433 8C04 NMVY43YFSQAYQTRGRSPPKV Nieznany

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Basic Networking
Technologies

Terms you’ll need to understand:

✓ Media Access Control (MAC)

addressing

✓ Ethernet 802.3

✓ Ethernet_II

✓ Fast Ethernet

✓ Gigabit Ethernet

✓ Token Ring 802.5

✓ Fiber Distributed Data Interface

(FDDI)

✓ Copper Distributed Data Interface

(CDDI)

✓ Carrier Sense Multiple Access with

Collision Detect (CSMA/CD)

✓ Beaconing

✓ Ring insertion

✓ Ring monitor

✓ Dual homing

✓ H.323

✓ Signaling System 7 (SS7)

✓ Realtime Transport Protocol (RTP)

✓ RTP Control Protocol (RTCP)

✓ Quality Of Service (QOS)

Techniques you’ll need to master:

✓ Describing layer 2 MAC addresses

✓ Working with Ethernet, Token

Ring, and FDDI characteristics and
limitations

✓ Understanding basic multiservice

theory

○

Chapter 4

This chapter concentrates on the characteristics and limitations of the different
types of Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI)
technologies. After reviewing each of these technologies, the chapter briefly turns
to voice and video communications that can be delivered over existing data net-
works. These topics are called Multiservice Services by Cisco.
The following CCIE blueprint objectives as determined by the Cisco Systems
CCIE program are covered in this chapter:

➤ Data Link layer—MAC addressing and IEEE 802.2 standards

➤ Ethernet/Fast Ethernet/Gigabit Ethernet—Encapsulation, Carrier Sense Mul-

tiple Access with Collision Detect (CSMA/CD), topology, speed, controller
errors, limitations, and the IEEE 802.3 standards

➤ Token Ring—Token passing, beaconing, active monitor, ring insertion, soft and

hard errors, topology, maximum transmission unit (MTU), speed, limitations

➤ FDDI/CDDI—Dual ring, encapsulation, class, redundancy, dual homing,

medium (including copper and fiber), claims, station management (SMT),
limitations

➤ Voice/Video—H.323, codecs, Signaling System 7 (SS7), Realtime Transport

Protocol (RTP), RTP Control Protocol (RTCP), Quality Of Service (QOS)

Additional information is provided for completeness and in preparation for addi-
tional subjects as the CCIE program expands. We will begin by discussing what
makes up a MAC address.

MAC Addressing

All devices that operate over a physical LAN medium require a unique address,
called the Media Access Control (MAC) address. The MAC address is also some-
times referred to as the physical address, burned-in address (BIA), or hardware
address. A MAC address is assigned to each hardware device that connects to a
LAN, such as an Ethernet NIC. In Token Ring networks, the MAC address can
be set in software.

In IEEE 802 networks, the Data Link Control (DLC) layer of the OSI reference
model is divided into two sublayers: the Logical Link Control (LLC) layer and
the Media Access Control (MAC) layer. Figure 4.1 displays the location of the
LLC sublayer and the MAC sublayer in relation to the OSI model.

LLC Sublayer Functions

The LLC sublayer provides networks with connection or connectionless
enviroments. The LLC sublayer simply sits on top of all 802.x protocols and
provides a service to the Network layer.

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Basic Networking Technologies

Using IP as an example, we know that IP is a connectionless service, but the role
of the LLC sublayer is to identify that an IP packet is carried in the data portion
of the frame. The IP software then looks further into the frame to locate the
header information and the IP address.

MAC Sublayer Functions

The MAC sublayer simply provides access to the Physical layer, whether Ethernet
or any other medium is in use. To allow this communcaition each device must
have a unique address.

To enable all devices to have a unique address or MAC address, the network
interface cards have a unique MAC address located in Read Only Memory
(ROM). This unique address allows communication between devices regardless
of the physical medium. Let’s now describe the format of the MAC address.

MAC addresses are 48 bits long, and they are expressed primarily in two formats:

➤ 0060.7015.5e4d

➤ 00-60-70-15-5e-d4

The first byte, or octet, of a MAC address also contains two reserved bits that are
used to identify what destination device or devices are intended to be the recipent
of the frame:

➤ I/G—Individual/Group

➤ L/G—Local/Global

Layer 2 frames can be directed to one (I bit) or more devices (G bit). The Local/
Global bit defines whther the address is the burned in address or a locally as-
signed address.

OSI Model

IEEE 802 Network

Network

Data Link

Physical

Logical Link Control

Media Access Control

Figure 4.1

IEEE 802 DLC.

○

Chapter 4

Canonical vs. Non-Canonical

The IEEE refers to MAC addresses as Universal Addresses. The IEEE also specified

that when the bits are sent across the Ethernet Physical layer the least significant

bit is transmitted first. This is referred to as

non-canonical. Token Ring is canonical,

which means that the most significant bit is transmiited first. Let’s look at a simple

example of sending the number 1 (decimal) across an Ethernet network. The num-

ber 1 in binary is 00000001. The non-canonical format of this is 10000000. This

means that on the Ethernet wire the bits 10000000 will be reversed by the receiving

device back to 00000001.

The majority of modern networks use a 48-bit addressing scheme or plan. MAC
addresses are represented using the hexadecimal numbering system. The first 24
bits represent the manufacturer’s identification, vendor’s code, or the organiza-
tion unique identifier (OUI). The next 24 bits typically provide a serial number
assigned by the vendor. To illustrate, here is an example of a Cisco MAC address:

006070-155e4d

In the preceding address, 00-60-70 (24 bits) identifies Cisco as the manufacturer
or vendor code, and 15-5e-4d (24 bits) identifies the serial number assigned by
Cisco. Manufacturers such as Cisco may have more than one OUI. For instance,
Cisco Systems has more than 20 OUIs from the IEEE.

Frames sent to MAC addresses can be classified as being sent to either unicast,
multicast, or broadcast addresses:

➤ Unicast Frame—A frame destined for a specific device. In the destination

address, a unicast frame will appear as 0xxxxxxx in the first byte.

➤ Multicast Address—A special address reserved for communication among a

group of devices. For example, 1xxxxxxx in the first byte.

➤ Broadcast Address—An address destined for all devices on the wire. For ex-

ample, FF-FF-FF-FF-FF-FF in the destination field indicates all devices
must read the frame.

Note that all frames will have their source MAC address as a single node (unicast).

