Special topics on optical access network

Transcription

1 Special topics on optical access network Chapter 3 Access Network Architectures Based on TDMA PON 蘇銓清老師 1

2 This chapter explains the history and various flavors of PON. 2

3 Access Network Architectures Based 3.1 ATM PON 3.2 Ethernet PON Why Ethernet? 3.3 GPON on TDMA PON 3.4 Comparison of BPON/GPON and EPON approaches 3

4 3.1 ATM PON (1/3) In 1995, the Full Service Access Network (FSAN) was formed to create unified specification for broadband access networks. FSAN specify ATM as layer-2 protocol, which is called ATM PON (APON). The name APON was later replaced with BPON for broadband PON. 4

5 3.1 ATM PON (2/3) In 1997 the FSAN group submitted the BPON specification proposals to International Telecommunications Union Telecommunication Standardization Sector (ITU-T) for a formal ratification. ITU-T approved series of BPON-related recommendations: G.983.1~.8 5

6 3.1 ATM PON (3/3) The original BPON has 155/155 Mbps upstream/downstream bit rates. In 2001 BPON was amended to 155/622 Mbps and 622/622 Mbps upstream/downstream bit rates. 6

7 3.2 Ethernet PON (1/2) In January 2001, the IEEE formed a study group called Ethernet in the First Mile (EFM). Extend existing Ethernet technology into subscriber access area. Focus on both residential and business access networks. Provide a significant increase in performance. Minimize equipment, operation, and maintenance costs. EPON became one of the focus of EFM. 7

8 3.2 Ethernet PON (2/2) Ethernet PON (EPON) is a PON-based network that carries data traffic encapsulated in Ethernet frames as defined in the IEEE standard [802.3]. It uses a standard 8b/10b line coding (8 data bits encoded as 10 line bits) and operates at standard Ethernet speed of 1 Gbps. EPON utilized the existing specification, including the usage of existing full-duplex media access control (MAC). 8

9 3.2.1 Why Ethernet? (1/2) Ethernet technology has leapfrogged ATM. Ethernet has become a universally accepted standard, offering staggering economies of scale [Cla00]. High-speed gigabit Ethernet deployment is widely accelerating. 10-gigabit Ethernet products are becoming available. [Cla00] S. Clavenna, Metro optical Ethernet, Lightreading ( November

10 3.2.1 Why Ethernet? (2/2) Ethernet is easy to scale and manage. ATM PON may not be the best choice to interconnect two Ethernet networks. Ethernet gains new ground in MAN and WAN. More than 95 percent of enterprise LANs and home networks use Ethernet. ATM has high overhead for carrying variablelength IP packets. 10

11 Ethernet vs. ATM (1/4) Ethernet 38-byte encapsulation overhead 8-byte frame preamble 14-byte Ethernet header 4-byte FCS field At least 12-byte minimum interframe gap (IFG) should be left between two adjacent frames IFG is specified as a 96-ns time interval, which is equal to 12 byte-transmission times in 1 Gbps (1000BASE-X) Ethernet. 11

12 Ethernet vs. ATM (2/4) ATM encapsulation overhead (cell tax) Data is broken into multiple cells Multiple cell headers 8-byte ATM adaptation layer 5 (AAL5) trailer Variable-size padding to fill any remaining portion of the last cell 12

13 Ethernet vs. ATM (3/4) 13

14 Ethernet vs. ATM (4/4) For a particular IP datagram size distribution obtained in an access network [SG01] Ethernet frame encapsulation overhead: 7.42 % ATM cell encapsulation overhead: % It s better to use variable-size Ethernet frames to carry variable-size IP packets. [SG01] D. Sala and A. Gummalla, PON functional requirements: Services and performance, presented at IEEE 802.3ah meeting in Portland, OR, July Available at f. 14

15 Shortcoming of ATM A dropped or corrupted ATM cell will invalidate an entire IP datagram. The remaining cells carrying the portions of the same IP datagram will propagate further. Thus consuming network resources unnecessarily. Devices are more expensive than Ethernet. [Cla00] [Cla00] S. Clavenna, Metro optical Ethernet, Lightreading ( November

16 Ethernet advantage Newly adopted quality-of-service (QoS) techniques have made Ethernet networks support triple-play (Voice, Video, Data). Full-duplex Prioritization Virtual LAN (VLAN) tagging Ethernet is inexpensive. 16

17 3.3 GPON In 2001, FSAN undertook a new effort to Specify a PON system operating at bit rates exceeding 1 Gbps. Allowing a mix of variable-size frames and ATM cells. In , ITU-T has approved the new gigabit-capable PON (GPON) series of specifications. G.984.1, G.984.2, and G

18 3.4 Comparison of BPON/GPON and EPON Approaches (1/3) Both BPON and GPON architectures Were conceived by the FSAN group. Are optimized for TDM traffic and rely on framing structures with a very strict timing and synchronization requirements. EPON No explicit framing structure. Standard interframe spacing. 18

19 3.4 Comparison of BPON/GPON and EPON Approaches (2/3) Inter-timeslot time BPON: approximate 154 ns GPON: less than 49 ns EPON: less than 912 ns. Frame fragmentation BPON: AAL5 GPON: GPON encapsulation method (GEM) EPON: No 19

20 3.4 Comparison of BPON/GPON and EPON Approaches (3/3) Cost BPON & GPON: More expensive EPON: Cheaper 20