15th May 2000
Department of Computer Science and Engineering
Helsinki University of Technology
2. Macro Mobility Issues
2.1. Mobility Management in Mobile IP and GPRS Tunneling Protocol
3. Micro Mobility Issues2.1.1. Tunneling Methods to Support Mobility2.2. Convergence between Mobile IP and GPRS Tunneling Protocol
2.1.2. Tunneling End-Point
2.1.3. Default Router Functionality and Connectivity to External Networks
2.1.4. Registration and Tunnel Establishment2.2.1. IP Host Roaming and Accessing External Networks
2.2.2. PPP Support
2.2.3. Dynamic IP Address Allocation
2.2.4. Functions on Cellular Network Operator Level
2.2.5. Convergence Status in GPRS/UMTS Release '99 Standards
3.1. Mobility Management States
3.2. Paging Area Concept
3.3. Lower Layer Issues
3.4. Packet Routing
3.5. Handoff Decision
Macro Mobility Management
5.2. Micro Mobility Management
One key component in telecom networks has been an exchange which handles, e.g., call control, call switching, charging and call routing. On another hand, IP router has been one key building block of the Internet. The functionality of a typical IP router is very different from a telecom exchange. A basic IP packet forwarding applied in the Internet does not require a concept of end-to-end connection from router's point of view. The principles of resource reservation are also very different in the Internet and in the telecommunications networks. The Internet has typically worked by using a best effort traffic model. Every packet is treated (forwarded or discarded) equally. This has been a very simple and efficient model and several arguments has been stated against any need for a more complicated system . Telecom networks, on another hand, have traditionally made an explicit reservation of transmission resources for each call.
Unfortunately, the existing telecom exchanges are not very capable of supporting bursty and high speed data traffic. However, from commercial point of view, a reuse of existing fixed access technology is very important. E.g., asymmetric digital subscriber line (ADSL) is reusing the access infrastructure of the existing public switched telecommunications network (PSTN). The existing telecom exchanges do not have any role in packet forwarding from a ADSL subscriber to an IP network (or vise versa). There have also been several attempts optimize cellular networks for packet data. One such example is General Packet Radio Service (GPRS), which is an enhancement to the existing GSM network (Global System for Mobile Communications) .
IETF Mobile IP working group is aiming to understand data service in cellular systems. However, IETF is not focused only on cellular networks. One must also remember that not all wireless networks are cellular networks. If we compare the existing cellular technologies, e.g., to Wireless LANs (WLAN), we can point several differences. One important difference is that the existing cellular technologies are commonly built for wide area coverage. Other typical differences are the principles of resource reservation: cellular networks carry out explicit resource reservation over the radio interface. In this sense, cellular networks can be seen as a wireless extension to the PTSN. WLANs, on another hand, are reusing many of the principles applied in fixed local area network technologies (e.g. Ethernet).
A clear drawback of the existing cellular networks is a limited data rate: for example the most commonly applied GSM data service is only a 9.6 kbps circuit switched connection. The coverage offered by WLAN technology is limited, but the bit rates are higher. A typical existing WLAN implementation (compliant to IEEE 802.11) offers 2 megabits per second in a shared access manner .
Figure 1 present the architecture studied in this paper.
This report studies mobility management methods applied in GPRS and other
wireless, non cellular networks. On macro mobility level, GPRS tunneling
protocol and Mobile IP are compared. On micro mobility level, Cellular
IP is compared to the methods applied in GPRS. Where needed within the
scope of this study, characteristics of Wireless LAN radio access (IEEE
802.11) is compared to the characteristics of GPRS radio access.
Figure 1. Architecture studied in this paper.