Ethernet, Fast Ethernet, and
Gigabit Ethernet

Ethernet is one of the most popular local area network (LAN) technologies used
today. Ethernet can operate at three speeds:

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Basic Networking Technologies

➤ Ethernet—Allows transmission speeds of 10Mbps

➤ Fast Ethernet—Allows transmission speeds of 100Mbps

➤ Gigabit Ethernet—Allows transmission speeds of 1,000Mbps

10 Gigabit Ethernet is coming soon. The networking industry has formed
a coalition to make 10G a reality.

Originally, Ethernet started when the Xerox Corporation released a method of
allowing devices to share a common medium and communicate together. Table 4.1
shows a summary of Ethernet’s recent evolution.

In this section, we’ll review the three main Ethernet types, starting with a discus-
sion about traditional Ethernet.

Ethernet 802.3 and Ethernet_II

Ethernet has two versions available—Ethernet 802.3 and Ethernet_II. The main
difference between Ethernet 802.3 and Ethernet_II can be found within the
frame formats, as discussed later in this section. Original Ethernet and then the
second version called Ethernet_II was jointly developed by the Digital Equip-
ment Corporation, Intel, and Xerox Corporation, also know as the DIX Consor-
tium. Ethernet 802.3 is the standard defined by the IEEE.

The Ethernet specifications also define the frame sizes as follows:

➤ Minimum Ethernet Frame Size—64 bytes

➤ Maximum Ethernet Frame Size—1,514 bytes

Table 4.1

Ethernet history.

Date

Timeline Event

1972

Work begins on Ethernet by Xerox

1980

Ethernet released

1982

Version II released by DIX (Digital, Intel, and Xerox)

1985

IEEE 802.3 Ethernet Standard is released

1994

Transmission of Ethernet over twisted pair wiring is released

1995

Fast Ethernet

1998

Gigabit Ethernet

○

Chapter 4

When running IP, Ethernet_II is the default transmission method for
Cisco Routers.

Both Ethernet 802.3 and Ethernet_II are shared physical media technologies.
This means that when an end device on an Ethernet network needs to send data,
it first must wait to see if the physical medium is not being used before transmis-
sion can commence. The end device will listen to the wire using its ability to
carrier sense, which is part of the CSMA/CD. (CS stands for carrier sense.)

Further, while the data is transmitting, the end device must ensure no other de-
vice has transmitted at the same time on its receive interface. The sending station
will also listen to see if received data is different from the transmitted data. If it is
different, a collision occurred. Specifically, the end device is listening for a change
in voltage on the wire.

When a collision occurs, the sending end device transmits a jam signal (a random
signal used to inform all devices that a collision has been detected) and backs off
for a random value calculated with the back off algorithm. This method is called
Collision Sense Multiple Access with Collision Detection (CSMA/CD). As a result
of Ethernet’s shared-medium properties, Ethernet is sometimes referred to as
undeterministic. This is because end devices don’t know when they can send data
(that is, when the wire is clear), and end devices aren’t aware that another device
will transmit at the same time. A deterministic device is able to calculate the
maximum time that will pass before any end station will be able to transmit.

What is half-duplex and full duplex Ethernet? Half duplex Ethernet al-
lows only one device to send data or receive data at a time. Full duplex
Ethernet allows the capability of simultaneous data transmission be-
tween two devices. Full duplex Ethernet allows for better use of the
available bandwidth because both devices can send and receive data
at the same time.

To illustrate CSMA/CD in action, Figure 4.2 shows an Ethernet network with
four PCs (end devices) trying to communicate. The following occurs:

1. PC-1 listens and determines that no one else is sending data.

2. PC-1 starts to transmit information if the wire is clear. At exactly the same

time, PC-2 decides to send data after also determining that no other device
was using the media to send data.

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Basic Networking Technologies

4. At some point, the bits will collide, a collision will be detected by the colli-

sion detection circuitry within each PC’s NIC, and both devices will send a
jam signal and then back off for a random amount of time before attempting
to retransmit.

5. PC-1 sends data once more after completing Step 1 again. This will be tried

up to 15 times before an error is sent to the user applications.

When using a shared media, such as Ethernet, collisions are a part of Ethernet’s
operation, and they are considered normal. However, excessive collisions can cause
delays and reduce available bandwidth to end devices. Typically, after network
utilization goes above 12 percent, you will start to see excessive collisions. Exces-
sive collisions will result in time delays, and end user performance will be im-
pacted. When utilization reaches 30 percent, Ethernet networks will start to
experience longer delays and excessive collisions.

As mentioned earlier, there are two types of Ethernet—Ethernet 802.3 and
Ethernet_II. There are four frame type formats that are supported in Ethernet.
The frame formats vary for each of these Ethernet types. Figure 4.3 displays the

PC-1 has

data to send

PC-2 has

data to send

PC-3 has no
data to send

PC-4 has no
data to send

Collision

occurs

Figure 4.2

Using Ethernet devices to send data using CSMA/CD.

○

Chapter 4

four Ethernet frame formats, which can contain the Ethernet_II, Ethernet 802.3,
Ethernet 802.2, and the SNAP Ethernet frame.

Note: Previously, we mentioned that the minimum size of an Ethernet frame is 64
bytes. Now that we have introduced the preamble, it is crucial to note that while the
preamble is part of the Ethernet frame, it is not considered when determining size.
Thus, an Ethernet frame with a size of 64 bytes (minimum) or 1518 bytes
(maximum) has a preamble that is not counted in the frame size.

We will now cover what each field is responsible for in the four Ethernet frame types.

Ethernet_II has the following frame format parameters:

➤ Preamble (8 bytes)—The preamble is used to synchronize all stations.

➤ Destination Address (6 bytes)—The destination address can be unicast (spe-

cific device), multicast (group of addresses), or broadcast (all devices).

➤ Source Address (6 bytes)—The source address identifies the sender.

➤ Type (2 bytes)—The type field describes the protocol been carried in the frame.

➤ Data (46 to 1,500 bytes)—The data field carries end user information, such as

an email message.

➤ Frame Checksum (4 bytes)—This frame checks sequence and calculates all fields,

except the preamble and the frame checksum (FCS), to make sure the frame
is not corrupted.

