Content Overview The physical layer transmits bits from one computer to another and regulates the transmission of a stream of bits over the physical medium. This layer defines how the cable or antenna is attached to the network adapter and what transmission technique is used to send data over the medium. Repeaters, hubs, multiplexers, and network interface cards (NICs) operate at the physical layer. A NIC is primarily a Layer 2 device, but also operates at the physical layer of the OSI model. A NIC performs Media Access Control (MAC) sub-layer functions, but it also encodes bits and sends them out on the medium. Repeaters, hubs, and multiplexers have no interest in higher layer function and simply regenerate and propagate received bit-streams to outgoing ports. Problems that occur at Layer 1 are distinctly different from problems that occur at higher layers. The physical layer is the only layer with physically tangible properties such as wires, cards, and antennas. In this lesson, identifiable characteristics and commands will be used to isolate problems at the physical layer. Failures and suboptimal conditions at the physical layer do not merely inconvenience users. Networks that experience these kinds of conditions usually come to a grinding halt. Since the upper layers of the OSI model depend on the physical layer in order to function, a network technician must have the ability to effectively isolate and correct problems at this layer. Upon completing this module, the students will be able to:
Content 3.1 Characteristics of Physical Layer Failure Problems 3.1.1 Critical characteristics – connectivity Anytime that there is a physical layer failure, a loss of connectivity will be experienced. A user or a group of users affected by the problem will usually call the help desk complaining that everything is down. Less astute users may only complain that a specific service or application is not working. If a technician can log in to one of the affected devices and start gathering information, it will be evident that no component above the physical layer is operating. Unlike network failures, all cable failures are approached in approximately the same manner, whether the link is newly installed or has failed during operation. There are many instances where a poor quality link has been in service, but due to the operating environment and influences it has stopped working. These influences include visible damage to the cable, new electrical noise sources near the cable, or accidental movement of the cables. Most networks have converted from coaxial cable to Category 5 or Category 5e UTP links. However, there is still a surprising amount of coaxial cable in legacy network segments. Fiber-optic cable is approaching a price point where it will rival Category 6 UTP in overall cost. The overall cost of a fiber-optic cable installation includes materials, labor, and network adapters. All of these factors have become less expensive in recent years. There are different installation and maintenance issues for each cable type. Lab Activity Lab Exercise: Applying a Logical Layered Model to a Physical Network After completing this lab, the student will be able to understand the data flow in the classroom network based on the logical network model.
Content 3.1 Characteristics of Physical Layer Failure Problems 3.1.2 Critical characteristics – upper layer component operation With the exception of Layer 1, each layer of the OSI model depends on the layer below it for communication to work. In the event of a Layer 1 connectivity problem, no component above the physical layer will be operational. For example, name resolutions, ARP requests, and DHCP requests all rely on Layer 1 for connectivity. An example of upper layer protocol failure would be a corporate user attempting to access a file server from a workstation. Depending on the location of the fault, the following types of communication components may fail:
Content 3.1 Characteristics of Physical Layer Failure Problems 3.1.3 Noncritical characteristics – equipment indicators The Light Emitting Diodes (LEDs) on a device can give feedback for diagnosing the operational status of the device. When there is a physical problem with equipment, the LEDs of the failing device are usually off, flashing, or a different color than usual. Most LED link lights are now software controlled, so they are no longer reliable as a sole indicator of connectivity. If the link light is illuminated, it may or may not mean that a valid link is present. If the port is faulty, it may be possible to disconnect the cable and still have the link light illuminated. If the link light is off however, then it is still a fairly good indication that no link is present. For devices using multispeed interfaces such as 10/100 and 10/100/1000 NICs, it is not uncommon to see link lights at one end of a failing link, but not at the other. If a 10BASE-T device is connected to a switch that is ‘hard-set’ or only capable of 100BASE-TX, the link LED on the 10BASE-T device will often show that the link is active, but the network card will fail to communicate. The 100BASE-TX switch will not show link, and the 100BASE-TX Ethernet segment is unaffected. If a 100BASE-TX only station is connected to a shared media 10BASE-T hub, then it will not show a link. The workstation will cause somewhere between 33 percent and 100 percent collisions on the 10BASE-T collision domain. The 10BASE-T hub will show the link, and if it has status LEDs it will usually also show constant utilization and/or collisions with those status LEDs. To troubleshoot mismatched speeds, a more thorough examination than just looking at the link state LED is needed. If a cable fault is the cause of link failure, an auto-negotiating 10/100 device may successfully negotiate a link to 100BASE-TX, but it may not be able to subsequently establish and maintain signaling synchronization. This results in the link light blinking on and off as the Ethernet chipset repeatedly negotiates a connection speed, then fails to link. If the cable fault is not severe, then the link may come up and operate very poorly for a short time before the link fails and is again renegotiated. If the affected port is a parallel uplink between switches, this fault can cause spanning-tree problems and loss of logical connectivity across a broadcast domain.
Content 3.1 Characteristics of Physical Layer Failure Problems 3.1.4 Noncritical characteristics – power failures A major category of physical layer problems which may be encountered are power related. If power is not supplied to each network device within specific tolerances, then either a loss of service or damage to the unit may occur. Either the device will not have sufficient power to function, or it may be damaged, if the power fluctuates above the design specifications for that device. These fluctuations could include the following: Power failure
A power failure is characterized by a loss of power. Incoming power is subject to blackouts, which are complete power outages, often caused by