In GPRS (as in most cellular systems), network is divided into two parts: core network and radio access network. In GSM, radio access network is called base station subsystem. In core network, from packet forwarding point of view, GPRS specific functionality is divided into two network elements. From network topology point of view, Serving GPRS Support Node (SGSN) has direct connectivity to radio access network. Gateway GPRS Support Node (GGSN) has direct connectivity to the external IP networks (e.g. Internet and intranets). GPRS core network in general carries out many similar macro mobility issues which are supported by Mobile IP. This will be discussed in Section 2.1. Micro mobility issues will be further discussed in Section 3.
GPRS tunneling protocol (GTP) carries out many similar functions as Mobile IP. The following sections point out some similarities and differences when Mobile IP and GTP are compared from mobility management point of view.
There are different methods of implementing the tunnel over intermediate
routers. In Mobile IP, both home agent and foreign agent must support tunneling
datagrams using IP in IP encapsulation . Optionally foreign agent and
home agent may also support GRE and minimal encapsulation.  In GPRS,
GTP for user plane (which is tunneling protocol as such) is carried over
UDP/IP or TCP/IP . TCP/IP is used only if X.25 is carried over GPRS,
which most likely will not be widely used commercially. In case of IP over
GPRS, GTP carried over UDP/IP.
In cellular environment, radio resources are scarce. From a radio resource
perspective, using of a foreign agent care-of addresses is more efficient.
This is because in co-located care-of addresses, the tunnel is terminated
at the mobile node . If the tunnel is transported over the radio interface,
this means additional IP headed overhead which consumes limited cellular
radio capacity. GTP is similar to a foreign agent care-of address, as GTP
tunnel is not terminated on mobile station but on SGSN. In GPRS, a tunnel
is identified by a tunnel endpoint identifier and a SGSN/GGSN address.
In Mobile IP, the home agent is a router on a mobile node's home network
and it does not have to tunnel the packets which are destined to mobile
nodes that have direct layer 2 connection. In GPRS, there is no similar
"home network" concept from IP routing point of view. GGSN always tunnels
IP packets to SGSN.
According to GPRS specifications, it is possible for a mobile station
to be attached to the GPRS network even though it does not have an activated
GTP tunnel for data transport. This might be necessary for example when
a subscriber wants to be able to receive short messages without using network
resources (e.g. IP address). A function similar to Mobile IP registration
is built on GPRS specific data connection activation procedure. During
this procedure, a GTP tunnel is established between SGSN and GGSN. In Mobile
IP (RFC 2002), it is not explicitly described how a tunnels are established.
If IP-in-IP tunneling is used, there can be one tunnel for each IP
address pair (e.g. HA and FA).
Even GTP is not only a mobility management protocol. It also carries
out e.g. some session management and tunnel establishment functions. There
are some important additions to Mobile IP functionality beyond basic RFC
2002 which are very important when considering GTP replacement by Mobile
IP. The following sections describe some of the necessary additions.
One requirement for Mobile IP before it can be deployed in future cellular
networks is a need to interwork with and operate in co-existence with existing
protocols in the cellular networks . To allow Mobile IP to be a mobility
solution which supports many different kinds of access networks or technologies,
Mobile IP functionality shall be independent of the access network technology
. A separation of the authentication procedures is motivated by the
fact that radio resources are scarce, and an access network operator may
not want to allow Mobile IP signaling until the access network in itself
has accepted to provide resources for a mobile node .
In GSM/GPRS, radio access network consists of base stations and base station controllers. Cellular IP network consists different kinds of nodes. Base station is a Cellular IP node that has a wireless interface. Gateway is a Cellular IP node that is also connected to a regular IP network. Cellular IP Gateway serves also as the mobile host's foreign agent and it relays it's packets both up and downlink . Cellular IP nodes route IP packets inside the Cellular IP Network.
Cellular IP defines also many mobility management concepts which are similar to those applied in existing cellular networks. The following sections describe the micro mobility management in existing IETF documentation and compares it to GPRS. Where needed within the scope of this study, characteristics of WLAN radio access (IEEE 802.11) is compared to the characteristics of GPRS radio access.