(All lengths in bytes)

Ethernet_II Frame

Preamble
8

DA
6

SA
6

Type
2

Data
46 to 1500

FCS
4

Ethernet 802.3 Frame

Preamble
8

DA
6

SA
6

Length
2

802.2 or SNAP Header (see below)
46 to 1500

FCS
4

Ethernet 802.2 Frame

Preamble
8

DA
6

SA
6

Length
2

802.2 Field

DSAP
1

SSAP
1

CTRL
1

Data
46 to 1497

FCS
4

Ethernet SNAP Frame

Preamble
8

DA
6

SA
6

Length
2

AA
1

CTRL
1

Data
46 to 1496

Ethernet Type
1

FCS
4

SNAP Fields

Figure 4.3

Four Ethernet frame types.

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Basic Networking Technologies

The total length of an Ethernet frame must never exceed 1,518 bytes,
or a frame called a

giant will be generated. The smallest frame size is

64 bytes. A frame smaller than 64 bytes is called a

runt.

Ethernet 802.3 has the following frame format parameters:

➤ Preamble (8 bytes)—The preamble is used to synchronize all stations.

➤ Destination Address (6 bytes)—The destination address can be unicast (spe-

cific device), multicast (group of addresses), or broadcast (all devices).

➤ Source Address (6 bytes)—The source address identifies the sender.

➤ Length (2 bytes)—The length field describes data length.

➤ Data (46 to 1,500 bytes)—The data field carries end user information, such as

an email message.

➤ Frame Checksum (4 bytes)—This frame checks sequence and calculates all fields,

except the preamble and the frame checksum (FCS), to make sure the frame
is not corrupted.

The main difference between Ethernet_II and Ethernet 802.3 is that
Ethernet_II has a type field and 802.3 has a length field. If the contents
of this field exceed a value of 1,518, devices will know they are in
possession of a Ethernet_II frame and read the field as a type field. If
the value is less than 1,518, the field is treated as a length field.

Ethernet 802.2 has the following frame format parameters:

➤ Preamble (8 bytes)—The preamble is used to synchronize all stations.

➤ Destination Address (6 bytes)—The destination address can be unicast (spe-

cific device), multicast (group of addresses), or broadcast (all devices).

➤ Source Address (6 bytes)—The source address identifies the sender.

➤ DSAP (1 byte)—The Destination Service Access Point field together with

the SSAP define the source and destination protocol of the frame.

➤ SSAP (1 byte)—The Source Service Access Point field together with the DSAP

define the source and destination protocol of the frame.

➤ CTRL (1 byte)—The control field.

➤ Data (46 to 1,497 bytes)—The data field carries end user information, such as

an email message.

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Chapter 4

➤ Frame Checksum (4 bytes)—This frame checks sequence and calculates all fields,

except the preamble and the frame checksum (FCS), to make sure the frame
is not corrupted.

Note: With an IPX packet, the 802.2 header is set to E0 E0 03.

Ethernet SNAP header has the following frame format parameters:

➤ Preamble (8 bytes)—The preamble is used to synchronize all stations.

➤ Destination Address (6 bytes)—The destination address can be unicast (spe-

cific device), multicast (group of addresses), or broadcast (all devices).

➤ Source Address (6 bytes)—The source address identifies the sender.

➤ DSAP (1 byte)—The Destination Service Access Point field together with

the SSAP define the source and destination protocol of the frame. For a SNAP
frame, this field is set to AA.

➤ SSAP (1 byte)—The Source Service Access Point field together with the DSAP

define the source and destination protocol of the frame. For a SNAP frame,
this field is set to AA.

➤ CTRL (1 byte)—The control field.

➤ Data (46 to 1,496 bytes)—The data field carries end user information, such as

an email message. For a SNAP frame will include a type field that will iden-
tify the payload type like IP for example.

➤ Frame Checksum (4 bytes)—This frame checks sequence and calculates all fields,

except the preamble and the frame checksum (FCS), to make sure the frame
is not corrupted.

In the Control Fields (CTRL), 03 indicates Logical Link Control Type I or
datagram service. In Ethernet_II, the type field identifies the

payload or

end user data. Some common type field examples include the following:

➤ 0x0800—TCP/IP

➤ 0x6004—Local Area Transport (LAT)

➤ 0x8037—IPX

Table 4.2 summarizes a number of standards for Ethernet. Notice that Ethernet
802.3 and Ethernet_II run at 10Mbps. This was found to be too slow for today’s
networks, so a new standard was developed, called Fast Ethernet.

Table 4.2 displays three different physical Ethernet standards used in Ethernet
networks. The first two digits display the speed (in this case 10, it could be 100 or

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Basic Networking Technologies

1,000 for example), the word Base identifies this as baseband (BASE, where one
carrier frequency is used), and finally the last digit identifies the length and cable
type. For example,10BaseT is 10Mb Ethernet, Baseband, Unshielded twisted pair
and a maximum length of 100m.

Fast Ethernet

Fast Ethernet operates at 100Mbps. Fast Ethernet’s frame format and MAC
addresses are the same as Ethernet’s frame format and MAC addresses. The major
differences between Ethernet and Fast Ethernet is that Fast Ethernet can oper-
ate over many types of physical layer connections, including four pair twisted pair
cable. Another major difference is that Fast Ethernet, when cabled with twisted
pair cable, has a maximum network diameter of only 205 meters. Both Ethernet
and Fast Ethernet use CSMA/CD to gain access to media. Table 4.3 summa-
rizes a number of standards for Fast Ethernet (802.3u).

Today, an even faster version of Ethernet is available, called Gigabit Ethernet.
Gigabit Ethernet allows transmissions rates up to 1,000Mbps.

Gigabit Ethernet

Gigabit Ethernet is a recent development that enables transmissions up to
1,000Mbps. Gigabit Ethernet is an extension of the IEEE 802.3 standard and
was developed to meet the needs of an industry that demands more bandwidth
as time goes by. Gigabit Ethernet uses the same frame formats and MTU sizes,
and uses the CSMA/CD algorithm as well. 802.3z defines Gigabit Ethernet.
(For more information about Gigabit Ethernet, visit the IEEE Web site at
www.ieee.com.)

Table 4.2

Ethernet (802.3) characteristics.

Ethernet Standard

Characteristic

10BaseT

10Mbps over two pair twisted cable. Maximum length 100m.

10Base2

10Mbps over coaxial cable (RG58). Maximum length 185m.

10Base5

10Mbps over thick Ethernet. Maximum length 500m.

Table 4.3

Three Fast Ethernet (802.3u) characteristics.

Ethernet Standard

Characteristic

100Base-Tx

100Mbps over two pair twisted wire. Maximum length 100m.

