Transcript T325: Technologies for digital media

T325: Technologies for digital media
Block III - Part 2: Network architecture
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• Key protocols
• User services and the network architecture
• Authentication
Outline
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• Different protocols are used for 2G, 2.5G and
3G
• 2G and 2.5G : different protocol stacks are used for the
control plane and the user plane
• 3G : same protocols are used for the control place and the
user plane
Key protocols
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• The layered protocols approach
Key protocols
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• Each service provides data in blocks called data units.
• The set of blocks at each node is often known as the protocol stack.
• Each protocol has two end points
• PHY protocol: UE and Node B
• GPRS mobility management (GMM): UE and the SGSN
Key protocols
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• UE and SGSN are not adjacent on the diagram, which means the
GMM messages are carried over the intermediate nodes (Node B
and RNC) by lower level protocols; in this case the radio resource
control (RRC) and radio access network application protocol
(RANAP).
• These transport the GMM messages but do not alter them in
any way, so the intermediate nodes are invisible to the GMM
protocol.
Key protocols
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• Each block can be thought of as communicating with the block
of the same name at the other end point using the services of the
layers below.
• Radio Link control (RLC) block in the RNC communicates with the
RLC block in the UE, using the services of the medium access control
(MAC) and PHY layers.
• Blocks communicating in this way are said to be communicating on
a peer-to-peer basis and the two blocks are often referred to as peer
entities.
Key protocols
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• Protocol interworking: A node converts from one protocol to
another in each direction.
• Example: At the lowest level, the Node B has blocks labeled
PHY and ATM.
Radio access network
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• The blocks labeled PHY refer to the physical layer of the radio
interface, which comprises a number of different physical channels.
• In 3G, the primary means of sharing the allocated radio bandwidth
between these channels is W-CDMA
• The physical channels are used for:
• user traffic
• transport of Signaling messages
• management of the radio link itself.
• PHY interworks at the Node B with the ATM protocol
• ATM protocol is used to transport user traffic and Signaling messages
across the fixed part of the network from the Node B to the SGSN (and
the MSC for circuit-switched traffic) via the RNC.
Radio access network – Physical Layer
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• Next group of protocols to consider are those which are
used to manage the flow of data between the UE and the
radio access network and to control the allocation of
radio channels.
• The three most important of these are the RRC, RLC
and MAC protocols
• The end points for all of these protocols are the UE and
the RNC (except for one specific case which need not
concern us here).
• This means that in virtually all cases the; Node B does
not participate in these protocols  it simply forwards
the data to the UE
Radio access network
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Key protocols
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• MAC Layer : Principal functions
• Prioritization between:
• data flows addressed to a single piece of UE
• data flows addressed to different pieces of UE.
• Multiplexing data units arriving from the upper layers onto
the data units delivered to the physical layer and
demultiplexing flows in the opposite direction.
• Encryption (if this has not been carried out by the RLC
layer)
• Traffic volume measurement.
Key protocols – MAC Layer
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• RLC layer : Responsible for
• Segmentation of data units received from the next layer up
(the RRC layer) into smaller data units to be forwarded to
the MAC layer.
• Reassembling data units received from the MAC layer into
larger data units for forwarding to the RRC layer.
• If instructed to do so, the RLC layer will also routinely
acknowledge packets and retransmit those which are
reported by the receiver to contain errors.
Key protocols – RLC Layer
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• Three types of communication channels (3G
standards)
• Logical channels: data flows between the
RLC blocks
• Transport channels: the data flows between
the MAC blocks
• Physical channels: the data flows transmitted
across the radio interface by the physical layer
Key protocols
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• The top layer of the three is the RRC protocol, which
handles most of the major functions in the radio network,
including:
• Setting up of physical radio channels
• Reporting of radio measurements
• Transport of Signaling messages between the mobile and
the core network
• It also instructs the RLC layer whether or not to
acknowledge packets.
• The three protocols (MAC/RLC/RRC) and the physical
layer protocol PHY are classified together in the
standards as the access stratum – the level which
provides the UE with access to the core network.
Key protocols – RRC Protocol
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• Radio network controller needs to interact with the
core network.
• This is done using a protocol called RANAP, which is
used to interact with the SGSN (and also the MSC in the
circuit-switched domain).
• RANAP makes use of a version of the standard Signaling
protocol used in the circuit-switched domain, Signaling
System No. 7 (SS7).
• The version of SS7 used in RANAP has been modified to
run over ATM.
RAN-Core network interaction
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RAN-Core network interaction
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• At the very top of the stack is the non-access stratum (NAS),
which handles direct Signaling between the mobile and the
core network.
• In the 3G packet domain, the principal constituent of the nonaccess stratum is the GMM protocol.
RAN-Core network interaction
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• IP addressing in fixed networks is hierarchical
• Network/subnetwork address
• Host address
• This hierarchy is used for routing purposes
• Problem of Mobility in IP-based Mobile networks (e.g. 3G)
• If a device were to move from one subnetwork to another:
• It could be come invisible at the subnetwork routing level, since
the top-level routing would direct the packets to the previous
subnetwork.
• Since its IP address would almost certainly not match the
addressing scheme of the new subnetwork, messages from the
device might also be rejected by firewalls checking for valid
source addresses.
Support for IP in 3G networks
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• Solving the mobility problem for mobile data
communication, therefore, depends on using the location
information the network holds, in order to make the IP
addressing system continue to work as the mobile moves
from one subnetwork to another.
• A clever solution is for the network to put each IP packet
inside another IP packet.
• The outer packet can then be used to get the message to the
router to which the mobile device is currently connected, at
which point this router extracts the inner packet and sends it
directly to the mobile device.
• This approach is known as tunneling, and in GPRS and 3G
it is provided using the GPRS tunneling protocol (GTP).
Support for IP in 3G networks
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Support for IP in 3G networks
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• When an IP packet arrives at the GGSN from an external IP
network addressed to an active mobile:
• GGSN adds the GTP header and the UDP header and puts
the result in an IP packet addressed to the radio network
controller in whose area the mobile is located.
 GGSN sends the packet through the appropriate tunnel, which
ends at the relevant RNC.
• The RNC then extracts the inner IP packet and forwards it to
the user equipment using the RRC protocol and the lower
layers in the access stratum.
• The UE then handles the IP packet just as if it had received it
from a local IP router.
Support for IP in 3G networks
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• Packets from the UE to the GGSN follow a similar
process and are routed by the GGSN to a specific external
IP network depending on the session associated with the
user traffic tunnel through which the packet arrived.
• As the mobile moves around the network, it may pass
from an area controlled by one RNC to that of another 
If this happens, the two RNCs and the SGSN cooperate to
move the tunnel end point from the old RNC to the new
one.
• If the move includes a change of SGSN, then the tunnel is
also relocated from the old SGSN to the new one.
Support for IP in 3G networks
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• Inner level represents the packet to be sent through the tunnel
• Outer level consists of the headers added by the tunnelling
protocol.
• GTP uses the user datagram protocol (UDP) to send the
tunnelled packets and also adds its own control header, which
is used, among other things, to identify which tunnel is which.
Support for IP in 3G networks
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• Session information in the packet-switched side of the
network is stored in a series of tables maintained by the
SGSN and the GGSN called the packet data protocol
(PDP) context.
• Each session is directed to a particular network physically
connected to one GGSN, and this network is identified by
a uniform resource locator (URL) known as the access
point name (APN).
• The APN and the tunnel identifiers are held as part of the
PDP context tables.
• Typical APNs point towards the internet via a network
belonging to an internet service provider (ISP), or to a
corporate data network.
Support for IP in 3G networks
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• In addition to data transfers to single recipients, 3G also
offers a multicast streaming and download service called the
multimedia broadcast and multicast service (MBMS).
• This allows an IP multicast tree to be constructed within the
mobile network, eliminating the need to send multiple copies of
the data across the network.