In GPRS, ready state is supervised with a timer. A mobile station changes from ready state to standby state when the ready timer expires. The ready timer is reset when data is transferred between SGSN and mobile station . Also in Cellular IP, the mobile station returns to idle state when it has not received or transmitted any data packets for some time . Both in GPRS and in Cellular IP, a state transition timer is not statically defined. These kinds of parameters have typically been a part of cellular network planning. Thus in GPRS, SGSN may change the length of the ready timer by transmitting a new value .
Mobility management state functionality is important to minimize the
battery consumption in a mobile station. For example a GPRS mobile station
standby state has to monitor only a control channel. In IEEE 802.11 there
is no logical channel separation within one carrier. However, IEEE 802.11
standard directly addresses the issue of power saving and defines a mechanism
which enables stations to go into sleep mode for long periods of time without
losing information. The base station periodically transmits information
to power saving stations if they have frames buffered at the base station.
If there is a frame buffered at the access point, then the station sends
a polling message to the base station to get these frames. As the buffering
information is sent periodically, this function requires that stations
are synchronized . Some other radio technology specific issues are described
in Section 3.3.
In Cellular IP, paging area is a set of base stations. Idle mobile hosts crossing cell boundaries within a paging area do not need to transmit control packets to update their position . In GPRS similar functionality is provided by a routing area concept. GPRS mobile station executes mobility management procedures to inform the SGSN when it has entered a new routing area. The mobile station does not inform the SGSN on a change of a cell in the same routing area . Also Cellular IP has a similar functionality: an idle mobile host moving to a new base station transmits a paging update packet only if the new base station is in a new Paging Area .
In Cellular IP, wide area mobility occurs when the mobile host moves
between Cellular IP networks. The mobile host can identify Cellular IP
networks by the Cellular IP network identifier contained in the base stations'
beacon signals. The beacon signal also contains the IP address of the gateway.
When a mobile host has received this broadcast information, it can send
a registration request to the gateway . Mobile host can also send a
Mobile IP registration message to its home agent, specifying the gateway's
IP address as the care-of address. Alternatively, the gateway can register
at the home agent on behalf of the mobile host . In GPRS, a mobile station
can initiate a cell re-selection which may change the base station controller
(or both base station controller and SGSN). If SGSN is not changed, intra
SGSN routing area update procedure is carried out. If SGSN is changed,
inter SGSN routing area procedure is carried out. The latter naturally
changes the end point of a GTP tunnel which is associated to this mobile
In IEEE 802.11, one carrier provides a transmission medium which supports 2 Mbps per second in a shared access manner . In many ways, this transmission medium is similar to a segment in a fixed LANs. Instead of collision detection which is used in Ethernet, IEEE 802.11 applies collision avoidance with a positive acknowledge mechanism . From resource reservation point of view, IEEE 802.11 as such has many similarities to fixed LAN technology. Typically no explicit resource allocations are made, but the usage of radio resource is based on shared access principles. If a mobile host does not have anything to send or to receive, it uses radio resources only for control signalling. Also in GPRS, allocation of radio resources is based on the needs for actual packet transfer. A GPRS packet data traffic channel can be allocated for one mobile station at a time. Up to eight packet data traffic channels may be allocated for one mobile station at the same time. GPRS release '97 supports four coding schemes for packet data traffic channels. The data rates of these coding schemes are 9.05 kbps, 13.4 kbps, 15.6 kbps, and 21.4 kbps .
In GPRS, there are separate logical channels for transmitting control information and signaling. Some of them are applied in shared access manner but there are also logical channels which are associated only to one mobile station. Random access channel is used to allocate radio resources and it is applied only uplink. Access grant channel is used on opposite direction for radio resource assignment. Also paging has a separate logical channel, which is naturally a downlink only channel . In WLAN there is not this kind of logical channel structure. All control information and signaling is transmitted over the same shared channel.
