Path optimization in packet-based telecommunication network

Details Path optimization in packet-based telecommunication network Path optimization in packet-based telecommunication network

1999-12-17

2004-10-26

Ho, Duc (Department: 2665)

Multiplex communications

Data flow congestion prevention or control

Control of data admission to the network

C370S238000, C370S236200, C370S248000

Reexamination Certificate

active

06810013

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to connection-oriented packet-based telecommunication networks, such as ATM (Asynchronous Transfer Mode), and particularly to route optimization in such networks.
BACKGROUND OF THE INVENTION
At present, there are two very interesting development trends in telecommunication: mobile communications and broadband networks in which bit rates typically exceed 2 Mbit/s. An example of broadband networks is Broadband Integrated Services Digital Networks.(B-ISDN), the transfer mode of which is selected to be Asynchronous Transfer Mode (ATM). The ATM is a switching and multiplexing solution, particularly relating to a data link layer (i.e. OSI Layer
2
, from here on called the ATM layer). ATM enables an implementation of a connection-oriented packet network in the B-ISDN networks,
In ATM data transmission-the end user's data traffic is carried from a source to a destination by virtual connections. Data is transferred over switches of the network in standard-size packets of 53 bytes, the packets being called ATM cells. The structure of an ATM cell is illustrated in FIG.
1
. An ATM cell contains a header of 5 octets and an information field of 48 octets containing actual payload. The main object of the header is to identify a connection number for a cell sequence forming a virtual channel to a particular call. A physical layer (i.e. OSI Layer
1
) may comprise several virtual paths which are multiplexed in the ATM layer. The virtual paths are identified by a VPI (Virtual Path Identifier). Each virtual path may comprise a number of virtual channels that are identified by a VCI (Virtual Channel Identifier). The header contains also other fields, such as an HEC (Header Error Control), a GFC (Generic Flow Control), a CLP (Cell Loss Priority) and a PT (Payload Type). The ATM cell contains indirectly the information on the receiver's address, each cell thus being an independent data transfer unit. The number of cells transferred in a time unit is proportional to the user's bandwidth requirements.
The ATM is a connection-oriented traffic technique, but because there is no connection before it is established, a connection establishment request shall be routed from a source through the ATM network to a destination approximately in the same way as packets are routed in packet switched network works. After the connection has been established the cells travel along the same virtual path during the connection.
A third development trend is to introduce wireless data transmission (wireless ATM) and mobility into the ATM networks (wireless ATM). The present B-ISDN and ATM standards do not support as such the additional features required by wireless communication, but different solutions to implement for example mobility management in connection with wireless ATM have already been presented. The aim has been to add wireless data transmission and mobility to the ATM network without significant changes in the existing ATM standards and networks. Such solutions have been presented in the applicant's copending Finnish patent applications 971178 and 970602, for example.
In PLMNs (Public Land Mobile Network) radio interfaces have conventionally been narrowband interfaces. The transmission systems of mobile networks have conventionally been implemented by circuit-switched connections in a star or tree network configuration. In order to increase the capacity and flexibility of the transmission systems, the use of different broadband packet-switched transmission systems or ATM technique in mobile networks has also been proposed, for example in WO 9400959, EP 0366342 and EP 0426269. A possible future development trend is mobile systems having a broadband radio interface. Then a broadband transmission system of the mobile system is also needed, while a potential alternative is ATM technique.
In mobile networks and in a wireless ATM network a terminal does not have any fixed access point to the network, but the terminal and the access point may move in the network. When a virtual connection has been routed to the terminal through the ATM network, the routing must also be changed or extended from an old access point (a base station, for example) to a new one. This procedure is called handover or handoff. In a hard handover, the data transmission is interrupted when the connection is connected from one access point to another. In a soft handover the continuation of the data transmission is secured by the terminal having a connection both with the old and the new AP (Access Point) simultaneously during handover.
Handover and particularly several consecutive handovers can lead to non-optimal routing of a virtual connection. Non-optimal routing refers to a situation in which the virtual connection is not routed directly from one point to another, but the connection travels a longer winding path which is determined according to handover. In the worst case, the virtual connection path has a loop, which starts from and ends at the same network node. Unnecessary loops on the connection path cause delay and use network resources, which is why they should be avoided.
The following simple example illustrates how a routing loop could be produced in a wireless ATM network. Two base stations BTS
1
and BTS
2
have been connected to an ATM switch
20
. BTS
1
and BTS
2
are also equipped with ATM switching functions. Let us first assume that a mobile station MS has a radio connection with the base station BTS
1
and a virtual ATM connection between the base station BTS
1
and port
2
of the ATM switch
20
. Inside the ATM switch port
2
is connected to port
1
from which a virtual connection has been established (through the ATM network) to a second party. Next, a so-called path extension (PE) handover is performed from the base station. BTS
1
to base station BTS
2
. Let us assume in the example case that there is no direct ATM link between the base stations. Then BTS
1
first routes the connection back to port
3
of the ATM switch
20
from where it is routed through port
4
to the base station BTS
2
. The result is a non-optimal routing, when the connection path has a loop from the switch
20
to the base station BTS
1
and back. An optimal routing would require bypassing the loop by connecting the connection directly from port
1
to port
4
. Similar loops can be produced in any part of the ATM network.
In order to be able to remove the above described loops, they should first be detected. In the ATM networks a global identifier, which is transferred in the ATM cells, is used for the connection and the loops can be detected by this identifier. If the network node detects that the same global identifier appears at two output ports, it may conclude that the matter referred to is a loop that should be removed. However, in cases where a global identifier is not available, it is difficult to detect that a loop exists. Furthermore, when the loop has been detected, the optimal connection path should be switched in such a manner that transferable cells do not disappear or that their order does not change.
SUMMARY OF THE INVENTION
An object of the invention is thus to develop a method and an equipment implementing the method so that said problems will be solved.
The objects of the invention are achieved with a method for optimizing a connection path in a packet-based telecommunication network in which an access point of a terminal to the network and thereby the routing of a connection path can change during connection. The method is characterized by
adding a special packet to an end-to-end packet flow in a network node along the connection path, said network node being able to locally identify said special packet,
detecting the looping of the connection path back to said network node if said special packet later returns to said network node,
optimizing the connection path by switching the connection path to bypass a loop inside said network node.
The invention further relates to an arrangement to optimize a connection path in a packet-based telecommunication networ

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