information. The script mentions a primer on the S1-MME interface, indicating its importance in the network's control mechanisms.

💡Topologically Incorrect Address

A topologically incorrect address is an IP address that does not match the actual network topology. In the script, it is mentioned that the IP address assigned by the P-Gateway to Mehmet is not topologically correct, meaning it cannot be directly routed to the eNodeB where Mehmet is connected. This concept is important for understanding how LTE networks use tunnels to route packets to mobile devices.

💡Tunneling

Tunneling in the context of LTE refers to the process of encapsulating a data packet within another packet to send it over a network. This is necessary when dealing with topologically incorrect addresses, as explained in the script. The script describes how tunneling is used to route packets from the internet to a mobile device like Mehmet's smartphone, even when the IP addresses are not directly routable.

Highlights

The video covers the LTE attach process, which is essential for a mobile device to register and communicate within a network.

The main identity of a mobile device in the network is not the phone number, contrary to common belief.

Call flow is crucial for understanding the procedures captured in the network specification.

The concept of Access Stratum and Non-Access Stratum are introduced, explaining their differences and purposes.

Mobility Management and Session Management are two key concepts used in LTE.

The video provides an overview of the LTE architecture, including E-NodeBs, Serving Gateway, and PDN Gateway.

The HSS stores user subscription information, and the MME acts as the 'brains' of the system for mobility management.

The process of a mobile device getting an IP address and the significance of a constant IP address for communication are discussed.

The provided IP address by the network is not topologically correct, which requires special routing methods.

Tunneling is used for routing packets in a mobile network with non-topologically correct addresses.

The role of the P-Gateway in connecting to the internet and the process of tunneling packets to the mobile device are explained.

The importance of the GTP-U protocol in the LTE network for user plane traffic is highlighted.

The video explains the signaling process required to set up tunnels and connections in the LTE network.

The signaling plane involves three logical connections between the mobile device and the E-NodeB for communication.

The S1-MME interface and the S1-AP protocol are discussed for their role in signaling between the E-NodeB and the MME.

The video describes the signaling connection between the MME, Serving Gateway, and P-Gateway for setting up user plane tunnels.

The entire attach procedure is summarized as the setup of network 'plumbing' for the attached case.

