Extensiones

Introduction

There is no doubt that most of the present transmission networks are based on SDH/SONET technology. Although the demand growth for higher bit rates and the increase in traffic growth in communication networks have caused the introduction of WDM/DWDM

technology so it can cover most the demands, but still traffic aggregation is continuously done by SDH/SONET systems. Even in networks with more than 500Gb/s capacity, almost 90% of the traffic is aggregated on STM-16 interfaces (Fig.6.2.7).

(Gb/s) 0 10 20 30 40 50 60 70 80 90 100 100 200 300 400 500 600 700 ST M -1 6 C ir c u it s Network Capacity (% )

Fig .6.2.7-Fraction of circuits STM-16 of aggregated network capacity

In addition, most of the present and future service's request from the transmission networks has been forecasted over STM-1 and STM-4 interfaces. This shows the importance of SDH/SONET systems in the future.

On the other hand, one of the important challenges in developing transmission networks is the growth of data traffic compared with voice traffic. This challenge has become evident when most of data traffic produced by subscribers is done via Ethernet protocol. Ethernet optimizes traffic transport, in other words the role of SDH systems in future transmission networks has been adapted with Ethernet. At the present time, the transport of Ethernet over SDH systems has the following problems:

- Lack of flexibility

The defined bit rates for Ethernet are 10Mb/s, 100Mb/s, 1Gb/s and 10Gb/s and for SDH the bit rates are 155Mb/s, 622Mb/s, 2.5Gb/s and 10Gb/s.

If Ethernet traffic has carried on SDH system, part of the capacity will become useless. For example (Fig.6.2.8), a fast Ethernet service (100Mb/s) should be carried via STM-1 interface, which means that 30% of the transmission network capacity is lost.

STM-4 STM-1 E1 100 Mb/s ( Internet Connection) 500Kb/s ( Internet Connection) Fig.6.2.8-Lack of flexibility - Low band with utilization over a ring

In an SDH ring, to transport certain traffic on part of the ring between two nodes, the needed capacity will occupy the whole ring. This causes low utilization of bandwidth (Fig.6.2.9).

10 Mb/s over E3 10 Mb/s over E3

3 E3 ATM Port

10 Mb/s over E3

155Mbps of bandwidth allocated For 30Mbps of data (utilization=20%)

SDH/SONET Ring 100% full SDH Ring (155 Mb/s)

Fig. 6.2.9 - Low bandwidth utilization - Variety in platforms

A variety in services comes from various technologies and hardware. In other words, new services in central offices add a new set of hardware on a rack that results in increase of auxiliary costs.

Next Generation SDH

The above-mentioned problems introduce a new aspect to solve them, which is called “Next Generation SDH”. The development and deployment of SDH system offers new services using new protocols such as RPR, GFP, VC and LCAS.

Changing services landscape shows that Ethernet services have its importance in the future. The evolved Ethernet offers higher bit rates (10Mb/s, 100Mb/s, 1Gb/s, 10Gb/s), but speed is only part of the story. Ethernet still has its own disadvantages:

Poor facility utilization No edge to edge Qos Slow services restoration

Standard capacity (spanning tree)

In addition, in a transmission network some transparent disadvantages appear in transport circuits and the number of nodes over SONET/SDH ring is limited up to 16 nodes. Ethernet architecture

- Meshed Ethernet

In a meshed Ethernet architecture, STP (Spanning Tree Protocol) limits protection response to minutes, and Qos is applied at each hop.

- Ethernet Rings

In Ethernet ring, there is a limitation in the number of nodes and protection schemes. Also, packets are processed at each hop and no high-priority transit allowed. The attempt to solve the above problems or offer Ethernet services on present ring architectures has resulted in emergence of RPR (Resilient Packet Rings).

- RPR

RPR is a new media access control protocol based on a ring topology that has developed by 1EEE802.17 working group. The goal behind it is to provide an efficient use of network bandwidth and a resilient network with < 50 ms recovery time. Also, it supports up to 128 nodes in a ring.

In RPR, packets take the shortest path to the destination. The entire ring belongs to one subnet; this reduces many of the inter-subnet issues. RPR supports multicasting such that targeted multicast group nodes will copy the multicast source and pass them through the ring to the next node. Also, using RPR, the multicast source nodes will remove the multicast packets.

