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issues may arise; for example, mixing UTRAN nodes that support only ATM with R5
nodes supporting IP. Another example might be a core network upgraded to IP with the
UTRAN still using ATM. Since ATM itself provides models for IP transport it is perfectly
acceptable for IP messages to be moved transparently across an ATM link usually using
AAL5. The added complication with UMTS is that protocols based on SS7 transport
and ALCAP do not expect to be run over IP. To circumvent this problem for signalling
transport, an SS7/SCTP/IP bearer has been proposed. This can carry both RAN signalling
such as NBAP and RANAP as well as the transport control protocol ALCAP. For user
data (carried in R99 over AAL2) AAL2 over UDP has been proposed.
There are two possibilities for internetworking between IP and ATM: dual stack or by
using an internetworking unit to translate between the nodes. The dual stack con¬guration
is shown in Figure 9.43 for the Iub interface. In this case the RNC will use ATM signalling
with the R99 base station and IP with the R5 base station.
With the Iub interface the dual stack option is the simplest and most effective solution
for interworking. This is because each BTS is expected to support only IP or ATM and

IP UDP CPS CPS CPS CPS CPS
CPS payload
header header header payload header header payload

UDP payload

Figure 9.42 AAL2 over UDP
9.8 IP IN THE RADIO ACCESS NETWORK (RAN) 619


Release 99 BTS Release 5 RNC Release 5 BTS
ATM Stack IP Stack
User User User User
NBAP NBAP
NBAP
data NBAP
data data data

ALCAP ALCAP
STC STC AAL2 AAL2
SSCF SSCF SCTP UDP UDP SCTP
SSCOP SSCOP
IP IP
AAL2 AAL5 AAL2 AAL5
Datalink Datalink
Iub Iub
ATM/L1 ATM/L1 Physical Physical


Figure 9.43 RNC dual stack IP/ATM (Iub)


RNC-B
R5
RNC-A RNC-C
IP ATM
R99 R5
ATM IP



Figure 9.44 Interworking IP and ATM RAN domains


talk to only one RNC. For the Iur interface the dual stack option may not be the most
suitable solution. Figure 9.44 shows why this is the case. The diagram shows two network
domains: RNC-A supports only IP and RNC-C only ATM. RNC-B is connected to both
networks and uses a dual stack to support connections to both networks. A can talk to B,
C can talk to B but with the con¬guration shown A cannot talk to C. This is because the
radio network layer signalling coming from A will be terminated at B; if it is not destined
for B then it will be discarded. This is because B is not con¬gured to forward messages
between the two networks but only to provide connectivity from B to other RNCs directly
connected. To provide full connectivity between the two domains, an interworking unit
(IWU) is required.
An IWU (Figure 9.45) provides protocol translation between the IP and the ATM
domains. The IWU sits between the two domains forwarding the packets from IP to

Release 99 RNC Interworking Unit Release 5 RNC
User User
RNSAP User User RNSAP
RNSAP RNSAP
data data
data data

SCCP ALCAP ALCAP SCCP
MTP-3b STC STC MTP-3b M3UA AAL2 AAL2 M3UA
SSCF
SSCF SCTP UDP UDP SCTP
SSCOP SSCOP
IP IP
AAL2 AAL5 AAL5 AAL2
Datalink Datalink
Iur Iur
ATM/L1 ATM/L1 Physical Physical


Figure 9.45 Interworking unit at Iur
620 RELEASE 5 AND BEYOND (ALL-IP)


ATM and vice versa. Since the transport addressing schemes used on the two domains
are quite different, SS7 codepoints are used as a global addressing scheme across the whole
network. The operation of the IWU is complex since it must not only forward messages
between the two domains, but must also take part in bearer establishment procedures. The
operation of the IWU is still work in progress and is discussed in TS 25.933.



9.9 MULTIPROTOCOL LABEL SWITCHING (MPLS) IN
UMTS
MPLS combines the complexity and high levels of functionality of IP routing, QoS
and security mechanisms with the speed and ef¬ciency of layer 2 switching. The basic
operation of MPLS is as follows. Before transmission a path, referred to as a label switched
path (LSP), is established across the network between sender and receiver using a network
layer or QoS protocol (e.g. OSPF or RSVP). The packets are then switched across this
path using a ¬xed-length label. Since the switching is packet by packet at layer 2 using a
¬xed-length label, it can be done at great speed and implemented in hardware. Services
that can be provided by MPLS include IP routing, QoS and virtual private networks using
MPLS tunnels.
Although MPLS is focused on the delivery of IP services over an a ATM network, the
speci¬cation is ¬‚exible and allows for:

• transfer of data over any combination of layer 2 technologies
• support of all layer 3 protocols.

Since UMTS is designed to deliver multimedia type services and will be using IP
extensively, in particular as operators migrate to R5, the use of MPLS to provide QoS
in the IMS is strongly indicated. 3GPP discusses the application of MPLS to provide
QoS when using IP transport in the RAN (24.933). This is illustrated in Figure 9.46.



IP/MPLS Backbone

Compressor/
Narrowband
BTS
RNC Decompressor
link
Node




Class of service 1 LSP
Class of service 2 LSP
Class of service 3 LSP

Figure 9.46 MPLS in the RAN
9.9 MULTIPROTOCOL LABEL SWITCHING (MPLS) IN UMTS 621


The RNC is connected to the BTS via a broadband routed IP/MPLS cloud, with the last
mile being via a narrowband link, for example an E1 line. MPLS in this case is used to
provide different classes of services for the UTRAN traf¬c. The compressor/decompressor
node provides IP/UDP header compression to optimize the use of the bandwidth on the
narrowband link.


9.9.1 MPLS terminology
Prior to a discussion of the speci¬cs of MPLS operation, it is useful to de¬ne the key
terms used in conjunction with MPLS:

• Label: a short ¬xed-length identi¬er indicating a path.
• Forwarding equivalence class (FEC): a group of packets that should be forwarded the
same way and therefore can be assigned the same label.
• Label binding: mapping between a label and a FEC.
• Label information base (LIB): a database containing label bindings.
• Label switched router (LSR): any router with MPLS functionality.
• Label edge router (LER): LSR at the edge of an MPLS domain.
• MPLS domain: a set of nodes under one administrative domain capable of performing
MPLS routing.



9.9.2 MPLS forwarding
With conventional routing, as a packet crosses the network, each router examines the
header to extract all the relevant information to decide how to forward the packet. Usually,
for IP, it is only the destination IP address that is relevant, but other components can also
have relevance. Since this header inspection is done at each router, it limits the speed and
ef¬ciency of operation.
For MPLS, the LER that accepts the packet to the MPLS domain maps the layer
3 header information to a label using the label binding. This label is then used for
subsequent forwarding decisions. Therefore, once the label is chosen, it is put at the front
of the packet and the header need not be examined again throughout the MPLS domain.
At each intermediary LSR the label is mapped to a FEC; at a minimum this will de¬ne
a new label value and an outgoing interface at the LSR, but it may also control how
the packet is queued and scheduled in the case of QoS provision. Within the RAN, the
RNC and the compressor/decompressor node will act as LERs. The topology of an MPLS
domain is shown in Figure 9.47.
Each LSR receiving a packet will look up the label binding in its LIB, add the new label
then forward the packet on the given interface. This process is illustrated in Figure 9.48.
The packet is received at the LER at the left of the diagram with an IP destination address
of 130.24.45.6. The edge router looks up the IP destination address in its LIB, adds the
622 RELEASE 5 AND BEYOND (ALL-IP)


MPLS Domain
LIB
MPLS Edge Router MPLS Edge Router
Routing table with
Adds label Strips label
preconfigured paths
LIB LIB
LIB
LSR
LIB
LER LSR LER
Non-MPLS
Network LSR



Figure 9.47 MPLS domain



Destination address
130.24.45.6

Label edge
Label edge
043 IP packet router
router
1
192.7.6.0
Label information base
Label switch
Destination Outgoing Outgoing 2
router
address lable interface
3
192.7.6.0 7 1
Label information base
128.4.0.0 10 1
Incoming Outgoing Outgoing Label edge
145.20.0.0 43 1 lable lable interface router
007 020 1
130.24.0.0 43 1
043 024 3 128.4.0.0
010 022 2
IP packet
024
100 120 1


1 145.20.0.0
Label edge
router
2
IP packet
Label information base
130.24.0.0
Incoming Outgoing Outgoing
lable lable interface
024 - 1


Figure 9.48 MPLS forwarding



label value (43) and then forwards the packet on interface 1 to the LSR in the centre. The
LSR then looks up the value 43 in its own LIB and determines it must forward the packet
on interface number 3 with a new label value of 24. The edge router at the lower right
then has to forward the packet. It looks up the label binding and ¬nds that the packet
is to leave the MPLS domain (indicated by a null entry for the outgoing label). It then
examines the IP header and determines that the packet should be forwarded on interface
number 2. Note that it is only at the edge of the MPLS domain that the layer 3 header
has to be examined.
FURTHER READING 623


9.9.3 Label switched paths (LSP)

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