One important functionality which is carried over radio interface is
broadcasting of system information. GPRS applies a separate logical broadcast
channels whereas IEEE 802.11 applies beacon frames. All three standards
discussed in this paper (Cellular IP, GPRS and IEEE 802.11) define some
broadcast information. For example, Cellular IP base stations must periodically
transmit beacon signals to allow for mobile hosts to identify an available
base station. In each beacon signal, there is a paging area identifier,
which enables mobile hosts to notice when they move into a new paging area
. Similar functionality is also applied in GPRS for routing areas .
Other typical broadcast information in cellular networks include network
identification, cell identification and parameters for cell re-selection.
In Cellular IP, broadcast information include base station's layer 2 address
and gateway's IP address.
In GSM/GPRS radio access network IP packet is encapsulated over GPRS specific frame structures. Routing of these frames is partially based on topology. One base station is always connected to one base station controller. One base station is always connection to one SGSN. This topology information is used to route the uplink frames. When mobile station is in ready state, SGSN is aware on which cell mobile station is attached. Also base station controller is aware on which cell mobile station is attached. Based on this information it is possible to route the downlink frames to a mobile station in ready state.
In Cellular IP, paging cache is maintained by some Cellular IP nodes and it is used to route packets to mobile hosts. By deploying two caches (paging cache and routing cache), the granularity of location tracking can be different for idle and active mobile hosts. Mobile hosts that are not actively transmitting or receiving but want to be reachable for incoming packets, let their route cache mappings time out but maintain paging cache mappings. IP packets addressed to these mobile hosts will be routed by paging caches. Paging caches have a longer time-out value than route caches and are not necessarily maintained in every node. On the path from the gateway toward the mobile host, the paging packet is broadcast by all nodes it passes. The paging packet is an ordinary IP packet. The set of cells that are reached by the paging packet forms a paging area . In GPRS, if a mobile station is in standby state, SGSN pages it before it transfers data to that mobile station. The paging procedure moves the mobile station to ready state and allows the SGSN to forward downlink data towards the mobile station.
Packet acknowledgment and buffering functionality typically have interactions. If a packet is not acknowledged in certain period of time, it can be retransmitted if it is still stored in a buffer. Buffering can be carried out on different network elements. It is typically preferable that buffering is not carried out very far away from that place where handoff takes place. As described previously, IEEE 802.11 acknowledges on MAC layer. IEEE 802.11 MAC layer is typically implemented in the base station. Thus, one obvious location where buffering can be implemented is base station. How much buffering is to be applied, should be a flexible network configuration parameter. There are certain limits how much delay different higher level protocols and applications can tolerate. For example Voice over IP and streaming technologies require a real time data at a receiver with a strict tolerance. If this tolerance is missed, buffering does not bring any additional benefit. On another hand, some other protocols might tolerate longer packet delays but not any packet loss.
In Cellular IP, when a mobile host switches to a new base station it sends a route update packet to make the chain of cache bindings to point to the new base station. Packets that are traveling on the old path will be delivered to the old base station and will be lost. Although this loss may be small it can potentially degrade TCP throughput. This kind of handoff, when the mobile switches all at once to the new Base Station is called "hard" handoff. To improve the performance of loss sensitive applications, another type of handoff may be introduced, called "semi soft" handoff. During semi soft handoff a mobile host may be in contact with either of the old and new base stations and receive packets from them. Packets intended to the mobile host are sent to both base stations, so when the mobile host eventually moves to the new location it can continue to receive packets without interruption .
