Data communications Day 2

I Error correction via Retransmission

1.1 Stop-and Wait ARQ (Automatic Repeat reQuest)
The sender stops and waits for a response from the receiver after each data packet.

1.2 Continuous ARQ (Automatic Repeat reQuest)
The sender does not wait for an acknowledgement after sending a message, it immediately sends the next one.

• The packets that are retransmitted may be only those containing an error (called Link Access Protocol for Modems [LAP-M]. This is better because it is more efficient.
• The first packet with an error and all those that followed it (call Go-Back-N ARQ).

II. TCP Sequences

The client on either side of a TCP session maintains a 32-bit sequence number it uses to keep track of how much data it has sent. This sequence number is included on each transmitted packet, and acknowledged by the opposite host as an acknowledgement number to inform the sending host that the transmitted data was received successfully.

III Network Layer 3 IP Layer

3.1
What does a DNS server do? - to translate name and IP address and vice versa

What does a DHCP server do? - to assign an IP address for a local network computer.

What is the purpose of the default gateway? - to connect to Internet (out of the local area network).

3.2
Output the physical and IP address mappings
arp -a

Show a record of the sites you have visited
ipconfig /displaydns > c:\temp\dnscache.txt

3.3
Show TCP/IP statistics and details about TCP and UDP connections
netstat -a

To view PC's routing table
netstat -r

3.4
Test a PC's connectivity to the network
ping 127.0.0.1 to check that CP/IP layers are working

3.5 Maximum Segment Size
1460 bytes + 20 bytes typical TCP header length + 20 bytes typical IP header length + 18 bytes Ethernet header and CRC = 1518 bytes. That is the maximum allowable packet size on an Ethernet network.


References
[1]. Jerry Fitzgerald, Alan Dennis and Alexandra Durcikova, Business Data Communications and Networking, 11th Edition, John Wiley & Sons, Inc.