ITL - Internet Teaching Laboratory

Physical Layer

Overview and Precautions

All physical media require care in handling. Here are some precautions you should take: The physical media in use in the networking lab are: All of these media use a structure in which there is one "pin" (it is peculiar to label an optical connection a "pin", but the argument still applies) of the connector that transmits and one that receives.

This picture is only conceptual. Most of the connections have many more pins than just these two for transmit and receive.

If the connector at the other end of the cable has the same pin arrangement, then in order for the transmit at one end to connect to the receive at the other, and vice versa, the cable that connects them must have transmit and receive crossed. That is, within the cable pin x at one end connects to pin y at the other and vice versa. Such a "crossover" cable looks like this (even though the lines in the cable in the diagram do not cross one another, this is a crossover cable):

Some devices reverse the trasmit and receive pins internally, so a straight-through cable is appropriate. That is, within the cable pin x at one end connects to pin x at the other end, and the same for pin y. A straight-through cable looks like this:

Some devices provide a switch that selects one arrangement or the other for the pins of a particular connector. It is common for an ethernet hub to have a bank of ports for twisted pair, one of which has a selectable polarity.

The cable in use in the lab is one of the following:

The straight through and crossover cables are used for ethernet connections, and the rolled cables are used for serial console connections.

Twisted Pair Ethernet

There are two kinds of interfaces in the lab that accept RJ45 connectors for twisted pair ethernet cables - interfaces with RJ45 connectors, and interfaces with AUI connectors to which tranceivers can be attached that accept RJ45 connectors. The steps for attaching a tranceiver to an AUI port are as follows:

In the lab the convention is used that red cables are ethernet crossover cables and all other colors except black are straight-through cables. The cables that terminate as jacks on the racks are straight-through. When you connect two ports with the same polarity you use a crossover cable. When you connect two ports with opposite polarity you use a straight-throught cable. Here is a list of the polarities of the RJ45 ports in the lab. The polarity of the network interface cards on the computers is designated as "normal":

computer network interface card normal
3COM 2500/3500 8 port ethernet reversed
3COM 3500 management port normal
Asante AH2072 12 or 24 port segment reversed
Asante AH2072 AUI port with tranceiver normal
Cisco 2900 port reversed
Cisco 7000 AUI port with tranceiver normal

When the proper cable connects two ports a LED on each port lights to show that link between the ports has been established. The NICs on the computers each have two LEDs - a green one that lights when link is established for 100BaseT and a yellow one that lights when link is established for 10BaseT.

Optical Fiber

Although there is little risk from optical ports, you should avoid looking directly at the light emitted by the ports. Whenever possible unused ports should have covers on them.

The connectors for optical fiber may be round or square. The round connectors are type ST (think of "T" for "tube", the cylindrical shape of these connectors) and the square connectors are type SC (think of "C" for "cube"). Currently the only optical patch cables in the lab are type ST multimode fiber.

There are two kinds of interfaces in the lab that accept connectors for optical fiber ethernet cables - interfaces with connectors, and interfaces with AUI connectors to which tranceivers can be attached that accept fiber connectors. Most of the optical ports in the lab use ST connectors, and all the fiber cables have ST connectors. There are a few ports on the CoreBuilder 3500s that accept SC connectors, and there are currently no cables that can be connected to these ports.

Care should be taken when connecting the fiber cables to the ports. The housing at the end of the cable rotates so that a notch on the housing aligns with a key on the port. The housing has a spring loading so that once the notch is aligned with the key the connector can be pressed towards the port and turned to lock it in place.

You must connect the transmit connector of one port to the receive connnector of another. If you connect the fiber correctly a LED on each port will light to show that link has been established.

Serial Network Connections

The Cisco 7000 routers support two kinds of interfaces that use serial cables - High Speed Serial Interface (HSSI) and Fast Serial Interface (FSIP). The only cables in the lab that connect to these interfaces are crossover cables. The cables are distinguishable from each other by the connectors. The connectors for HSSI are SCSI connectors, whereas those for Fast Serial are DB60 connectors.

When a HSSI cable is connected to HSSI interfaces at both ends a LED on each interface will light to indicate a connection. These cables are symmetric - one end is the same as the other. Since HSSI interfaces are synchronous, they require a clock signal. For each HSSI interface the following command must be issued in interface mode:

     router(config-if)# hssi internal-clock

FSIP interfaces have 4 LEDs. The LED labeled "CON" will light when a FSIP cable is successfully connected to FSIP interfaces at both ends. These cables are NOT symmetric. One end is labeled DCE and the other DTE. The cable can be connected in either direction, but clock signal must be provided by the interface at the DCE end. For the FSIP interface at the DCE end of the cable the following command must be issued in interface mode:

     router(config-if)# clock rate r

where "r" is one of the valid clock rates (which can be discovered by issuing the command with a "?" in place of "r". All 4 LEDs will be lit when there is a successful connection with clock signal.