There is one potential reasoning why GPRS, Cellular IP and IEEE 802.11
all support mainly mobile controlled handoff. All of them use "on demand"
resource reservation principle. This is often reasonable as those resource
reservation principles that are applied for circuit switched bearers do
not provide optimal radio resource usage for packet switched traffic. Thus,
it is not that straightforward to apply similar handoff mechanisms for
packet data as for circuit switched bearers. In general, network has to
be aware, what kind of bearer or resource reservation is associated to
each mobile station. Actually we come back to a previously presented issue
in this report. Internet has typically worked by using a best effort traffic
model. This has been a very simple and efficient model and several arguments
has been stated against any need for a more complicated system . Similar
argument may also be applied for wireless Internet access. However, this
argument might not be as applicable in cellular environment where radio
resources are scarce. In hot spot areas, where higher bit rate WLAN technologies
are used, this argument is at least more applicable.
GTP is an integral part of first phase GPRS implementations. One possible
scenario is that Mobile IP will replace GTP at some point of time. There
are several issues beyond RFC 2002, which are needed to fulfil this. Many
of these are related on how to access existing external networks. One such
GPRS specific mechanism is Access Point Name. Access Point Name is visible
to the end user and it has a global significance between operator networks.
These all are issues, which make the replacement of functionality more
challenging. Backward compatibility with GPRS specific functionalities
such as Access Point Name can be one of the trickiest parts of GTP replacement.
Cellular IP is currently an Internet draft. It can be stated that Cellular IP is more applicable in those environments where hosts are very mobile and the network consists of cells with small radius. The core part of Cellular IP is mobility management states and paging area concept. In cellular network, the main reason for introducing this kind of functionality is reducing the control signaling and limiting the power consumption of the mobile hosts. Cellular IP alone can reduce signaling load but as such it cannot reduce significantly power consumption. It is up to radio access technology how a mobile host can apply a sleep mode functionality. For example in IEEE 802.11 this is carried out by buffering frames in base stations and broadcasting information about these frames periodically in beacon frames. However, from implementation point of view, it can be discussed how well PCMCIA card technology can use this sleep mode functionality. PCMCIA cards consume typically power even if they are only activated from operating system’s point of view.
One key question from micro mobility management’s point of view is that
how much functionality really should be carried out in IP protocol level.
A typical radio access technology provides layer 2 functionality with error
correction, buffering, retransmission and acknowledgement. Handoff functionality
can also be built using layer 2 switching functionality. Additional IP
level functionality beyond to basic Mobile IP functionality should be justified
with clear benefits relative to additional complexity. For example hierarchical
mobility management can be used to limit the need for subsequent registrations.
Cellular IP can be used to reduce load for control signalling. Some key
functionality such as power consumption reduction requires interaction
with layer 2 functionality. There are also arguments which are more related
to installed base issues than pure technical protocol issues as such. The
existing corporate environment relies heavily on layer 2 switching (Ethernet).
For example WLAN implementations in the near future will most likely be
optimised with this kind of environment. Bringing IP level functionality
including management of routing tables at a very low level network element
is certainly possible but a significant change to existing situation.
| Geoff Huston, ISP Survival Guide. John Wiley & Sons Inc, New
York, 1999. ISBN 0-471-31499-4
< Not available online >
 Markus Peuhkuri, IP Quality of Service, 5.10.1999.
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Services and System Aspects;
 Brenner, P., A Technical Tutorial on the IEEE 802.11 Protocol, 18.7.1996,
 IETF Mobile IP Working Group, IP Routing for Wireless/Mobile Hosts.
Checked at 10th March 2000.
 C. Perkins, IP Mobility Support, RFC 2002, October 1996.
 A. Campbel et al, Cellular IP, January 2000.
 3rd Generation Partnership Project; Technical Specification Group
Services and System Aspects;
 B. Aboba et al, The Network Access Identifier, RFC 2486, January
 J. Solomon et al, Mobile-IPv4 Configuration Option for PPP IPCP,
RFC 2290, February 1998.
 W. Simpson et al, The Point-to-Point Protocol (PPP), RFC 1661,
 Eva Gustafsson et al, Requirements on Mobile IP from a Cellular
Perspective, June 1999.
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GSM 03.64 version 126.96.36.1998.
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