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As we saw in Section 2.3.3, when transporting IP packets over SONET/SDH (PoS), the entire SONET/SDH payload has to be dedicated to IP packets. Unlike PoS, virtual concatenation permits the bandwidth of a SONET/SDH frame to be divided into several subrate payloads, each of which can carry different type of traffic (see Figure 2.26). The
12 OC-3/ STM-1
Figure 2.26 SONET/SDH virtual concatenation.
capacity of the OC-48/STM-16 payload is split into an OC-12/STM-4 subrate payload, which is used to carry voice; the rest is split into 12 OC-3/STM-1s, which can be used to carry data either individually or virtually concatenated.
2.8.2 Link Capacity Adjustment Scheme (LCAS)
The number of subrate payloads allocated to an application is typically determined in advance. However, the transmission rate of the application can indeed vary over time. In view of this, it can be useful to dynamically vary the number of subrate payloads allocated to an application. This can be done using the link capacity adjustment scheme (LCAS). In LCAS, signaling messages are exchanged between the originating and terminating SONET/SDH node to determine and adjust the number of required subrate payloads. LCAS makes sure that the adjustment process is done without losing any data.
1. Consider the DS1 and E1 signals:
a) How many voice calls are multiplexed in a DS1 signal?
b) How many voice calls are multiplexed in an E1 signal?
c) Why is there a difference in the number of voice calls carried in a DS1 signal and in an
2. Consider a time slot in the DS1 frame. Every sixth frame, the 8th bit is robbed and is used for signaling. What is the data rate of this signaling channel?
3. What is fractional T1?
4. In SONET, identify the differences between a section, a line, and a path.
5. In SONET, what is a virtual tributary group? What is a virtual tributary? Give an example of
a virtual tributary.
6. Explain how an add/drop SONET/SDH multiplexer works.
7. Two IP routers are connected via an OC-3c link by using packet over SONET (PoS). Assume that 33% of the resulting HDLC frames are 50 bytes long, and the rest are 1500 bytes long. What is the rate of transmission of IP packets?
8. Consider the 1:1 and 1:N protection schemes in a point-to-point fiber link. Which of these two schemes provides better protection? Why?
9. Consider the two-fiber bidirectional line switched ring (2F-BLSR) shown in Figure 2.18. What is the total available capacity of the ring for the transmission of the working traffic from ADM 1 to ADM 2:
SONET/SDH AND THE GENERIC FRAME PROCEDURE (GFP)
a) When all fibers are working?
b) When fibers 2 and 8 fail?
c) When fiber 12 fails after fibers 2 and 8 have failed?
10. Explain how the virtual concatenation scheme works. Why does it have to be implemented only at the originating and terminating nodes?
The Asynchronous transfer mode (ATM) architecture was standardized by ITU-T in 1987 as the preferred architecture for the broadband integrated services data network (B-ISDN). The broadband integrated services data network was conceived as a future high-speed network that would have replaced the telephone network and other data networks. It would have provided a single network for the transport of voice, video, and data. The term asynchronous transfer mode was chosen in contrast to synchronous transfer mode (STM), which was proposed prior to the standardization of ATM and which was based on the SONET/SDH hierarchy. The term transfer mode means a telecommunication technique for transferring information.
Despite the enormous technological advancements in networking, the integration of voice, video, and data on to the same network is still elusive. Currently, ATM is a mature technology that is primarily used in the backbone. For instance, it is widely used in the backbone of Internet service providers (ISPs) and it has been deployed to provide point-to-point and point-to-multipoint video connections. It is also used in cellular telephony to carry multiple voice connections using the ATM adaptation layer 2 (AAL 2). ATM is used for circuit emulation, which is a service that emulates a point-to-point T1/E1 circuit over an ATM network. ATM is also used in access networks such as ADSL-based residential access networks and ATM passive optical networks (APON). ATM is not visible to the networking users, as is, for instance, the ubiquitous TCP/IP protocol. In view of this, it is often mistaken as a network that it is no longer in use - which is absolutely not the case!
ATM constituted a novel departure from previous networking architectures, and it has built-in mechanisms that permit it to transport different types of traffic with different QoS. Until the advent of multi-protocol label switching (MPLS) architecture in the late 1990s, ATM was the only networking technology that provided QoS on a per connection basis. The reader is encouraged to develop a good understanding of ATM and its congestion control schemes, before proceeding to Chapter 6 on MPLS.