Physical Interface Properties

The physical properties of a network interface are the characteristics associated with the physical link that affect the transmission of either link-layer signals or the data across the links. Physical properties include clocking properties, transmission properties, such as the maximum transmission unit (MTU), and encapsulation methods, such as point-to-point and Frame Relay encapsulation.

The default property values for an interface are usually sufficient to successfully enable a bidirectional link. However, if you configure a set of physical properties on an interface, those same properties must be set on all adjacent interfaces to which a direct connection is made.

Table 1 summarizes some key physical properties of device interfaces.

user@host# set interfaces em0 mtu 1400

In telecommunication transmission, the bit error rate (BER) is the percentage of bits that have errors compared to the total number of bits received in a transmission, usually expressed as 10 to a negative power. For example, a transmission with a BER of 10^–6 received 1 errored bit in 1,000,000 bits transmitted. The BER indicates how often a packet or other data unit must be retransmitted because of an error. If the BER is too high, a slower data rate might improve the overall transmission time for a given amount of data if it reduces the BER and thereby lowers the number of resent packets.

A bit error rate test (BERT) is a procedure or device that measures the BER for a given transmission. You can configure a device to act as a BERT device by configuring the interface with a bit error rate and a testing period. When the interface receives a BERT request from a BER tester, it generates a response in a well-known BERT pattern. The initiating device checks the BERT-patterned response to determine the number of bit errors.

Clocking determines how individual routing nodes or entire networks sample transmitted data. As streams of information are received by a device in a network, a clock source specifies when to sample the data. In asynchronous networks, the clock source is derived locally, and synchronous networks use a central, external clock source. Interface clocking indicates whether the device uses asynchronous or synchronous clocking.

Most networks are designed to operate as asynchronous networks. Each device generates its own clock signal, or devices use clocks from more than one clock source. The clocks within the network are not synchronized to a single clock source. By default, devices generate their own clock signals to send and receive traffic.

The system clock allows the device to sample (or detect) and transmit data being received and transmitted through its interfaces. Clocking enables the device to detect and transmit the 0s and 1s that make up digital traffic through the interface. Failure to detect the bits within a data flow results in dropped traffic.

Short-term fluctuations in the clock signal are known as . Long-term variations in the signal are known as .

Asynchronous clocking can either derive the clock signal from the data stream or transmit the clocking signal explicitly.

This topic contains the following sections:

• Data Stream Clocking
• Explicit Clocking Signal Transmission

All packets or frames within a network can be damaged by crosstalk or interference in the network's physical wires. The frame check sequence (FCS) is an extra field in each transmitted frame that can be analyzed to determine if errors have occurred. The FCS uses cyclic redundancy checks (CRCs), checksums, and two-dimensional parity bits to detect errors in the transmitted frames.

This topic contains the following sections:

• Cyclic Redundancy Checks and Checksums
• Two-Dimensional Parity

The MTU values are by default without any MTU configurations. If the MTU value is set, then the formula IFF MTU (IP MTU) = IFD MTU (Media MTU) – L2 Overhead is applicable. See Table 2 for default MTU values.

Table 2 lists MTU values for the SRX Series Firewalls Physical Interface Modules (PIMs).

SRX Series devices support jumbo frames up to 9192 bytes.

Jumbo frames are Ethernet frames with more than 1500 bytes of payload (maximum transmission unit [MTU]). Jumbo frames can carry up to 9000 bytes of payload.

You configure jumbo frames at the physical interface by using the following command:

set interface interface-name mtu mtu-value

Example:

The supported range for configuring an MTU packet size is 256 through 9192 bytes. However, all interfaces do not support 9192 bytes. For more information on the supported interfaces, see MTU Default and Maximum Values.