 The individual pieces of user equipment subscribing to the
relevant services form leaves on the multicast tree, so that each receives
a copy of the data stream or the downloaded data.
• The streaming service is particularly suitable for supporting
mobile TV, and the download service can be used to distribute
multimedia messages (for example, for advertising purposes).
Support for IP in 3G networks
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• Most networks offer additional facilities
associated with the voice service, known as
supplementary services.
• There are many of these, but they include:
• Call forwarding, call on hold, notification of a waiting
incoming call, conference calls, prevention of calls to or
from a mobile handset, voice mail
• 3G networks allow video calls
• The user requests a video call using the normal phone number
and the network attempts to set it up with the called party.
User services and the network architecture
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• Second most popular service offered by mobile networks
(after voice calls) is texting, or SMS (short message
service).
• The text messaging service is also provided by the
circuit-switched part of the network
• In the circuit-switched domain, the Signaling required to
set up and clear down calls uses the ISDN SS7 protocol,
which can be seen as an early packet-based protocol.
• This is transmitted over one of the ISDN time slots which
is reserved for Signaling.
• Text messaging makes use of the SS7 protocol to deliver
the messages, avoiding the need to reserve bandwidth to
transmit a message.
Text messaging
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• Internet Access: the user is connected to the internet via
an internet service provider working in partnership with
the mobile network.
• Access to corporate networks through a virtual private
network (VPN): typically utilize an IPsec tunnel in order
to provide an additional layer of security.
• IPsec is a set of protocols for making IP communication
more secure.
• Multimedia Messaging Service (MMS): allows still
pictures, video clips and audio messages to be sent, as
well as much longer text messages than SMS permits.
• Streaming services present audiovisual content to the
mobile device for buffering and immediate replay.
Data services
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• Two types of video streaming service:
• Mobile TV
• User can choose from a number of channels but is
constrained by a programme schedule
• Content-on-demand.
• Content is downloaded to the mobile device and can
be replayed several times
• The content provider may restrict the number of
times the content can be played or specify an expiry
date beyond which it is no longer available.
Data services
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• Internet Access:
• The user is provided with a standard APN, which is a URL specific to
the mobile network and which needs to be selected as a setting on a
mobile device or on a USIM card associated with the device
• The APN points towards the GGSN interface connected to the ISP’s
network, and the mobile device can use this to set up web browsing
sessions.
How data services are provided?
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• Multimedia Messaging:
• Multimedia messaging is similar in one way to text messaging, in
that it makes use of a multimedia messaging centre (MMSC)
which is used to store and forward messages.
• Unlike the SMSC, however, the MMSC is a fully IP-based
device which can only be accessed through an IP network.
• When the user wants to send a message, the user equipment sets
up a session using an APN (URL) pointing to the MMSC.
How data services are provided
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• In 2G, authentication is the process used to prove to
the network that the mobile device is genuine and
allowed to use the network.
• In 3G, the process is mutual; it also allows the mobile to
prove to itself that the network is genuine.
• In both 2G and 3G, authentication is a function of the
non-access stratum
• Takes place between the user equipment (actually the USIM
card) and the core network without the intervention of the
radio access network.
• The RAN does not itself discriminate between users who
are associated with a particular operator and those who
are not. This means that all devices capable of operating
on the frequency in question can connect to the RAN.
Authentication
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Authentication
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1. On receipt of an authentication request, the SGSN sends a request to
the AuC to begin the authentication process.
2. The AuC does this by calculating an authentication vector (AV),
which is a sequence of, which includes the following five
components:
• RAND : a random number .
• AUTN : the authentication token, which depends among other things on a
sequence number SQN, which the UE and the AuC synchronise between
them, and the random number RAND .
• XRES : an expected result, to be used by the SGSN to check the
authentication response from the UE .
• CK : a temporary cipher key, to be used to encrypt the user data until the
next authentication request .
• IK : a temporary integrity key, to be used to verify the integrity of
Signaling messages to and from the UE.
3. SQN and RAND are used to prove that the AV has been freshly
generated, to prevent replay attacks based on captured AVs.
Authentication
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