preparing for CCNA --note ( 1)

Typically, a Cisco router has the following types of memory components:
1. RAM (or DRAM): This type of memory is used for working storage. This is where part of IOS image, routing tables, packets, and others are stored during normal working of a router. RAM is erased when the router is powered off.
2. ROM (Read Only Memory): ROM stores bootstrap code for booting Cisco IOS. It points to the functional IOS image so that the router can complete its start-up configuration successfully. It is somewhat analogous the BIOS in PC. ROM also contains a small portion of IOS.
3. Flash memory: Flash memory contains the fully functional IOS images. It is the default location for getting IOS image at boot time. You can loosely compare the IOS image with operating system (say DOS).
4. NVRAM: NVRAM stands for Non Volatile RAM. NVRAM stores the initial or startup configuration file.


In Carrier Sense Multiple Access With Collision Detection (CSMA/CD), a node transmits only after listening to the media, and ensuring that no transmission is taking place. Further, sending nodes detect when a collision occurs and stop transmitting immediately, backing off for a random amount of time before trying again. This results in efficient use of the media since the bandwidth of transmitting the entire frame is not wasted.


The following are some Presentation Layer standards:
Graphic and Visual Image: PICT, TIFF, JPEG
Movies and Sound: MIDI, MPEG, Quick Time
The following are Session layer standards:
NFS, SQL,RPC,X-Windows.