Performance Evaluation of TCP over ATM in Broadband Satellite Networks

 

 

 

By -  Ajit Gundale

__________________________________________________________________________________________________________________________

 

TCP is reliable, connection-oriented, end-to-end error, flow control and congestion control protocol. It is widely used protocol for data transfer in packet form in the Internet. It is designed to work independently of lower layer, such as ATM, to transfer data. Its basic implementation is unsuitable for high speed and long delay networks and therefore modifications are suggested to enhance its performance. The proposed extensions are use of large window size in order to accommodate high BDP value of high speed & long delay network and TCP time stamp for precise estimation of round trip time (RTT). Various TCP schemes are suggested to improve TCP behavior on high BDP networks by slow start phase at the beginning or retransmission policy to have multiple lost segment repetition in one round trip time.

 

ATM is a scalable cell switching and multiplexing technology for Broadband Integrated Services Digital Network (B-ISDN). It is widely implemented in LANs and WANs. The TCP over ATM provides maximized performance when transmitter and receiver both are synchronized properly and congestionless situation on the channel. The performance of TCP over ATM is characterized by a metric throughput.

 

The low throughput of TCP in ATM networks is mainly due to the reason- The loss of any cell in a packet corrupts the corresponding whole packet, TCP retransmits this corrupted packet, the packet generates a bundle of cells, this process is repeated and finally generates vast retransmission duplicates. The other reason is that TCP should wait for time-out period, which is greater than Round Trip Time (RTT) and is usually too big, in order to retransmit the lost packet when a cell (or a packet) is dropped during its transmission. And if it is a long distance communication then actual throughput is hardly to get.

 

TCP Application sends data in large packets. These packets must be segmented to fit into the ATM cell structure before being put on the network. The effective throughput of TCP over ATM networks may be quite lower than that over packet-based networks, when ATM cells are dropped at congested ATM switches. Most cell losses occur at congested switches in ATM networks while physical links usually have very low bit error rate (BER).

 

When TCP-ATM application is used in satellite networks, congestion may occur in ATM switches. Thus ATM Congestion Control mechanism is necessary to have maximum throughput. Due to unavailability of required bandwidth or output queue size, cells are dropped. By selecting proper TCP parameters and output queue size,  % cell drop can be reduced giving improved throughput. Hence it is important to study congestion control of TCP over ATM in satellite network. ATM rate-based flow control mechanism effectively reduces cell loss and buffer requirement in ATM switches, and helps to enhances overall performance in ATM networks.

 

Following are the observations during studies of performance of TCP over ATM in satellite networks.

 

 

Optimization of TCP parameters in ATM networks with

Background Traffic applied

 

                   This simulation is done to analyze the performance of a TCP over ATM in satellite network. The analysis consists of optimization of TCP parameter such as Throughput, Retransmission Percentage, and Maximum Segment Size. 

Network shown below is selected to find various TCP parameters:

         

 

 

 

Network Parameter Detail:

 

TCP Application Parameters:

 

Bit Rate

155 Mbps

Buffer Size

Variable

Max. Segment Size

Variable

Buffer Management Algorithm

EPD

 

 

BTE parameters:

 

Max. Output queue Size

Unlimited

Max. Input queue Size

Unlimited

 

 

Rate Based Switch Parameter:

 

Delay to Process

0 msec.

Output Queue Size

Unlimited

ATM congestion control algorithm

Applied

 

 

Selection of Maximum Segment Size (MSS)

 

 

                   Here TCP Buffer Size is kept constant and Max Segment Size is varied and effect is seen on Throughput and maximized throughput is observed for certain MSS.

 

 

Constant Parameters

Buffer size

(bytes)

9000

Variable Parameters

Max. Segment Size

(octets)

512,5000,9180, 10000

 

 

 

Graph below shows Effect of variation of Max. Segment Size on Throughput

 

 

Selection of TCP Buffer size

 

                        Here TCP Buffer Size in network is varied and its effect is seen on Throughput and Retransmission Percentage.

 

Constant Parameters

Max. Segment Size

 (octets)

9180

Variable Parameters

Buffer size

(bytes)

7000,8000,9000,

32000

 

Graph below shows Effect of variation of TCP Buffer size on Throughput

 

Graph below shows Effect of variation of TCP Buffer size on amount of  percent Retransmission

 

 

 

 

CONCLUSION:

 

ü      Background traffic helps in proper utilization of the link. When traffic from TCP application is less and bandwidth is available this background traffic utilizes the remaining bandwidth.

 

ü      From simulation it is observed that the Maximum Throughput for TCP connection with background traffic in Satellite based network is available with TCP buffer size of 9000 octets and Maximum Segment Size of 9180 octets.           

 

           

ü      For above optimized values minimum Retransmission is observed all TCP applications.

 

ü      For Maximum Segment Size set to 9180 octets the Maximum Throughput is achieved, it concludes that MSS is approximately equals to TCP buffer size.