100Base-T4

100Mbps over four pairs of category 3, 4, or 5 cable.
Maximum length 100m.

100Base-Fx

100Mbps over two strands of fiber. Maximum length 400m.

○

Chapter 4

Verifying Ethernet Operation

Now that we’ve talked a little bit about theory, let’s look at a Cisco router’s statis-
tical display on a 10Mbps Ethernet interface and review the fields you need to be
aware of that are provided to you in a show interface command. Listing 4.1 pro-
vides a sample Ethernet statistical display taken from a Cisco router.

Listing 4.1

The show interface Ethernet0 command.

Ethernet0 is up, line protocol is up

Hardware is Lance, address is 0000.0c92.2ed4

Internet address is 10.99.34.50/24

MTU 1500 bytes,BW 10000 Kbit,DLY 1000 usec,rely 255/255,load 1/255

Encapsulation ARPA, loopback not set, keepalive set (10 sec)

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters never

Queuing strategy: fifo

Output queue 0/40, 0 drops; input queue 0/75, 0 drops

5 minute input rate 2000 bits/sec, 2 packets/sec

5 minute output rate 1000 bits/sec, 2 packets/sec

533880 packets input, 74463913 bytes, 0 no buffer

Received 524894 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

0 input packets with dribble condition detected

94282 packets output, 8713055 bytes, 0 underruns

1 output errors, 141 collisions, 2 interface resets

0 babbles, 0 late collision, 230 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

The following list highlights the most important fields relative to troubleshoot-
ing and understanding how Ethernet is operating:

➤ MTU—Maximum transmission unit.

➤ BW—Interface bandwidth, measured in Kbps.

➤ DLY—Interface delay, measured in microseconds.

➤ rely—Reliability of the interface; 255 out of 255 means the interface is 100

percent reliable.

➤ load—Interface load; 255/255 means the interface is 100 percent loaded.

➤ ARP type—Type of Address Resolution Protocol assigned.

➤ packets input—Total number of error-free packets received by the system.

➤ bytes—Total number of bytes received by the interface. (Note that this is on

the same line as packets input.)

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Basic Networking Technologies

➤ no buffers—Number of packets discarded because the router had no available

buffers to store the packet before delivery. (Broadcast storms can typically
make this number high, because the router might not be able to handle the
amount of packets received.)

➤ Received broadcasts—Broadcast or multicast packets received by the interface.

➤ runts—Packets less than the minimum 64 bytes required for Ethernet.

➤ giants—Packets greater than the maximum allowable frame size in Ethernet.

Maximum frame size is set to 1,500 on Cisco router (MTU 1500).

➤ input errors—Number of runts, giants, no buffer available, CRC, frame, over-

run, and ignored counts.

➤ CRC—Cyclic redundancy checksum. Calculated by the source station and

checked by the router.

➤ frame—Number of frames received that have incorrect checksum errors.

➤ overrun—Number of times the receiver hardware was unable to hand re-

ceived data to a hardware buffer because the input rate exceeded the receiver’s
ability to handle the data.

➤ ignored—An internal condition on the router that indicates how many times

the interface runs low on internal buffers.

➤ input packets—A dribble bit error that indicates that a frame is slightly

too long.

➤ with dribble—Number of packets that have been seen by the router that are

slightly larger than the maximum frame size.

➤ packets output—Total number of messages transmitted by the system.

➤ bytes—Total number of bytes put out by the interface.

➤ underruns—Number of times the transmitter (Tx) has run faster than the

router can handle.

➤ output errors—Sum of all errors.

➤ collisions—Number of collisions detected by the router on the local Ethernet.

➤ interface resets—Number of times the interface has been reset. Interface re-

sets can occur manually with the clear interface E0 command or due to an
error condition on the segment, such as excessive broadcasts.

The preceding fields are important when troubleshooting Ethernet networks from
the viewpoint of a Cisco router. For example, an interface that reports a high
number of collisions is indicative of a device that might be faulty on the network.

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Chapter 4

Note: The interface command show interface fastethernet followed by the interface
number displays the same statistical display shown by the show interface Ethernet0
command. The notable difference is the bandwidth parameter, which is set to
100000 Kbit as opposed to 10000 Kbit for Ethernet.

The overriding benefits of Ethernet are that it’s cheap and easy to install. Further,
with Gigabit Ethernet’s recent developments, the future looks good. Now, let’s
turn to the most common networking technology used in the late 1970s and
’80s—Token Ring.

Token Ring 802.5

Token Ring networking was developed by IBM and Texas Instruments in the
1970s in response to the growing popularity of the personal computer. The IEEE
committee defined Token Ring in 802.5 to provide a standard to be used by non-
IBM devices.

In a Token Ring network, a device must have possession of the token frame before
it can transmit data onto the ring. Possession of the token frame allows a device
to send data onto the ring. Without a token frame, devices are not permitted to
transmit. Hence, Token Ring networks are sometimes called deterministic, be-
cause the possession of a free token determines whether a device can transmit
across a medium. Figure 4.4 displays a typical scenario where a group of devices
attached to the ring must wait for a free token before data can be sent across the
network. The free token is placed onto the ring once a device has finished send-
ing data. Token Ring networks are deterministic because each station has equal
access to the token and, therefore, equal access to the network. The token rotates
through the ring in a predictable fashion.

In a Token Ring network, a station must wait for the token to be available before
it can send data on to the ring. After a device possesses the token, the device can
then send data. The data is circulated around the ring until the destination device
has copied the frame and returned the frame into the ring. Then, the sending
device must remove the frame from the ring and place the free token back onto
the ring. The exception to this rule is in the case of early release, when the receiv-
ing station can release the free token.

Token Ring 802.5 can run at two speeds—4Mbps and 16Mbps. To modify the
ring speed on a Cisco router, you use the following IOS command:

ring-speed <4 or 16>

In summary format, the main characteristics of Token Ring 802.5 are:

➤ Star topology

➤ Star cabling

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Basic Networking Technologies

➤ Data rates are 4Mbps or 16Mbps

➤ Logically a ring

➤ Full duplex modes are supported on Cisco routers only when there are just

two stations on the ring. Full Duplex Token Ring allows two devices to trans-
mit simultaneously without the existence of a token.

Note: Token Ring is phsically a star topology. Therefore, Token Ring is sometimes
referred to as a star.

Now, let’s look at the Token Ring frame format defined in 802.5.

Token Ring Frame Formats

There are two Token Ring frame formats defined in 802.5—a token frame with
no data and a frame that contains data (that is, a busy token). A token frame with
no data contains the following fields:

SD AC ED

Token

Ring

Token frame

This PC has

the free token

Possession of token
permits data transfer

Figure 4.4

Token Ring data transfer.

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Chapter 4

A token data frame (a token carrying data) can contain the following fields:

SD AC FC DA SA RIF Data FCS ED FS

Notice that the Token Ring data frame format has more fields when compared to
an Ethernet frame. This makes Token Ring a little more robust, as you can see in
the following descriptions:

➤ SD (Starting Delimiter, 1 byte)—Indicates the start of the frame and is repre-

sented as JK0JK000 in hexadecimal. Don’t worry too much about this field,
but it merely is used to indicate any Manchester code violations.

➤ AC (Access Control, 1 byte)—Contains parameters that define the priority (1

bit that indicates whether the frame is a data or free token). A bit that indi-
cates if the active monitor has seen the frame. The active monitor is a device
on the ring that maintains the ring. The AC field is represented as
PPPTMRRR, where the P bits are priority bits, T bits identify whether the
frame is a token or data frame, M identifies the monitor bit, and RRR speci-
fies the reservation bits.

➤ ED (End Delimiter, 1 byte)—Indicates the end of the frame. This field is set

to JK1JK11E.

➤ FC (Frame Control, 1 byte)—Indicates which type of frame is arriving.

➤ DA (Destination Address, 48 bits)—Indicates the destination MAC address.

➤ SA (Source Address, 48 bits)—Indicates the source MAC address. If the first

bit of the source address is set to 1, a routing information field (RIF) will be
present.

➤ RIF (Routing Information Field)—Describes the routing information field,

which can be up to 18 bytes in length.

➤ Data (>0 bits)—Contains user data.

➤ FCS (Frame Check Sequence, 32 bits)—Checks the FC, DA, SA, and Data fields.

➤ FS (Frame Status, 8 bits)—Indicates if the frame was recognized by another

device and copied. This field is represented as AC00AC00, where A is set to
1 when the address is recognized and C is set to 1 when the frame is copied.
The R bits are reserved. The A and C bits are copied, because there is no
redundancy check or CRC made on this field.

An important fact is that Token Ring supports two broadcasts frame
types—FF-FF-FF-FF-FF-FF and C0-00-FF-FF-FF-FF.

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Basic Networking Technologies

Figure 4.5 illustrates a variety of bit combinations and serves as a very handy
diagram for troubleshooting Token Ring networks.

Token Ring has a number of built-in stations that monitor and maintain the
ring. These stations enable the ring to recover from faults and error conditions,
such as when there are no free tokens circulating a ring for an extended time. The
stations ensure that the token is always available and report problems to network
protocol analyzers, if they are present. Table 4.4 summarizes the functions per-
formed by Token Ring stations.

Token Frame

(No data)

P P P T M R R R

D D 0 0 C C C C

Data

FCS

0 0 0 0 0 0 0 E

A C 0 0 A C 0 0

• P bits indicate

priority.

• T bit indicates

whether the
frame is a
token (bit set
to 0) or data
frame if this
bit is set to 1.

• R bits are

used for
reservation
purposes.

• D bits are used

to identify
whether the frame
is a LLC (01) or
MAC Frame (00).

• The C bits indicate

what type of
management frame
the frame is:
0000 Express buffer
0010 Beacon frame
0011 Claim token
0100 Ring purge
0101 Active monitor
0110 Standby

monitor present

• If the E bit is set
to 1, this frame
will have a FCS
error and must
be retransmitted
by source device.

• If the A bit is set

to 1, the address
has been
recognized.

• C is set to 1 if

the frame has
been copied.

• Both A/C bits are

duplicated,
because the FCS
does not cover
this field.

up to 17800 bytes

Field length
in bits unless
specified

Figure 4.5

Token Ring frame formats.

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Chapter 4

Token Ring networks elect two devices called the

active monitor and

standby active monitor. Any device on the ring can perform this func-
tion. The function performed by the active monitor is basically to ensure
the integrity of the ring and ensure no device is holding onto the token
free frame forever. In case a token free frame is lost or corrupted, for
example, the active monitor will clear or purge the ring and issue a new
token. The standby active monitor waits for a failure on the active monitor.

If there’s a token failure (such as a station waits for a token but does not see it for
a specified frame or a faulty device does not release the token or continually sends
data irrespective of other devices), a special frame advertises that there is no to-
ken available. This process is called beaconing. When beaconing occurs, the ring
is down, and all stations will re-insert into the ring. Beaconing is not a desirable
process for a network, because it indicates a possible hardware fault. During the
beaconing process, a Token Ring station is unable to send data.

Let’s now look at how a Token Ring station attaches, or inserts, itself into a ring
using a procedure called the ring insertion process.

Ring Insertion Process

All devices on a Token Ring will go through a process to insert into the ring. The
steps involved in the ring insertion process are as follows:

1. Phase 0: Lobe Media Check/Physical Insertion—The device runs a loopback

test to ensure that any frame that is sent is received.

Table 4.4

Function performed by Token Ring stations.

Service

Description

Active Monitor

Can be any station on a ring. The main function is to provide
timing information and maintenance functions. One of the main
functions of the Active Monitor is to ensure that frames will not
circulate the ring forever. A bit in the Token Ring frame called
the

monitor bit ensures that the frame will only circulate the

ring once.

Standby Monitor

Can be any station on a ring. This station monitors the current
active monitor and replaces it if it becomes available.

Ring Error Monitor

Typically a network analyzer. This monitor collects errors and
other data seen on the Token Ring.

Beacon

A special frame that indicates a problem on the ring. A beacon
is sent out by a by a station when a free token frame has not
been seen for an amount of time.

Ring Purge

A recovery action performed by an active monitor in instances
when a recovery of the ring is required.

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Basic Networking Technologies

2. Phase 1: Monitor Check—The device sends a signal (called a phantom voltage)

to flip the old mechanical switches.

3. Phase 2: Duplicate Address Verification—The device sends a frame to ensure

that there is no duplicate MAC address.

4. Phase 3: Neighbor Notification—Adjacent neighbors are found, and the new

Token Ring device will notify the downstream neighbor of its MAC ad-
dress.

5. Phase 4: Request Initialization (parameters from the ring)—The device requests

initialization parameters. A station already on the ring (known as a ring pa-
rameter server) sends required values, such as ring number and speed.

After the preceding five steps have been accomplished and no errors have been
found, the new device becomes a participant in the Token Ring. That is, the new
device becomes eligible to transmit data and waits for a free token before trans-
mitting any data.

Token Ring Errors

If errors are encountered on a Token Ring network, they can be classified into
two general types—soft and hard errors. Soft errors do not require a full recovery
of all stations and can be tolerated by the ring. For example, a corrupt frame (one
with a bad checksum, for instance) is a soft error. On the other hand, hard errors
are serious errors that cause network disruption. For example, a beacon frame is a
hard error.

Note: Token Ring’s popularity in the 1970s and ‘80s means that the technology will
be around for a while. There is no real justification to rip out all the Token Ring
infrastructure and spend thousands of dollars to install Ethernet networks. New
installations are tending to be Ethernet also becasue of the cost in relation to Token
Ring. Token Ring cards are more expensive than Ethernet cards, because Token
Ring stations require a considerable amount of circuitry to accommodate the extensive
management and ring operation.

Token Ring Reliabilty

The IEEE 802.5 specification defined the field called the Frame status field as
displayed in Figure 4.5.

The A or address recognized field and the C or copied field are used together to
ensure the status of the frame. The A field is used to identify if the frame has been
recognized by the destination device. The A bit ensures that the frame has been
seen by the destination device. The C bit identifies whether the frame had been
copied from the ring into the local buffer by the destination device. You must

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Chapter 4

remember that in Token Ring, the data frame is only stripped from the ring by
the originating device or the active monitor. The following bulleted summary de-
scribes the possible combinations used in Token Ring to ensure the status of a frame:

➤ A C—Description.

➤ 1 1—Frame has been recognized and copied.

➤ 1 0—Frame has been recognized but could not be copied by the destination

device.

➤ 0 0—The frame has not been recognized or copied.

➤ 0 1—Not a possible condition.

Token Ring Display

Now, let’s examine a Cisco Token Ring interface and review the fields. Listing 4.2
presents a sample display from a Cisco router that displays a Token Ring interface.

Listing 4.2

The show interface Tokenring0 command.

TokenRing0 is up, line protocol is up

Hardware is TMS380, address is 0000.308f.3655 (bia 0000.308f.3655)

Internet address is 137.10.9.1/24

MTU 4464 bytes, BW 16000Kbit, DLY 630usec, rely 255/255 load 1/255

Encapsulation SNAP, loopback not set, keepalive set (10 sec)

ARP type: SNAP, ARP Timeout 04:00:00

Ring speed: 16 Mbps

Multiring node, Source Route Transparent Bridge capable

Source bridging enabled, srn 150 bn 1 trn 140

proxy explorers disabled, spanning explorer enabled

Group Address: 0x00000000, Functional Address: 0x0800011A

Ethernet Transit OUI: 0x000000

Last input 00:00:06, output 00:00:07, output hang never

Last clearing of "show interface" counters never

Queueing strategy: fifo

Output queue 0/40, 47 drops; input queue 0/75, 0 drops

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

1027036 packets input, 139907117 bytes, 0 no buffer

Received 885583 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

2589508 packets output, 2212474871 bytes, 0 underruns

0 output errors, 0 collisions, 5 interface resets

0 output buffer failures, 0 output buffers swapped out

6 transitions

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Basic Networking Technologies

Notice the preceding display is similar in some respects to an Ethernet display
(shown earlier in this chapter in Listing 4.1). Following is a brief summary of
relevant Token Ring fields shown in Listing 4.2:

➤ MTU—Specifies the maximum transmission unit.

➤ BW—Specifies the bandwidth of the interface in Kbps.

➤ DLY—Indicates the delay of the interface (usec or micro seconds).

➤ rely—Specifies the reliability of the interface (255/255 equals 100 percent

reliability).

➤ load—Indicates the load on the interface (255/255 equals 100 percent loaded).

➤ Encapsulation—Specifies the Token Ring encapsulation as SNAP.

➤ ARP type—Specifies the type of address resolution protocol assigned.

➤ Ring speed—Specifies the ring speed (4 or 16).

➤ {Single ring/Multiring node}—Specifies whether the router is permitted to

collect and use source routing information or RIF.

➤ Group Address—Specifies the group address of interface, if any. Used in

multicasts.

➤ Functional Address—Specifies special addresses that are used by Token Ring

stations. There are eight functional addresses in Token Ring.

➤ Last input—Indicates the last time a packet was seen on the interface.

➤ packets input—Specifies the total number of error-free packets received by

the system.

➤ bytes—Specifies the number of error free bytes.

➤ no buffer—Specifies the number of received packets discarded because there

was no available memory to allocate to store the frame or frames temporaily
in the buffer space.

➤ Received broadcasts—Specifies the total number of broadcast or multicast pack-

ets received by the interface.

➤ runts—Calculates the number of frames that have been received less than the

minimum frame.

➤ CRC (cyclic redundancy check)—Increments when errors are found. Checksums

are calculated by the source station and checked by the router.

➤ input errors—Sums all errors together seen by the interface.

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Chapter 4

➤ overrun—Specifies the number of packets received incorrectly due to check-

sum errors.

➤ ignored—Indicates how many times the interface runs low on internal buffers.

➤ packets output—Specifies the total number of messages transmitted by

the system.

➤ underruns—Specifies the number of times the transmitter (Tx) has been run-

ning faster than the router can handle.

➤ output errors—Sums all errors together.

➤ collisions—Indicates when a software error occurs on the router (although

Token Ring has no concepts of collisions). This number is zero when there
are no software errors.

➤ interface resets—Specifies the number of times an interface has been reset.

This can be set manually with the clear interface Tokenring command, or it
can be set by an error condition on a segment, such as a beaconing. Beaconing
is a mechanism used to notify all devices of a hardware fault.

➤ transitions—Indicates the number of times the interface has changed from

active to inactive.

The Maximum Transmission Unit or MTU in Token Ring is 17,800 bytes.

The fields displayed by a Token Ring interface on a Cisco router are important
when troubleshooting a network that might be exhibiting some form of error.

FDDI and CDDI

Fiber Distributed Data Interface (FDDI) and Copper Distributed Data Inter-
face (CDDI) were developed by the American National Standards Institute
(ANSI). Typically, FDDI is used in a network backbone, where high bandwidth
is required. Copper Distributed Data Interface (CDDI) was introduced in re-
sponse to a demand to run FDDI over copper wire. FDDI uses two counter-
rotating rings running at 100MB. The counter-rotating rings are used for
redundancy purposes in the event one of the rings fails. Table 4.5 summarizes
FDDI’s properties and limitations.

There are a number of devices that can reside on an FDDI ring. Figure 4.6 shows
a diagram that represents the types of stations that can exist on an FDDI ring.

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Basic Networking Technologies

FDDI

FDDI to Ethernet bridge

Single attached

station

Dual

attached

station

Dual

attached

file server

Figure 4.6

FDDI station types.

Table 4.5

FDDI’s properties and limitations.

FDDI Property

Limitation

Speed

100Mbps

MTU

4,500 bytes

Distance

100km for each ring.

Frame format

Similar to Token Ring, with a few minor differences; a free
token is required to send data.

Ring monitor

Can be performed by more than one station.

Number of rings

Two counter rotating rings; the second ring is used as a
backup.

Electrical interference

Fiber is not affected by electromagnetic radiation, and it’s

and security

secure from unauthorized tapping. This is the also true for
Ethernet and Token Ring transmissions, when fiber is used.

Maximum number of

500

station per ring

○

Chapter 4

In Figure 4.6, you can see Cisco routers that are dual attached (connected to both
rings) and Cisco router devices that are single attached stations (attached to one
ring). Stations that connect to both rings are sometimes referred to as dual homed.
In the event of a failure of the primary ring, the secondary ring takes over. Only
stations that are dual attached will recover.

FDDI Frame Formats

FDDI supports two frame formats—one for free tokens and another for busy or
data tokens. The format for a free token frame is as follows:

Preamble, Starting Delimiter(SD), Frame Control(AC),

End Delimiter(ED)

The next format shows the frame format used for busy or data tokens:

Preamble, SD, FC, DA, SA, Data, FCS, ED, FS

Following are the field descriptions used in FDDI frame formats:

➤ Preamble (8 bits)—Signals the start of a frame (typically 16 bits).

➤ SD (Starting Delimiter, 8 bits)—Indicates the start of a frame.

➤ FC (Frame Control, 8 bits)—Indicates the type of frame that is arriving.

➤ DA (Destination Address, 48 bits)—Indicates the destination MAC address.

➤ SA (Source Address, 48 bits)—Indicates the source MAC address. If the first bit

of the source address is set to 1, a routing information field is present (RIF).

➤ Data (>0 bits)—Contains user data.

➤ FCS (Frame Check Sequence, 32 bits)—Checks FC, DA, SA, and Data fields.

➤ ED (End Delimiter, 8 bits)—Indicates the end of a frame.

➤ FS (Frame Status, 8 bits)—Indicates a frame’s status.

Let’s now look at how an FDDI station attaches, or inserts, itself into a ring
using a procedure called the ring initialization process.

Station and Ring Initialization Process

When a new FDDI ring is installed, there must be a process in which a token is re-
leased so that all stations have an opportunity to send user data. At least one station
must be present for this to occur. Before a station can enter a ring, the station
must go through a number of states before connection is guaranteed. Table 4.6
describes the initialization states in order of operation.

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Basic Networking Technologies

The ring initialization process determines which station will transmit a free to-
ken. The FDDI station also determines the token rotation time (TRT). TRT is
the amount of time a token will take to circulate around a ring. This guarantees
that all stations will have a process to indicate if the token is lost. The beacon
process is used to recover lost tokens or serious ring faults. This process can occur
when there is a physical hardware fault, for example. The FDDI lower layers
have special management frames called station management (SMT) frames that
communicate with the physical entities of a station to ensure proper ring and
station operation. Figure 4.7 summarizes the FDDI model.

Note: The relative cost of FDDI compared to Ethernet or Token Ring has kept the
implementation to a minimum.

Multiservice Services

Running voice and video over existing networks is cost effective on Cisco routers.
Some Cisco routers now come with voice functionality. The next few sections
briefly describe some of the technologies you need to know about before you take
the Cisco exam, including the following:

➤ H.323

➤ CODECs

➤ Signaling System 7 (SS7)

Table 4.6

Station initialization procedure.

Initialization States,
in Order of Operation

Description

Break state

The station sends a certain symbol, called a

quiet symbol, to its

neighbor. This causes the neighbor to break its connection, as
well.

Quiet line state

A station enters this mode to signal initialization.

Connect state

A continuous stream of halt symbols is sent.

Next state or signal state

The station exchanges port information, such as port type and
compatibility. Port type can be single attached or dual attached
ports, for example.

Idle line state

Before a station can receive data, the station must enter
this state.

Link confidence test

An exchange between an existing station and a new station
occurs. Test frames are exchanged to ensure the integrity of
MAC frames.

Join state

The station joins the ring.

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Chapter 4

➤ Realtime Transport Protocol (RTP)

➤ RTP Control Protocol (RTCP)

➤ Quality Of Service (QOS)

H.323

H.323 is the International Telecommunications Union—Telecommunications
(ITU-T) standard for realtime multimedia communications and conferencing
over packet-based networks. H.323 is designed for network compatibility and
works over and across existing infrastructures, such as LAN, WANs, the Internet,
ISDN, and POTS (telephone network). H.323 can be deployed over IP. A sum-
mary of the H.323 standard is as follows:

➤ Delivers high-quality and scalable multimedia-based conferencing

➤ Can be used over existing networks (IP)

➤ Can be used for long-distance voice calls

Logical Link Control (LLC)

OSI Model

FDDI Model

Media Access Control
- Addressing
- Cyclic redundancy checks
- Data delivery

Physical Layer Protocol (PHY)
- Clocking speeds
- Framing
- Codecs

Physical Layer Medium Dependent (PMD)
- Electrical specifications
- Connector types
- Fiber standards (FDDI)
- Copper standards (CDDI)

Station Management (SMT)
- Fault isolation
- Recovery
- Communication with

other stations

Figure 4.7

FDDI model.

○

Basic Networking Technologies

➤ Allows more cost-effective use of ISDN

➤ Supports intranets and can provide more reliable connections

CODECs (Coder-Decoders)

A CODEC is an integrated circuit device that typically transforms analog sig-
nals into a digital bit stream and digital signals back into analog signals. When
voice over IP is used, for example, an alogirthim is used to compress and decom-
press the speech or audio signals.

