Relation between Signal Level, Noise Level and RSSI

[Raghavendra]

Hi,

I have following two data, which display Signal Level = -79dBm, Noise Level 
= -88dBm and RSSI of 177.

Can any one tell me whats the relation between these three? any 
equation..etc. basicaly I want to know how RSSI value is determined as 177?

 # iwconfig eth0
eth0      XXXXX  ESSID:"xxxx"  Nickname:""
          Mode:Managed  Frequency:2.412 GHz  Access Point: 00:1D:7E:0E:3F:54
          Bit Rate:54 Mb/s   Tx-Power=15 dBm
          Retry limit:9   RTS thr=2347 B   Fragment thr=2346 B
          Encryption key:off
          Power Management:off
          Link Quality:0/100  Signal level:-79 dBm  Noise level:-88 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:2350
          Tx excessive retries:169  Invalid misc:41836   Missed beacon:0

# cat ./sys/class/net/eth0/wireless/level
177

Also if any one have idea about
-90 ~ -80dbm: level 1
-79dbm ~ -69dbm: level 2
-69dbm ~ -59dbm: level 3
-59dbm ~ 0: level4.
what these level stands for?

-Raghu