The resilience or proactive span protection automatically avoids failed spans within 50 ms. RPR supports both latency/jitter sensitive traffic such as voice & video services and

comes from spatial reuse. Unlike SDH/SONET, bandwidth is consumed only between the source and destination nodes. Packets are removed at their destination, leaving this bandwidth available to down stream nodes on the ring. RPR supports topologies of more than 100 nodes per ring and automatic topology discovery mechanism.

Packet over SDH

At the present time, transports of Ethernet traffic over SDH via HDLC protocols are not quiet efficient. The main protocols that encapsulate and frame data traffic for transport over next generation SDH infrastructure are as follows:

GFP (Generic Framing Procedure)

GFP is a traffic adaptation protocols, which is an ITU-T standard (ITU-T G704I).

It enables mapping of any data type to a SONET/SDH byte-synchronous channel. The GFP recommendation defines procedures for transporting various variable length client frames over legacy SDH transport and enables multiplexing of different client signals on to a single transport.

VC (Virtual Concatenation)

This protocol allows, instead of making a continuous allocation for a client signal, the transport path is created using a concatenation of smaller transport channels with a defined capacity like STM-1. VC compensates differential network delays up to 32ms. Only termination nodes need to support this feature.

LCAS (Link Capacity Adjustment Scheme)

It is a method to modify VCG size at the end points of transport path by using a specific signaling procedure. The signaling messages are transported in H4 byte. LCAS controls hitless addition/ removal of STM-N’s (VC-n’s) to/from VCG under management control. It also addresses the dynamic management of bandwidth for data transport services over SONET/SDH. LCAS works best on point-to-point links and supports virtual channel protection through “load sharing” on STM-n’s.

T raffic T yp e 1 0 M b p s E th e rn et 1 G b p s F ib re C h a n n e l 1 0 0M b it/s F as tE th e rn e t 2 0 0M b it/s (E S C O N ) 1 G b it/s E th e rn e t S T S -1 (2 0 % ) S T S -2 1 c (8 5 % ) S T S -3 c (6 7 % ) S T S -6 c (6 6 % ) S T S -2 4 c (8 3 % ) V T -1 .5 -7 v (8 9 % ) S T S -1 -1 8 v (9 5 % ) S T S -1 -2 v (1 0 0 % ) S T S -1 -4 v (1 0 0 % ) S T S -1 -2 1 v (9 2 % ) V C -3 (2 0 % ) V C -4 -1 6 c (3 5 % ) V C -4 (6 7 % ) V C -4 -4 c (3 3 % ) V C -4 -1 6 c (4 2 % ) V C -4 -7 v (9 5 % ) V C -3 -4 v (10 0 % ) V C -4 -2 v (66 % ) V C -3 -2 v (10 0 % ) V C -1 2 -4 6 v 1 00 % ) V C -4 -6 v(95 % ) V irtu a l V C -1 2 -5 v(9 2 % ) V irtu al C o n tigu o u s C o n tigu o u s S O N E T S D H

Tabele.6.2.2 GFP, Virtual Concatenation & LCAS Transport Efficiency New architecture of SDH/SONET platform

As mentioned so far, rapid change in services result in deployment of various systems and hardware at central offices. This made the suppliers to offer new platforms for (ATM, IP, Ethernet services) that are capable of producing various signals in the transmission network (STM-n or λ_).

These architectures are as follows: Single switch architecture

In this case a single fabric switch is defined on a system and interface adaptation is used over input/output card.

Hybrid multi-switch Architecture

In this case I/O traffic directed to a fabric switch based on service type. Multi layer SONET/SDH architecture

In this case ATM, IP switching is identified on I/O cards. This architecture identifies the cross connect and add/drop function over one platform and introduces new topology for new SDH metropolitan networks.

In the evolution of networks, the significant principle is to save and utilize the existing infrastructure. Fiber ring network is the most deployed network and the infrastructure should adapt with evolved trends. Traffic patterns are changing from voice towards data, and

networks based on SONET/SDH inherent planned and implemented for transport of voice traffic (or circuit service).Saving and utilization of SONET/SDH network depends on the change of transport capabilities in data traffic.

Next generation SDH includes RPR implementation, utilization of VC, GFP, LCAS protocols, and same platforms can play effective role in development and operation of SONET/SDH networks.