written 8.3 years ago by | • modified 8.3 years ago |
Mumbai University > Computer Engineering > Sem6 > Mobile Communication and Computing
Marks: 10M
Year: Dec 2015
written 8.3 years ago by | • modified 8.3 years ago |
Mumbai University > Computer Engineering > Sem6 > Mobile Communication and Computing
Marks: 10M
Year: Dec 2015
written 8.3 years ago by |
With the advent of WLANs, a lot of research went into increasing the performance of TCP in wireless and mobile environments, some of its outcome are I-TCP and SNOOP-TCP, Mobile-TCP etc. Let us study Mobile-TCP.
M-TCP (mobile TCP) has the same goals as similar to its variants i.e. I-TCP and Snoop-TCP. It too wants to improve overall throughput, to lower the delay, to main end-to-end semantics of TCP.
But, it is mainly enhanced to address problems related to lengthy or frequent disconnections.
Basic TCP methodology:
When a node does not receive an acknowledgement back form the host, it carries out retransmission.
A TCP sender tries to retransmit data controlled by retransmission timer which doubles up with each unsuccessful attempt. (upto a maximum of one minute)
A sender tries to retransmit an unacknowledged packet every one minute and gives up after 12 minutes.
If in I-TCP, the mobile host is disconnected, then in such a situation, the FA will keep of buffering more and more data packets.
In case of a handover following this disconnection, we have more data to be transmitted to new FA.
Snoop-TCP also suffers from similar such problems.
Mobile TCP:
The M-TCP splits up the connection into two parts:
The Supervisory Host (SH) adorns the same role as the proxy (Foreign Agent) in I-TCP.
The SH is responsible for exchanging data to both the Standard host and the Mobile host.
Here in this approach, we assume that the error bit rate is less as compared to other wireless links.
So if any packet is lost, the retransmission has to occur from the original sender and not by the SH. (This also maintains the end-to-end TCP semantic)
The SH monitors the ACKs (ACK means acknowledgement) being sent by the MH. If for a long period ACKs have not been received, then the SH assumes that the MH has been disconnected (maybe due to failure or moved out of range, etc...).
If so the SH chokes the sender by setting its window size to 0.
Because of this the sender goes into persistent mode i.e. the sender’s state will not change no matter how long the receiver is disconnected.
This means that the sender will not try to retransmit the data.
Now when the SH detects a connectivity established again with the MH (the old SH or new SH if handover), the window of the sender is restored to original value.
Advantages:
Maintains the TCP end-to-end semantics. (No failed packet retransmission is done by the SH .All job handled by original sender)
Does not require the change in the sender’s TCP.
If MH disconnected, it doesn’t waste time in useless transmissions and shrinks the window size to 0.
No need to send old buffer data to new SH in case of handover (as in I-TCP).
Disadvantages:
M-TCP assumes low bit error which is not always true. So, any packet loss due to bit-errors occurring, then its propagated to the sender.
Modifications are required for the MH protocol software.