FDDI Connections

The FDDI cables are pairs of multimode fiber with the ends permanently attached to a single connector. When the cables are not connected to a piece of equipment the pins that emit and detect the optical signals should be covered with rubber caps. Be sure to remove these caps before plugging in the cables. Put the caps aside in a safe place and put them back on the cables when the cables are not in use.

The FDDI interfaces on the Cisco 7000 routers have two connectors. These are labeled "PHY A" and "PHY B". When two or more routers are connected the FDDI cables should form a ring. One cable connects "PHY A" of router 1 to "PHY B" of router 2, another cable connects "PHY A" of router 2 to "PHY B" of router 3, and so on until a cable connects "PHY A" of the last router to "PHY B" of router 1 to complete the ring. A router connected in this manner is a "Dual Attachment Station" or DAS. Cables and concentrators for "Single Attachment Stations" or SAS are not available in the lab.

The FDDI interfaces have a matrix of 6 LEDs to indicate their status. Before they are connected by cables the FDDI interfaces will either have none of these LEDs lit or all of them lit. If none are lit the interface is administratively up and ready for use. If all are lit the interface is administratively down and must be enabled through an interface command. If the interface is adminstratively up, when a connection is made between "PHY A" on router 1 and "PHY B" on router 2 the LED on router 1 for "DAS" and "PHY A" will light, and the LED on router 2 for "DAS" and "PHY B" will light.

Token Ring

There are token ring interfaces in some of the Cisco routers. Some of these have adapters to convert the DB9 connector to an RJ45 connector. For these ports standard straight through twisted pair cable can be used to connect the interface port to a port on the MAU.

In order to enable the token ring interface, first set a ring speed, and then administratively enable the interface: 

     router#configure terminal
     router(config)#interface tokenring 4/0
     router(config-if)#ring-speed 4
     router(config-if)#no shutdown

Serial Console Connections

There are console ports on all the network equipment to allow the equipment to be managed. These console ports all use serial cables to connect to to the serial port on a computer. The console ports differ in three ways:

All console ports and serial computer ports that use either DB9 or DB25 connectors have adapters that convert the DB9 or DP25 to RJ45 so that the only cables needed to connect serial console or computer ports are rolled cables (see above) with RJ45 connectors. For some devices gender changers are needed (these are needed on the Asante AH2072 hubs). For changing polarity "null modems" are needed (these are also needed on the Asante AH2072 hubs).

Here are the characteristics of the console ports on the different devices. Note that the gender shown in the table is the gender of the port connector without any adapters. The polarity of the serial port on the computer is designated to be normal.

computer serial port normal DB15 male
3COM 2500/3500 reversed DB15 male
Asante AH2072 normal DB15 female
Cisco 7000 reversed DB25 female

Interfaces on Cisco Hardware

In order to be usable an interfaces on a Cisco switch or router must be:

Exercises
  1. Using the web look up the cost of the various cables in use in the lab. Remember these costs when handling the cables.

  2. Using one twisted-pair stright-through cable and one crossover cable, and without looking back at the table of polarities above, determine for each of the following types of ports what polarity the port has and whether it supports 10BaseT, 100BaseT, or both. Begin by connecting the interface cards themselves to confirm that their leds show the speed of the connection.

  3. Get 2 computers communicating (either use ping or use the send and examine programs described in the section on programming with raw packets) with a direct connection (remember to make the connection at the distribution panel on the rack). Then determine the cabling needed to connect the computers

  4. What happens if you connect 2 computers through a hub but you also cable one port on the hub to another port on the same hub?

  5. What happens if you connect 2 computers through 2 hubs but you connect the hubs together redundantly (i.e. with two cables between them)?

  6. Connect a computer to the console port of one of the Asante AH2072 hubs and examine the menu structure and the information available about the hub. There is no password, so if you are asked for one just press Enter. One of the pieces of information you can examine is the number of collisions that have occurred. Connect two computers together through the hub whose console you are connected to and attempt to ping one computer from the other in an attempt to generate collisions in the hub. There is an option in ping to send packets as fast as ping can do so. You may wish to try this option to generate collisions. You might also get more than one computer sending ping packets through the hub.
| Sonoma State University | CS Department | Computer and Engineering Science | Internet Teaching Laboratory |