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The 3GPP architecture for data enhances the 2.5-generation General Packet Radio Service model, an intermediate step for transporting data over GSM networks. Two new nodes are defined in GPRS: the serving-GPRS support node (SGSN) and the gateway-GPRS support node (GGSN). The SGSN is at the same level as the mobile switching center, in that it interfaces to the "data" world in the same way that the mobile switching center interfaces to the circuit-switched PSTN (public switched telephone network). The GGSN, meanwhile, communicates with the SGSN to transport the data packets to and from external IP networks, such as the Internet.
For data traffic, the radio-access network (RAN) node communicates with the SGSN, which transfers the IP datagrams from the mobile station or node inside GPRS Tunneling Protocol (GTP) packets. GTP runs over UDP/IP and is used for communication between the SGSN and the GGSN, as well as between the RAN and the SGSN. The SGSNs and GGSNs are connected via a public land mobile network, which is based on IP.
Both IP version 4 and IPv6 are supported in this model. With IPv4, it is more flexible to assign IP addresses dynamically - often in the GGSN, which may in turn interface with a Radius remote-authentication server for authentication and address information. The end node thus communicates directly to the GGSN to obtain its IP address, and subsequently sends packets to be routed to the Internet via the GGSN. The node can connect to multiple GGSNs, depending upon the Internet service provider supplying the connectivity to the Net. The expanded address space of IPv6 allows each node to have a permanent IPv6 address, so there is no need to obtain one from the GGSN.
Mobility within the network is handled via the existing (GPRS-based) mechanisms. This includes changes to the IP home-location register, a database with information about the mobile subscriber, location, addressing and so on. Thus, the current UMTS model relies quite heavily on the existing GPRS model for mobility.
To support mobility with other networks, the Mobile IP model is used. A node has a home network and a visited/foreign network. The node registers its presence to a node in its home network called the home agent. When visiting a foreign network, the node takes on a "care-of" address by communicating with a foreign agent and informs the home agent about this address. To communicate with the end node, hosts send packets to the home agent, which tunnels them to the care-of address, to be processed by the mobile node. The foreign agent, meanwhile, is responsible for advertising the care-of address. In the UMTS model, a GGSN can act as a foreign agent, providing care-of addresses. It can also function as a foreign agent for a visiting mobile station.
When a node is on its home network, it typically obtains an IP address from the GGSN and sends an IP packet with the destination address of a node on the Internet. The packet is sent to the radio-access network node and relayed via GTP to an SGSN. The SGSN relays the packet to the appropriate GGSN, which in turn routes it to the Internet based on the IP address. It is only at the GGSN that the IP packet header is examined for routing purposes. Until this point, the IP packet is just tunneled. If the node is on a foreign network, the care-of address is obtained from the GGSN/foreign agent and the home agent is informed. The home agent tunnels incoming packets to the GGSN/foreign agent and forwards them to the mobile station.
There is an initiative within 3GPP to move to an all-IP network, where even voice calls from the mobile terminal are based on a voice-over-IP paradigm. The Session Initiation Protocol is a strong contender for the call setup in this model. This necessitates the use of media gateways, media gateway control functions and a new entity called the call-state control function, which is analogous to the call-control function in a circuit-switched environment. Thus, the voice traffic will be carried over a media gateway to a circuit-switched network or transported end to end as VoIP.
PDSN (CDMA)
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Like how 3GPP has done ... what about ANSI !!
Compared with the 3GPP IP architecture, the 3GPP2 model is very simple, since it does not use an existing architecture like GPRS. The entire data network architecture is IP based and does not rely on modifying a legacy infrastructure, such as the home-location register in 3GPP. For mobility, the 3GPP2 architecture uses the Mobile IP model.
The data link is based on the Point to Point Protocol. In the future, a simple data link-layer protocol will be used. The IP layer involves two types of access methods: Simple IP and Mobile IP. In Simple IP, the IP address is dynamically assigned from the network and the Internet access is performed directly. With Mobile IP, the home agent can reside in the IMT-2000 service provider network or in a private network.
Unlike the 3GPP model, there is only one network node to service data traffic from mobile stations: The packet data serving node (PDSN) establishes, maintains and terminates PPP sessions to the mobile station. It also provides an IP address for Simple IP - this could be done after interacting with an external entity like a Radius server. Like the GGSN in 3GPP, the PDSN also provides a foreign-agent functionality. It can establish secure connections (via IP Secure) to the home agent. The PDSN maintains the PPP connection when the mobile station moves from one zone to another, interacting with an entity called the packet-control function, with which it establishes a Layer 2 connection. This handoff interaction is completely transparent to the mobile station. In the reference model, this is treated as an IP-based radio network node (RN), which is similar to the RAN in 3GPP.
The 3GPP2 model envisages end-to-end IP connectivity. Even the RN-to-PDSN interface can be based on IP transport or on frame relay/asynchronous transfer mode, using ATM Adaptation Layer 5. A logical connection is established between the RN and PDSN. A user moving from one RN to another will not see the PPP session break; rather, a the logical connection will be moved from the current RN to the new RN.
As in the 3GPP VoIP model, multimedia (including voice) can be transported over an IP infrastructure, while using gateways to communicate with external networks like the PSTN. However, it is bound to take some time to iron out issues related to the data transport model and architecture.
There are some advantages to using an end-to-end IP model. One of the more significant ones is the mapping of quality-of-service parameters from the IP packet through multiple nodes in the network. Another advantage is the use of familiar AAA mechanisms, including Radius. Moving to an all-IP multimedia network using mechanisms like SIP will allow a unified infrastructure that also addresses mobility.
That, at least, is the theory. The split between the 3GPP and 3GPP2 camps is one reason why the move to an all-IP world in the 3G arena is likely to be delayed. It is not easy for equipment vendors to develop a unified network node system that will support the functionality of an SGSN/GGSN and that of a PDSN. Also, the incessant debate about which access method (wideband-CDMA vs. cdma2000) will only serve to delay the 3G migration.
How are they are related
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The Packet Data Serving Node, or PDSN, is a component of a CDMA2000 mobile network. It acts as the connection point between the Radio Access and IP networks. This component is responsible for managing PPP sessions between the mobile provider's core IP network and the mobile station (read mobile phone). It is similar in function to the GGSN (GPRS Gateway Support Node) that is found in GSM and UMTS networks.
Although PDSN is thought of being similar to GGSN in a conceptual sense, but logically its a combination of SGSN and GGSN in the CDMA world. It provides:
Mobility management functions provided by SGSN in the GPRS/UMTS networks) Packet routing functionality (provided by GGSN in the GPRS/UMTS networks)
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