Interpreting some alrt.log output

Platform: Oracle 8.1.7 EE on Solaris 8

No particular problem, just poking around in the alert log to get a better understanding of exactly what normal output is telling me. Here is a chunk of one. My comments/observations/questions are enclosed in parenthesis. Apologies for any line wrap. Would anyone care to set me straight if my obervations are wrong? Thanks.

Tue Apr 13 20:11:57 2004
Beginning log switch checkpoint up to RBA [0x200.2.10], SCN: 0x0000.01957b81
Thread 1 advanced to log sequence 512
  Current log# 2 seq# 512 mem# 0: /u06/oradata/CMQM1240/redo201.log   Current log# 2 seq# 512 mem# 1: /u07/oradata/CMQM1240/redo202.log

(we just switched to log group #2. This is the 512th log switch since
the db was put in archive log mode. ??)

Tue Apr 13 20:11:57 2004
ARCH: Evaluating archive log 1 thread 1 sequence 511 ARCH: Beginning to archive log 1 thread 1 sequence 511

(Since we are now using rdo log group #2 for seq. 512, we can start
archiving sequence 511, presumably from rdo log group #1

Tue Apr 13 20:11:57 2004
ARC0: Evaluating archive log 1 thread 1 sequence 511 ARC0: Unable to archive log 1 thread 1 sequence 511

      Log actively being archived by another process

(A second archive of seq. 511? Why?)

Tue Apr 13 20:11:57 2004
Creating archive destination LOG_ARCHIVE_DEST_1: '/u08/arch/CMQM1240/arch_1_511.arc'
ARCH: Completed archiving log 1 thread 1 sequence 511

(Created and completed writing the archive log for seq. 511)

Tue Apr 13 20:26:45 2004
ARCH: Possible network disconnect with primary database

( huh??)

Tue Apr 13 20:27:15 2004
Completed checkpoint up to RBA [0x200.2.10], SCN: 0x0000.01957b81 Wed Apr 14 08:46:42 2004
Beginning log switch checkpoint up to RBA [0x201.2.10], SCN: 0x0000.01969b1d

(checkpoint and log switch, but notice the time elapsed between the
two)

Thread 1 advanced to log sequence 513
  Current log# 3 seq# 513 mem# 0: /u06/oradata/CMQM1240/redo301.log   Current log# 3 seq# 513 mem# 1: /u07/oradata/CMQM1240/redo302.log Wed Apr 14 08:46:42 2004
ARC0: Evaluating archive log 2 thread 1 sequence 512 ARC0: Beginning to archive log 2 thread 1 sequence 512 Creating archive destination LOG_ARCHIVE_DEST_1: '/u08/arch/CMQM1240/arch_1_512.arc'
ARC0: Completed archiving log 2 thread 1 sequence 512

(this time there was no second attempt to archive)

Wed Apr 14 09:02:10 2004
Completed checkpoint up to RBA [0x201.2.10], SCN: 0x0000.01969b1d Received on Wed Apr 14 2004 - 13:36:19 CDT