Transcripts

this video we'll cover the process of

Lte attach it's also called the process

of registration by which a mobile comes

into a network gets an IP address and is

able to communicate I mean that's after

all is your main goal of trying to have

a mobile phone is that you can

communicate with other people

so in this lecture what we are going to

cover are the following aspects we will

cover what's your identity in the

network you may think that your identity

is your phone number but you'll be

surprised to know that that's not the

main identity by which the network knows

you we will look at a call Flow and this

is important to to take a look at

because this is one of the main ways in

which the specification captures the

various procedures in the network and

it's an example of such a procedure

the concept of access stratum and

non-axis stratum what these are and how

they are different from each other and

why they exist

um and then also what is mobility

management and what is session

management these are two important

Concepts that are used in LTE and you

should be aware of these in addition if

you want to take a look more at the

protocol Stacks that are two separate

short videos one on GTP primer which is

one of the main protocols that is used

in the LTE network and also there's a

separate primer on the S1 mme interface

and you may be interested in taking a

look at those in order to get a little

bit more overview of these two

interfaces

as we had covered in the previous

lecture the main elements of the LTE

architecture are shown in the figure

here and we can see that the main

elements that we do have are these base

stations which are called e node B's and

they are all anchored at something

called the serving Gateway which anchors

the mobility between them and then you

have the P Gateway or the PDN Gateway

which is the connection from the

internet so all packets for the UE comes

to this PDN Gateway and then from the

PDN Gateway make their way through to

the UE and back and forth these were the

main elements in the user plane and then

in the control plane we had said that

the HSS is the one which stores the

subscription information of the user and

then

subscription information and then we had

this key element the mme which is kind

of the brains of the system or the

mobility management entity which acts

also as a proxy for authenticating the

user to the HSS and also for handling

the mobility in between the base station

so as you you move from one base station

to the other it's the mme which kind of

controls some of this Mobility it is it

is essentially the the as we said the

brains of the system now into this

network works in our protagonist Mehmet

he's got a cell phone it's got a Sim and

this as you can see looks like an old

cell phone this looks like before the

iPhone ages but that's fine he's now

upgraded to a smartphone and he wants to

communicate with Aisha Aisha is his

friend and she's connected somewhere to

the internet and she probably has

another mobile phone or she's com she's

connected via a laptop we don't care

about that so Mehmet wants to

communicate with Aisha and for that the

network will have to provide them with

an IP address and now the the IP address

that the network provides him is kind of

pointing to the packet gateway to which

which is connected to the internet and

here is a globally routable address

which gets netted into a private address

which is the one which is used inside

the network

now the address which this network

provides is remain going to remain

constant unchanged as Mehmet moves in

the network another important thing that

happens is that and the reason that the

address has to remain constant is that

if its address changes then the

communication will break between Aisha

and Madness and that's one of the kind

of properties the way today's protocol

Stacks are working

another important thing to kind of keep

in mind is that the address that P

Gateway provides to Mehmet is not

topologically correct and when we say

topologically correct what does that

mean

correct

that means is that if this address which

is here and Mehmet is connected to a

base station over here and the IP

address or or the sub Network or to

which the space station belongs is not

going to be is not going to be something

that this packet here will get routed to

so his his private IP address of the

base station could be something like

10.40.50.0 or dot one which is as you

can see with the with the

um

Mosque of 24 bits that means these are

the important bits over here and that's

the sub Network in this e-node Beast is

attached to whereas the IP address of

this mobile has

10.40.40.12 which doesn't belong to this

network so uh it is not it cannot be

routed directly to this base station and

that is what's called topologically

incorrect address because it cannot be

routed to uh to the base station where

the where Mehmet is connected and the

reason that the network doesn't give a

topologically correct address is that it

is pretty sure it's a mobile network

that that Mehmet very soon is going to

be moving into another base station or

there is a high probability that he

would move and even if I give me the

topologically correct address here that

means belonging to dot 50.25 for example

which would be topologically correct

very soon he will end up in a base

station where his address is not

topologically correct

so when you have addresses which are not

topologically correct you need to kind

of figure out how to route a packet so

that it can reach the destination

so for that what is done in the IP world

are something called tunnels and there

are two tunnels that gets created

for in order to Route the packet which

comes from the internet to the mobile

and these two tunnels here are are the

tunnels between the P gate Venus Gateway

and between the S Gateway and the base

station and then over the air it's not

an IP connection it is just uh bytes

being sent over over a modem so now

let's take an IP packet that Aisha sends

to mehmed saying hi that's coming from

my IP address of Aisha down to the IP

address of Mac meth and it comes all the

way to the P Gateway now the P Gateway

when it sees the IP address of mermaid

knows that it needs to send this packet

down to Mad Men but this packet cannot

will not be routable over this interface

so what he needs to do or what the P

Gateway needs to do is to Tunnel this

packet that means to put it inside

another IP packet which now has the

destination address of the s Gateway and

says that it is coming from the P

Gateway in this way by putting this

packet which is coming from Aisha to

mehmed inside this outer packet it is

able to send the packet at least halfway

there to Mammoth and that's reaching the

S Gateway now the S Gateway is the end

hosts so not only you need to have this

address over here to enable to Route the

packet but you also need to get the

packet to S Gateway that means you

either you need a transport level

protocol and the transport level

protocol which is being used for routing

of packets or tunneling of packets is

UDP so there is a UDP protocol running

between P Gateway and S Gateway and

there is a well-known destination Port

which is 2152 which is the gtpu GTP user

plane protocol at which there is a

process in the serving Gateway which is

listening for packets coming GTP packets

coming over here so it so so the packet

is now able to get to a particular

process running in the S Gateway which

is listening on Port 2152 but since

there can be a large number of mobiles

that are connected to a base station and

connected to uh and and being forwarded

by the S Gateway not only do you need to

get to the right Port you also need to

say that this packet is actually for

mehmed and in order to do that there is

another identifier inside the UDP uh

packet in in What's called the GTP you

header GTP user plane header which is

called the tunnel identifier so this

identifies magnet so this says that this

packet is actually for Mehmet and not

for Yusuf or for Ali or for Ashok or for

John so each one has a different tunnel

ID if all of them if all of these

packets needs to be forwarded via

serving gateway to to do magnets so

um so we have a general overall

structure a little bit of how packets

are being tunnel from the P gateway to

the S Gateway and uh we will now kind of

continue on so we we've got the packet

reached halfway there to uh to metmet

but now they still need to get from uh

mehmed from Escape way to the E node B

and we don't have the keynote B written

here as gateway to the E node B so now

what this s Gateway does is it takes out

this packet from here and puts it into

yet another tunnel and this time the

tunnel uh the the tunneling is to the E

node beats going from the serving

Gateway again at the E node B There is a

a process which is listening on the

socket UDP socket 2152 and it gets that

packet from gets this packet coming from

the S Gateway and also there is a tunnel

ID here in this case on this interface

coming into the E node B the tunnel

identifier which identifies magnet is

some other value which is X which is

different from which can be different

from from the value used in the S

Gateway and so once it comes to with

this identifier to this e node B the E

node B knows that this packet is for

matmit it map so this e node b gets that

packet and then sends it out over the

radio interface using a data radio

better down to uh to magmet and over

here this is not an IP in IP tunnel link

this is just taking the the packet and

sending down to Matt method and mehmed

gets this packet so this is the way in

which we are getting packets coming all

the way from the internet from from

Aisha all the way down to Mehmet and

back and forth in the other way map is

going to send an IP packet here which

will go into another set of tunnels and

then going and then finally from the P

gateway to I say we will cover some more

details of gtpu at least the concepts of

gtpu a little bit in the Annex video for

on GDP

um so this was a user plane but then in

order to set up all these tunnels you

need to do some signaling they are

obviously not present when magnet didn't

come into the network in order to set up

these connections you need to have some

signaling uh plane and there are three

signaling connections logical

connections between mahmet and the E

node B they're called signaling radio

bearers uh these are just three logical

bearers and so don't get worried why

there are three and not one well all

three of these constitute the signaling

connection between mehmed's phone and

the e-node B

but this only gets magnets communication

only up to the E node B the E node B

also needs to communicate with the mme

and not only for forwarding for example

authentication messages to the mme but

also saying hey I've got a new mobile

what do I do about it and for that there

is a per UE connection between the E

node B and the mme over the S1 mme

interface and the application protocol

that is used over here is the S1

application protocol S1 AP so there is a

connection for the signaling messages

for forwarding not only the messages

from Mehmet to the mme but also for

other e node B and mme signaling

messages over this S1 mme interface

using the s1ap protocol

in addition you also need to have

communication between the mme and the

serving Gateway for setting up this

tunnel as you can see the signaling

connection doesn't there is no direct

signaling connection between the E node

V and S Gateway it all goes via the mme

and that's why to go over the S1 AP

protocol first and then the mme will do

signaling to the serving Gateway over

another signaling connection which is a

GTP control plane or gtpc and then there

is a signaling connection between the

serving Gateway and the P Gateway which

for example gets used for setting up

this user plane tunnel

so essentially

the network Plumbing at the end of the

attach procedure so so the entire

attached procedure is used for setting

up this uh this plumbing and this

connection for the attached case