DDR SDRAM Explained

DRAM is asynchronous, meaning it operates independently of the CPU and the system bus’ clock cycle. SDRAM is synchronous, meaning its operation is dependent on the data bus’ clock cycle (as is the CPU). This synchronicity translates to faster data transfers between memory and the CPU.

SDR (Single Data Rate) SDRAM sends one bit per clock cycle. DDR (Double Data Rate) SDRAM sends two bits per clock cycle (on both the rising and the falling edges of each clock cycle). DDR2 achieves speeds twice as fast as DDR by using an internal clock that runs at double the speed.


The maximum transfer rate for a memory module can be calculated with the following formula:

Maximum Transfer Rate = (clock rate in MHz) x (number of bits) / 8 (the reason for dividing by 8 is to convert bits to Bytes)

Since DIMM modules transfer 64 bits at a time, “number of bits” will always be 64. Since 64 / 8 = 8, we can simplify this formula to:

Maximum Theoretical Transfer Rate = clock rate x 8


DDR2-400 / PC2-3200 Memory Module

Has a 400MHz clock cycle (real clock = 200MHz, doubled), so it can send/receive 400M bits per second

To determine Bandwidth (Transfer Rate), multiply:

400M (Hz) x 8 (bits) = 3200 MB/s


IPv6 Neighbor Discovery Process (IPv6 Neighbor Discovery, SLAAC, Default Gateway assignment)

IPv6 Neighbor Discovery Process

  • Determines the link-local address of a neighbor-devices (all devices on the same  link/LAN)
  • Verifies the reachability of a neighbor
  • Keeps track of neighboring devices


IPv6 Neighbor Solicitation Message

  • Sent on the local link when a node wants to determine the link-layer address of another node on the same local link (see the figure below). When a node wants to determine the link-layer address of another node, the source address in a neighbor solicitation message is the IPv6 address of the node sending the neighbor solicitation message. The destination address in the neighbor solicitation message is the solicited-node multicast address that corresponds to the IPv6 address of the destination node. The neighbor solicitation message also includes the link-layer address of the source node.


Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6_basic/configuration/xe-3s/ip6b-xe-3s-book/ip6-neighb-disc-xe.html



IPv6 Link-layer addresses appear to be the same as Link-local addresses (are they TECHNICALLY the same?)

Packet Tracer and Java

Packet Tracer in Cisco Academy assessments may be blocked by Java security restrictions. To ensure you can complete the Packet Tracer assessments, perform the following steps:

1a) Click the Windows Start button, then type java, then select Configure Java (the Java Control Panel appears)

1b) Click the Security tab of the Java Control Panel


1c) Click the Edit Site List button (the Exception Site List appears)


1d) Click the Add button (a new location field appears)

1e) In the Location field, enter https://www.netacad-data.net:443

1f) Click the Add button (a new location field appears)

1g) In the Location field, enter https://ev-iip.netacad.net

1h) Click the OK button

1h) Ensure that your Java Control Panel appears like the screenshot above (with https://netacad.com and https://.netacad.net in the Exception Site List).

2) Go to http://skills.netacad.net/check/check.html and follow the on-screen instructions to ensure your device meets the minimum requirement needed to run assessments that contain Cisco Packet Tracer activities.

Technical Instructor