 

 

ü      For background traffic, if the number of TCP connections is increased, the overall link utilization also improves.

 

Thus following parameters are assigned with optimized values in future simulation studies:

 

 

TCP Buffer Size

9000 octets

TCP MSS

9180 octets

Switch Slot rate

155 Mbps

 

 

Above parameter values ideal BTE and switches are assumed.

 

 

 

TCP over ATM in Satellite Network

 

The following simulation put focus on optimization of B-TE switch parameters and effect of asymmetry on forward and reverse rate in satellite network. For simulation of both Generic and Rate based switches are considered.

 

Following network is assumed for simulation.

 

 

 

 

Parameters for TCP Application:

l      Bit Rate:                                 141 Mbps

l      Buffer Size:                           9000 bytes

l      Mean Processing Time:        3 msec

l      Max .Segment Size : 9180 octets

 

 

B-TE parameters:

l      Max. Output queue Size        10

l      Max. Input queue Size           10

 

 

Switch Parameter:

l      Processing delay                   0.3 msec.

l      Output Queue Size                kept variable

l      Buffer Mgt. Algorithm       Triggered

l      Congestion Algorithm          Applied

 

1. (a)  Generic switch format Network

 

Effect of variation of Switch Output Queue Size.

 

Here Switch Output Queue Size of network is varied and its effect is seen on     % Cell Drop of the Switch.

 

 

Constant Parameters

Uplink and

Downlink

Link rate

( Mbits /sec)

141

Variable Parameters

Switch

Output Queue size

(cells)

1,5,10,25,50

 

 

 

 

 

Graph below shows effect of variation of Switch Output Queue size on Cell Drop.

 

 

 

 

 

 

 

1. (b) Rate Based switch Format Network

 

Effect of variation of Switch Output Queue Size.

 

Here Switch Output Queue Size of network is varied and its effect on % Cell Drop of the Switch is observed.

 

Constant Parameters

Uplink and

Downlink

Link rate

(Mbits/sec)

141

Variable Parameters

Switch

Output Queue size

(cells)

0,5,10,50

 

 

 

 

Graph below shows Effect of variation of Switch Output Queue size on Cell Drop.

 

 

 

Above graph shows % Cell Drop in accordance with the Switch Output Queue Size. Here the effect of Delay on Cell Drop is observed.

 

 

 

2. (a) Performance of TCP over ATM in asymmetric satellite network-

 

Effect of variation of Uplink and Downlink Link rate based switch format

 

 

Constant Parameters

Uplink

Link Rate

(Mbps)

50

Downlink

50

Variable Parameters

Switch

Output Queue size

(cells)

0, 5, 10, 50

 

 

 

 

 

 

Above graph shows variation in the throughput for different switch output queue size

 

 

 

2. (b) Performance of TCP over ATM in asymmetric satellite network-

 

 

Effect of variation of Uplink and Downlink Link in Rate based switch format

 

 

 

Constant Parameters

Uplink

Link Rate

(Mbps)

25

Downlink

50

Variable Parameters

Switch

Output Queue size

(cells)

0,5,10,25,50

 

 

 

 

 

 

Above graph shows variation in the throughput for different switch output queue size

2. (c) Performance of TCP over ATM in asymmetric satellite network-

 

 

Effect of variation of Uplink and Downlink Link Rate

 

 

Constant Parameters

Uplink

Link Rate

(Mbps)

50

Downlink

25

Variable Parameters

Switch

Output Queue size

(cells)

0,1,5,10,25,50

 

 

 

 

 

 

Above graph shows variation in the throughput for different switch output queue size
CONCLUSION:

 

Satellite Based TCP Network Simulation:

 

ü      This simulation is done to study %Cell drop by varying Switch Output Queue Size. If the incoming cells exceed the limit of queue size or there is no bandwidth available on output link then Cells are dropped. 

 

ü      It is found that the Minimum % Cell Drop is achieved when Switch Output Queue Size is set at 50 Cells which is sufficient enough to handle all the incoming data from TCP connections using ATM Congestion Control mechanism for Rate based Switches. As Switch Output Queue Size doubles the Cell Drop reduces approximately by 2%.

 

ü      In asymmetric satellite networks, for given switch output queue size minimum cell loss at the side of switches is observed when uplink is larger than downlink. The variation of Switch Output Queue Size reduces % Cell drop by 1%. Thus we get Minimum Cell drop when Switch Output Queue Size is set at 50 cells.

 

Thus optimized results are observed when different parameters are assigned values given in the table

 

 

 

 

TCP Buffer Size

9000 bytes

Max. Segment Size (MSS)

9180 0ctets

Switch Output Queue Size

50 cells

Switch Slot rate

155 Mbps

Switch Delay to Process

3 msec

Switch ATM Congestion algorithm

Triggered