LTE Attach Part 1: Goals of LTE Attach
Summary
TLDRThis 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
📱 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.
🌐 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.
🔄 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.
Mindmap
Keywords
💡LTE Attach
💡Identity in the Network
💡Call Flow
💡Access Stratum
💡Non-Access Stratum
💡Mobility Management
💡Session Management
💡GTP Primer
💡S1-MME Interface
💡Topologically Incorrect Address
💡Tunneling
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
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