I.366.2 VOICE TRUNKING FORMAT OVER MPLS 315
TDM-MPLS frames, so that to provide a low end-to-end transfer delay and a low packet loss probability.
12.9 I.366.2 VOICE TRUNKING FORMAT OVER MPLS
As we have seen, AAL 2 can be used to multiplex many voice calls over the same ATM connection. To that effect, the AAL 2 SSCS for trunking described in Section 12.5 is
needed in order to convert the voice traffic and signals into packets at the transmitter, and extract the voice traffic and signals from the packets at the receiver. These packets
are in the form of CPS-packets, which are transmitted over ATM. This implementation agreement, assumes the presence of an AAL 2 service-specific convergence sublayer for
trunking, but instead of carrying the CPS-packets over ATM, they are carried over MPLS. The implementation agreement, therefore, is only concerned with the transport of AAL 2
CPS-packets over MPLS, and in view of this, it is commonly referred to as AAL 2 over MPLS A2oMPLS
. The reference architecture for this implementation agreement is shown in Figure 12.21.
The A2oMPLS functionality is implemented in a gateway GW, which can be a line card in a device that implements the AAL 2 SSCS for trunking. The device is attached to
one or more LSRs, and the gateways are interconnected over the MPLS network via bidirectional point-to-point LSPs.
In the AAL 2 CPS, the CPS-packets are packed into CPS-PDUs, and each CPS-PDU is carried in a separate ATM cell. In the A2oMPLS architecture, multiple CPS-packets
can be placed onto the same frame, known as the A2oMPLS frame, and transported over an LSP. The structure of the A2oMPLS frame is shown in Figure 12.22. The following
fields have been defined:
GW GW
GW
MPLS Network
LSR LSR
LSR LSR
PSTN PSTN
Figure 12.21 The A2oMPLS reference architecture.
Outer label
CPS-packet CPS-packet
. . .
Reserved Length
Seq. number 10-15
16-31 Inner
label A2oMPLS
header 0-9
Figure 12.22 The A2oMPLS frame structure.
316 VOICE OVER ATM AND MPLS
• Outer label
: This is the MPLS encapsulation that is required if the underlying layer 2 network is PoS or Ethernet.
• Inner label
: This is an optional MPLS label encapsulation that is used to increase the number of multiplexed voice calls onto the same LSP. This label is only meaningful to
the transmitting and receiving GWs, and is not used by the LSRs in the MPLS network. •
A2oMPLS header : The header consists of a reserved field bits 0 to 9, a length field
bits 10 to 15, and a sequence number field bits 16–31. The length field is used in the same way as in the TDM-MPLS header described above. It is used to indicate the
length of the A2oMPLS frame header and payload in case padding is employed to meet minimum transmission unit requirements of layer 2 of the MPLS network. It must
be used if the A2oMPLS frame length plus the layer 2 overhead is less than 64 bytes, and it must be 0 if this length exceeds 64 bytes. The 16 bits sequence number can be
used to guarantee ordered frame delivery.
• CPS-packets
: These have the same structure as shown in Figure 3.20. In A2oMPLS up to 248 different voice calls can be established over the same LSP.
The optional inner label can be used in order to increase the total multiplexed voice calls over the same LSP. Recall from Section 3.7.2 that each voice call transported over an
AAL 2 connection is associated with a channel identifier CID. The CID value is carried in each CPS-packet so that it can be associated with the correct voice call. CID values
are allocated as in AAL 2 see Section 3.7.2.
The CPS-packets are carried in a A2oMPLS frame in any arbitrary order. The procedure of populating an A2oMPLS frame is similar to the one in Section 3.7.2 used to populate
a CPS-PDU. That is, at the transmitter, CPS-packets are placed in the payload of an A2oMPLS frame until either the frame reaches a maximum size or a timer expires. The
frame is then transmitted over the LSP to the receiver A2oMPLS, where the CPS-packets are retrieved from the frame’s payload and delivered to the appropriate user.
PROBLEMS
1. What are the main sources for jitter and what is its effect on the QoS for voice over packet? 2. Explain how the QoS of a phone call over a connection-oriented network, such as ATM and
MPLS, can be guaranteed. 3. Explain, how an ISP provides QoS for a phone call over an IP network.
4. Explain the difference between switched mode and non-switched mode described in the ATM trunking using AAL 2 for narrowband services
specification. 5. The AAL 2 SSCS for trunking provides silence removal. Describe how this is done. Hint:
describe the relevant User function and the SSCS SID packet. 6. Describe the demodulationremodulation function of the AAL 2 SSCS for trunking used for the
transport of facsimile. 7. A T1 signal is transported using the TDM transport over MPLS using AAL 1 implementation
agreement. Calculate the delay to fill in a TDM-MPLS frame using the unstructured data transfer scheme and the percent overhead assuming that n subframes are transported in each TDM-MPLS
frame, for n = 1, 2, . . ., 31. The overhead consists of the TDM-MPLS overhead and a 30-bit MPLS label encapsulation. The percent overhead is the total number of transmitted overhead
bytes divided by the entire length of the TDM-MPLS frame plus the MPLS label encapsulation.
Bibliography
Most of the material in this book, except for Chapter 10 which deals with the new OBS architecture, comes from existing standards. In this section, we give a bibliography of
relevant standards and other references organized per topic. Browsing the Internet for further information is also encouraged.
SONETSDH AND GFP CHAPTER 2
The SONETSDH specifications can be found in References 1 and 2. References 3 and 4 describe how PPP frames can be transmitted over SONETSDH. Finally, References 5
and 6 describe the generic framing procedure and the link capacity assignment scheme.
1. Synchronous Optical Network SONET: Physical Interface Specifications, ANSI T1.105.06, 2000.
2. Network Node Interface for the Synchronous Digital Hierarchy SDH, ITU-T Recom- mendation G.707, October 2000.
3. PPP over SONETSDH, IETF RFC 2615, June 1999. 4. PPP in HDLC-Like Framing, RFC 1662, July 1994.
5. The Generic Framing Procedure GFP, ITU-T Recommendation G.7041, October 2001.
6. Link Capacity Adjustment Scheme LCAS for Virtual Concatenated Signals, ITU-T Recommendation G.7042, October 2001.
ATM NETWORKS CHAPTERS 3, 4, AND 5
The specifications for the ATM architecture and its adaptation layers are described in References 7 to 11. The classical IP and ARP over ATM specification is described in
Reference 12. References 13 and 14 describe how multicasting can be implemented over ATM. This topic is not covered in this book, but it can be found in my earlier book An
Introduction to ATM
. References 15 and 16 give the specifications for congestion control in an ATM network, and References 17 and 18 describe the signaling protocol for setting
up a point-to-point connection over the UNI. Finally, References 19 and 20 describe the signaling protocol for setting up a point-to-multipoint connection over the UNI and the
Connection-oriented Networks Harry Perros
2005 John Wiley Sons, Ltd ISBN: 0-470-02163-2
