beautypg.com

Link rx pwr col link rx pwr col – GarrettCom DS50 User Manual

Page 27

background image

Magnum DS50 / DS60s Dual Speed Hubs Installation and User Guide (06/02)

Each Fast Ethernet device component also has an associated BT delay, which

depends on the physical signaling system employed. Table 3.2b shows each Fast Ethernet

device component and the associated BT delay. Note that there is only one DTE pair

associated with any device-to-device path.

Table 3.2b: Worst case round-trip delay for Fast Ethernet device components*

Component

Round-trip delay in Bit Times (BT)

2 TX DTEs

100

2 FX DTEs

100

1 FX and 1 TX DTE

100

2 T4 DTEs

138

1 T4 and 1 TX or FX DTE

127

Class I Repeater

140

Class II Repeater with any
combination of TX and FX ports

92

**Note, the delay is only

80

Bit Times

for the MagnumDS50 / DS60, ports 1-5.

*Worst case delays taken from IEEE Std 802.3u-1995.

To determine whether a prospective network topology adheres to the collision

domain diameter specification, the following formula should be applied to the worst case path

through the network. The worst case path is the path between the two Fast Ethernet devices

(DTEs) which have the longest round trip time.

PDV = (sum of cabling delays) + (sum of repeater delays) +

(DTE pair delay) + (safety margin)

PDV is the Path Delay Value of the worst case path. For the network to adhere to

IEEE standard, this value must be less than 512 BT. The safety margin is specified in BT and

may be a value between 0 and 5. This margin can be used to accommodate unexpected delays,

as seen when using extra long patch cables. A safety margin from about 2 to 4 BT is

recommended.

A typical example of a

PDV calculation is shown to the

right. The example is illustrated in

Figure 3.2a. Here, an integrator

wishes to cascade two industry-

standard Fast Ethernet hubs with

100m Category 5 user segments

(i.e., from computer to hub) and

LINK
RX

PWR COL

LINK
RX

PWR COL

100 m cat. 5 cables

to user devices

Cat. 5 cable for

cascading

Industry-
Standard
100Mb/s Hub

Industry-
Standard
100Mb/s Hub

23

www GarrettCom com

.

.

This manual is related to the following products: