Comparison to reservation based protocols

This section compares protocol performance to reservation based protocols. About ten reservation protocols based on Slotted Aloha and Reservation Aloha were proposed in [11]. The protocols operate with one control channel and

C

?1 data channels. The architecture is based on one tunable transmitter and one tunable receiver per node. The access mechanism is as follows. A node

i

transmits a control packet on the control channel if a data packet is generated at the node. An idle node listens to the control channel and tunes to the data channel indicated in the control packet if the control packet is successful and the packet is destined for it. At the end of control packet transmission, the data packet is transmitted. If a

node attempts to transmit to a node currently receiving on another channel, the transmission is unsuccessful. This is termed receiver collision [76]. The analysis in [11] assumes infinite population and does not consider receiver collisions but are considered in the following analysis. Immediate feedback about the success of the transmitted packet is assumed and the packet is retransmitted till it is successful. Time is slotted on control packet boundaries and data packet length is assumed to be

L

time slots.

In this paper, we consider protocol 6 (P6) since it offers the best throughput-delay characteristics compared to other Slotted Aloha based protocols presented in [11]. Each channel withP6has its own cycle of duration

L

+1 slots. If a node has a data packet to transmit, it randomly chooses a data channel from

C

?1 channels, waits for the beginning of the cycle on that channel, transmits the control packet followed by data packet irrespective of control packet success. Retransmissions are attempted until successful transmission of the data packet.

The advantages of using the architecture and protocols presented in this paper compared to that ofP6are summarized as follows:

Simpler low-cost architecture: Reduced architectural complexity and system cost – only

one transmitter-receiver pair per node is required and fast -tunable receivers are not required.

Higher channel utilization: All channels are utilized for data transmission – the impact

is higher for lower number of channels.

No receiver conflicts: No arbitration algorithm is required to resolve receiver conflicts

since receiver conflicts are eliminated with the pre-allocation approach.

Extensible system: Easily reconfigurable system with change in number of nodes and

Packet delay forP6was defined as the number of control slots required to transmit a packet of length

L

slots. Since time is slotted on packet boundaries for I-SA and I-TDMA*, packet delay for P6 is normalized to the data packet length in the comparison to follow. Similarly, network throughput inP6was defined as the number of packets transmitted per channel cycle so the throughput ofP6is also been normalized to packet transmission time. The delay is now defined as the number of data packet slots needed to transmit a data packet and system throughput is the number of packets successfully transmitted across all channels in one data packet slot.

The architecture proposed in this paper is more cost-effective because tunable receivers are not required. Also, the delay characteristics of I-SA are better than P6 because no control packet is transmitted. Each (re)transmission of a packet inP6requires

L

+1 time units whereas only

L

slots are required in I-SA. Also, the number of available data channels for I-SA is

C

instead of

C

?1 as inP6. P6suffers from an initial cycle synchronization delay per packet equal to L+1

2 slots and is dependent on packet size. The maximum

theoretical throughput for infinite population achieved by I-SA is roughly 0

:

36

C

compared to 0

:

36(

C

?1)attained byP6.

P6 is simulated using discrete event simulation. Convergence is obtained on delay with a confidence of 95% in a less than 5% variation from the mean. The retransmission probability per cycle for P6 and per data slot for I-SA is taken as 0

:

05. The protocols are implemented with finite population and also consider receiver conflicts. Fig. 3.12 compares the performance of the three protocols in terms of normalized throughput and delay. Fig. 3.12(a) compares the effect of increasing

C

and fixed

M

for

M

= 32

;C

2 f8

;

16

;

32gand

L

=32. The value of

L

is empirically chosen since it yielded the maximum performance for P6. Fig. 3.12(b) compares the protocols for

C

= 4 and

M

= 32 and

The performance of I-SA is better than P6in terms of lower packet delay and higher maximum throughput. Under light traffic, the average delay using I-SA is

L

units, which is the length of the data packet. The average delay using P6 is L+1

2 +

L

+ 1 which includes cycle synchronization and delay for both data and control packets. P6 suffers from destination conflicts which increases delay and decreases throughput. For example, the maximum throughput for

C

= 8 and

M

= 64 is equal to 2.4 for I-SA and 2.0 forP6 compared to the theoretical bound of 2.9 for I-SA and 2.5 forP6. Similarly, the maximum throughput for

C

=32 and

M

=32 is 5.5 for I-SA and 3.9 forP6. In short, I-SA is able to outperformP6despite the lack of tunable receivers and proves to be a more cost-effective solution.

I-TDMA* is advantageous overP6because of its higher maximum throughput, higher system capacity and stability under heavy traffic. P6suffers from instability under heavy traffic like any Aloha based system. The packet delay is higher under light traffic using I-TDMA* because of its time multiplexed nature. The maximum theoretical throughput offered by I-TDMA* is

C

. This is verified in Fig. 3.12 where the delay using I-TDMA* is higher under light traffic but maximum throughput and system capacity is higher for I-TDMA*.

3.4

Summary

This chapter presented the pre-allocation protocols studied for a star coupled WDM network. Static and random access schemes were examined and extensively analyzed using analytic and simulation models. The results are summarized here. I-SA* was observed to have lower packet delay at lighter loads compared to I-TDMA*. However, the performance of I-TDMA* under heavy traffic is superior to that of I-SA* since its saturation point is

significantly higher. In particular, I-TDMA* is capable of achieving a maximum throughput equal to the number of channels in the system. Both I-SA* and I-TDMA* take advantage of an increase in the number of channels in the system. I-TDMA* is sensitive to increases in system size and insensitive to propagation delay. I-SA* is less sensitive to system size increase and highly sensitive to propagation delay due to acknowledgments.

The choice between I-SA* and I-TDMA* lies in the delay/throughput requirements and traffic characteristics. I-SA* is suitable for a system with large number of bursty users where the aggregate traffic is low and propagation delay is small. I-TDMA* is more suitable for an environment where number of nodes is small, traffic is steady and high, and propagation delay is large.

This chapter focused on the star coupled network without amplification mainly because of its power budget and fault tolerance advantages. Another topology widely studied for local networks is the bus topology. Poor power budget characteristics of bus based networks led to star being the primary topology choice for optic networks. However, the advent of optical fiber amplifiers in particular erbium-doped fiber amplifiers (EDFA) [2] has renewed interest in bus topology. The next chapter examines different protocol choices for the bus and star WDM networks with photonic amplification.

In document HIGH-SPEED COMMUNICATION PROTOCOLS FOR ALL-OPTICAL WAVELENGTH DIVISION MULTIPLEXED COMPUTER NETWORKS (Page 86-91)