Up-Link Switch At Port#1 For Cascading; Frame Buffering And Latency - GarrettCom Magnum 4K16 Installation And User Manual

Magnum 4K16 Switches Installation and User Guide
2.2.5

Up-link Switch at port#1 for Cascading

The port#1 is equipped with a Media Dependent Interface (MDI-X)
movable switch to simplify cascaded or up-link connections. It enables the first port's
RJ-45 cable to either connect to a user station (=) or to be cascaded to another hub (X)
with cross-over switch. (See Section 4.4 for more details about Up-link). Like all 4K16
ports, Port # 1 is a dual-speed switch port which will sense the speed of the connected
device. When the Up-link port is used to cascade two Magnum 4K16 together, the auto-
sensing feature will cause the connecting link to operate at 100Mb speed.
2.2.6

Frame Buffering and Latency

The Magnum 4K16 Series are store-and-forward switches. Each frame (or
packet) is loaded into the Switch's memory and inspected before forwarding can occur.
This technique ensures that all forwarded frames are of a valid length and have the
correct CRC, i.e., are good packets. This eliminates the propagation of bad packets,
enabling all of the available bandwidth to be used for valid information.
While other switching technologies such as "cut-through" or "express" impose
minimal frame latency, they will also permit bad frames to propagate out to the Ethernet
segments connected. The "cut-through" technique permits collision fragment frames,
which are a result of late collisions, to be forwarded to add to the network traffic. Since
there is no way to filter frames with a bad CRC (the entire frame must be present in order
for CRC to be calculated), the result of indiscriminate cut-through forwarding is greater
traffic congestion, especially at peak activity. Since collisions and bad packets are more
likely when traffic is heavy, the result of store-and-forward operation is that more
bandwidth is available for good packets when the traffic load is greatest.
To minimize the possibility of dropping frames on congested ports, each
Magnum 4K16 Switches dynamically allocates buffer space from a 512K memory pool,
ensuring that heavily used ports receive very large buffer space for packet storage.
(Many other switches have their packet buffer storage space divided evenly across all
ports, resulting in a small, fixed number of packets to be stored per port. When the port
buffer fills up, dropped packets result.) This dynamic buffer allocation provides the
capability for the maximum resources of the Magnum 4K16 unit to be applied to all
traffic loads, even when the traffic activity is unbalanced across the ports. Since the
traffic on an operating network is constantly varying in packet density per port and in
aggregate density, the Magnum 4K16 Switches are constantly adapting internally to
www GarrettCom com . .
9
(12/05)