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 13 summarizes some key physical properties of device interfaces.
Table 13: Interface Physical Properties
Physical Property |
Description |
|---|---|
bert-error-rate |
Bit error rate (BER). The error rate specifies the number of bit errors in a particular bit error rate test (BERT) period required to generate a BERT error condition. See Bit Error Rate Testing. |
bert-period |
Bit error rate test (BERT) time period over which bit errors are sampled. See Bit Error Rate Testing. |
chap |
Challenge Handshake Authentication Protocol (CHAP). Specifying chap enables CHAP authentication on the interface. See PPP Authentication. |
clocking |
Clock source for the link. Clocking can be provided by the local system (internal) or a remote endpoint on the link (external). By default, all interfaces use the internal clocking mode. If an interface is configured to accept an external clock source, one adjacent interface must be configured to act as a clock source. Under this configuration, the interface operates in a loop timing mode, in which the clocking signal is unique for that individual network segment or loop. See Interface Clocking. |
description |
A user-defined text description of the interface, often used to describe the interface's purpose. |
disable |
Administratively disables the interface. |
encapsulation |
Type of encapsulation on the interface. Common encapsulation types include PPP, Frame Relay, Cisco HDLC, and PPP over Ethernet (PPPoE). See Physical Encapsulation on an Interface. |
fcs |
Frame check sequence (FCS). FCS is an error-detection scheme that appends parity bits to a digital signal and uses decoding algorithms that detect errors in the received digital signal. See Frame Check Sequences. |
mtu |
Maximum transmission unit (MTU) size. The MTU is the largest size packet or frame, specified in bytes or octets, that can be sent in a packet-based or frame-based network. The Transmission Control Protocol (TCP) uses the MTU to determine the maximum size of each packet in any transmission. For MTU values on J Series interfaces, see MTU Default and Maximum Values. |
no-keepalives |
Disabling of keepalive messages across a physical link. A keepalive message is sent between network devices to indicate that they are still active. Keepalives help determine whether the interface is operating correctly. Except for ATM-over-ADSL interfaces, all interfaces use keepalives by default. |
pap |
Password Authentication Protocol (PAP). Specifying pap enables PAP authentication on the interface. To configure PAP, use the CLI or J-Web configuration editor. PAP is not available in the J-Web Quick Configuration pages. |
payload-scrambler |
Scrambling of traffic transmitted out the interface. Payload scrambling randomizes the data payload of transmitted packets. Scrambling eliminates nonvariable bit patterns (strings of all 1s or all 0s) that generate link-layer errors across some physical links. |
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.
 |
Note: Because truly synchronous networks are difficult to design and maintain, most synchronous networks are really plesiochronous networks. In a plesiochronous network, different timing regions are controlled by local clocks that are synchronized (with very narrow constraints). Such networks approach synchronicity and are generally known as synchronous networks. |
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 clock jitter. Long-term variations in the signal are known as clock wander.
Asynchronous clocking can either derive the clock signal from the data stream or transmit the clocking signal explicitly.
Common in T1 links, data stream clocking occurs when separate clock signals are not transmitted within the network. Instead, devices must extract the clock signal from the data stream. As bits are transmitted across the network, each bit has a time slot of 648 nanoseconds. Within a time slot, pulses are transmitted with alternating voltage peaks and drops. The receiving device uses the period of alternating voltages to determine the clock rate for the data stream.
Clock signals that are shared by hosts across a data link must be transmitted by one or both endpoints on the link. In a serial connection, for example, one host operates as a clock master and the other operates as a clock slave. The clock master internally generates a clock signal that is transmitted across the data link. The clock slave receives the clock signal and uses its period to determine when to sample data and how to transmit data across the link.
This type of clock signal controls only the connection on which it is active and is not visible to the rest of the network. An explicit clock signal does not control how other devices or even other interfaces on the same device sample or transmit data.
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.
On a link that uses CRCs for frame checking, the data source uses a predefined polynomial algorithm to calculate a CRC number from the data it is transmitting. The result is included in the FCS field of the frame and transmitted with the data. On the receiving end, the destination host performs the same calculation on the data it receives.
If the result of the second calculation matches the contents of the FCS field, the packet was sent and received without bit errors. If the values do not match, an FCS error is generated, the frame is discarded and the originating host is notified of the error.
Checksums function similarly to CRCs, but use a different algorithm.
On a link that uses two-dimensional parity bits for frame checking, the sending and receiving hosts examine each frame in the total packet transmission and create a parity byte that is evaluated to detect transmission errors.
For example, a host can create the parity byte for the following frame sequence by summing up each column (each bit position in the frame) and keeping only the least-significant bit:
Frame 1 0 1 0 1 0 0 1 Frame 2 1 1 0 1 0 0 1 Frame 3 1 0 1 1 1 1 0 Frame 4 0 0 0 1 1 1 0 Frame 5 0 1 1 0 1 0 0 Frame 6 1 0 1 1 1 1 1 Parity Byte 1 1 1 1 0 1 1
If the sum of the bit values in a bit position is even, the parity bit for the position is 0. If the sum is odd, the parity bit is 1. This method is called even parity. Matching parity bytes on the originating and receiving hosts indicate that the packet was received without error.
Table 14 lists MTU values for J Series devices.
Table 14: MTU Values for J2320, J2350, J4350, and J6350 Interfaces