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c) A new DOCSIS MAC frame begins immediately after the pointer_field, followed by ¬ stuff-bytes, followed by another DOCSIS MAC frame.
6. Explain the difference between FTTH and FTTB/C.
7. Explain how ranging is used in an APON to place each ONU at the same virtual distance.
8. In an APON, explain how the round robin algorithm can be modified so that grant permits are handed out only to those ONUs that have an ATM cell to transmit.
Voice Over ATM and MPLS
Voice over packet solutions have been developed for the IP network, ATM, frame relay, and MPLS. In this chapter, we explore the topic of voice over ATM and voice over MPLS. Both ATM and MPLS are suitable technologies for voice over packet because they can provide QoS, a necessary requirement for real-time traffic such as voice.
There are several reasons why it is beneficial to migrate voice from the TDM network to a packet-switching network. For instance, national and international operators typically have an extensive public switched telephone network (PSTN); they also operate data networks, such as IP with or without MPLS, frame relay, and ATM. Migrating voice to their data network permits them to maintain a single group of engineers and managers to run the network, rather than two separate groups, one for the PSTN and one for the data network.
Voice over packet technologies can also used by carriers, known as a competitive local exchange carrier (CLEC), and value added network suppliers, to provide communication services in competition with pre-existing operators, known as the incumbent local exchange carrier (ILEC). Typically, CLECs and value added network suppliers do not own the transmission infrastructure. In view of this, they either buy bandwidth from an ILEC, or form joint ventures with companies who have the right-of-way or have already deployed a network. In the case of a CLEC, therefore, integration of voice and data service makes sense due to cost and limited bandwidth availability. For instance, within the local loop, voice over packet permits a CLEC to provide voice and data services over cable modems, ADSL and APON, in addition to the voice and data services provided by the ILEC.
Voice over packet also provides a cost-performance solution for cellular operators who have to interconnect their cell sites and message switching centers (MSC). Finally, in enterprise (private) networks a significant portion of the traffic is voice. Since a company buys bandwidth at commercial rates, integrating voice and data is a cost-effective solution.
The ATM Forum has defined several specifications for transporting voice over ATM. These standards can be organized into two groups. The first group of specifications, referred to as ATM trunking for voice, deals with the transport of voice over ATM between two telephone networks. The second group of specifications deals with how to provide voice over ATM to a user at a desktop or to a user over ADSL. In this chapter, we describe two of the ATM trunking for voice specifications: circuit emulation services (CES) and ATM trunking using AAL 2 for narrowband services. Circuit emulation services emulate a TDM link, such as a T1 or E1 link, over an ATM network using AAL 1. The basic
Connection-oriented Networks Harry Perros © 2005 John Wiley & Sons, Ltd ISBN: 0-470-02163-2
VOICE OVER ATM AND MPLS
operation of circuit emulation services and AAL 1 is described in Section 3.7.1. The ATM trunking using AAL 2 for narrowband services specification is used to transport voice traffic between two distant private or public telephone networks
The MPLS and Frame Relay Alliance has so far defined two different specification for voice over MPLS. These two specifications use ATMís AAL 1 and AAL 2 protocols. The first specification deals with circuit emulation services over MPLS, and it makes use of AAL 1. The second specification deals with the transport of voice over MPLS and it uses AAL 2. Both specifications are described in this chapter.
Below, basic telephony concepts and signaling protocols are reviewed. Subsequently, we describe two ATM specifications: circuit emulation services (CES) and ATM trunking using AAL 2 for narrowband services. The latter specification is based on two specially designed AAL 2 service-specific convergence sublayers (AAL 2 SSCS for trunking and segmentation and reassembly SSCS [SEG-SSCS]). Because these two SSCS specifications play a very important role in AAL 2 trunking, they are also described in detail. The chapter concludes with the two specifications proposed by the MPLS and Frame Relay Alliance.
To understand the voice over ATM and MPLS solutions, the reader has to be familiar with basic telephony terms and signaling protocols that are used in the existing telephone system. In this section, we first review some of the basic terms used in telephony and describe the basic signaling steps involved in placing a call. Subsequently, we briefly present the channel-associated signaling (CAS) scheme, the signaling system no. 7 (SS7), narrowband ISDN (N-ISDN), and the digital subscriber signaling system no. 1 (DDS1).