Coding techniques have been standardized by the ITU-T, which develops the
standards for telecommunications technologies, such as coding and decoding.
The G series defines the characteristics such as bits per seconds and latency. For
example, the standard that defines how voice is transmitted over a Public Switched
Telephone Network (PSTN) line is the G.711 standard.

Signaling System 7 (SS7)

SS7 is the international standard for the common channel signaling system. SS7
defines the architecture, network elements, interfaces, protocols, and manage-
ment procedures for a network that transports control information between net-
work switches and databases. The North American version of SS7 is also
sometimes referred to as CCS7. SS7 is used between PSTN switches. The North
American standard is called the common channel signaling 7 (CCS7).

Realtime Transport Protocol (RTP) and RTP Control
Protocol (RTCP)

Realtime Transport Protocol (RTP) is a protocol that provides support for appli-
cations with realtime properties, such as video or voice over IP. Timing, loss de-
tection, and security are some of the functions carried out by RTP.

RTCP provides support for realtime conferencing for large groups within an
intranet, including source identification and support for gateways (like audio and
video bridges). RTP operates at the Transport layer of the OSI model.

Quality Of Service (QOS)

QOS provides a way in which information can be controlled and policed accord-
ing to a cost structure. It also provides a way for end users to ensure that they do
not oversubscribe to services so that service providers can enforce rate usage.
QOS will ensure end users will not overuse a service, and it performs the follow-
ing services:

➤ Provides dedicated bandwidth

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Chapter 4

➤ Avoids and manages network congestion

➤ Sets traffic priorities

Multiservice as defined by the CCIE blueprint does not detail precisely what is
required by the candidate and is treated lightly in this chapter. If you would like
to expand on H.323, SS7, RTP, and QOS, refer to the references listed in the
“Need To Know More?” section at the end of this chapter for some excellent
information freely available on the Internet.

○

Basic Networking Technologies

Practice Questions

Question 1

Gigabit Ethernet is specified by which IEEE standard?

❍ a. 802.2

❍ b. 802.3Q

❍ c. 802.1d

❍ d. 802.3z

❍ e. 802.5

The correct answer is d. Gigabit Ethernet is identified by 802.3z. Answer a is
incorrect, because 802.2 LLC does not identify Gigabit Ethernet. Answer b is
incorrect, because 802.3Q does not exist. Answer c is incorrect, because 802.1d
defines spanning tree. Answer e is incorrect, because 802.5 defines Token Ring.

Question 2

At what speeds can Token Ring 802.5 operate? [Choose the two best answers]

❑ a. 4MB

❑ b. 10MB

❑ c. 12MB

❑ d. 16MB

❑ e. 20MB

The correct answers are a and d. Token Ring 802.5 can operate at 4MB or 16MB.
Answers b, c, and e are incorrect, because 10MB, 12MB, and 20MB do not rep-
resent speeds in which Token Ring 802.5 can operate.

○

Chapter 4

Question 3

The 802.2 MAC address is represented by how many bits?

❍ a. 4 bits

❍ b. 12 bits

❍ c. 48 bits

❍ d. 24 bits

The correct answer is c. MAC addresses are represented by 48 bits. Answers a, b,
and d are incorrect, because 4 bits, 12 bits, and 24 bits are invalid bit lengths for
MAC addresses.

Question 4

Which of the following LAN standards use a possession of a token to send
data? [Choose the two best answers]

❑ a. Ethernet

❑ b. Token Ring

❑ c. FDDI

❑ d. Fast Ethernet

The correct answers are b and c. Token Ring and FDDI both use a free token to
send data. Answers a and d are incorrect, because Ethernet and Fast Ethernet use
CSMA/CD to transfer data.

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Basic Networking Technologies

Question 5

What is the ring speed of the following interface?

TokenRing0 is up, line protocol is up

Hardware is TMS380,address is 0000.308f.3655

Internet address is 137.10.9.1/24

MTU 4464 bytes, BW 16000 Kbit, DLY 630 usec,

rely 255/255, load 1/255

Encapsulation SNAP,loopback not set

ARP type: SNAP, ARP Timeout 04:00:00

Ring speed: 16

Multiring node, SRT Bridge capable

...

❍ a. 4MB

❍ b. 6MB

❍ c. 8MB

❍ d. 10MB

❍ e. 16MB

The correct answer is e. The line Ring speed: 16 indicates that the ring speed is
16Mbps. Answers a, b, c, and d are incorrect, because they represent incorrect
ring speeds. Remember, Cisco routers only support 4MB or 16MB ring speeds.

Question 6

What does it mean when the A and C bits in a token frame have been set to 1?

❍ a. The address was recognized, but due to memory requirements, it

was not copied.

❍ b. The address was recognized and copied.

❍ c. The address was copied only.

❍ d. The address was not recognized.

The correct answer is b. The A and C bits represent whether a frame that circu-
lated the ring was recognized (A) and copied (C). Figure 4.4, shown earlier in
this chapter, presents more details. Answer a is incorrect, because you are asked to

○

Chapter 4

explain what happens if the C bit is set to 1 also. If the device has insufficient
memory, the C bit would be set to 0 to indicate that the frame was not copied.
Answer c is incorrect, because the answer only defines the C bit. Answer d is
incorrect, because if the A bit is set to 1, then the address has been recognized.
Also, answer d does not satisfy the question in relation to what the C bit means.

Question 7

What is the maximum length in 10Base2 Ethernet?

❍ a. 200m

❍ b. 187m

❍ c. 185m

❍ d. 100m

❍ e. 500m

The correct answer is c. 10Base2 is Ethernet over coaxial cable, which has a
maximum length of 185m. Answer a is incorrect, because coaxial cable does not
specify a maximum length of 200m. Answer b is incorrect, because 187m is an
invalid IEEE specification. Answer d is incorrect, because 100m represents the
maximum length for 10BaseT or thin Ethernet. Answer e is incorrect, because
500m represents the maximum length for 10Base5 or thick Ethernet.

Question 8

In Token Ring, what will a station do if it has not seen any token for a speci-
fied period of time?

❍ a. Detect a collision

❍ b. Nothing

❍ c. Call the protocol analyzer

❍ d. Beacon

The correct answer is d. In a Token Ring, if a token is not seen, the station will
beacon. Answer a is incorrect, because there are no collisions in Token Ring net-
works. Answer b is incorrect, because the stations will try to recover from a prob-
lem. Answer c is incorrect, because the station will issue a beacon frame on the
network. This frame can be seen by a protocol analyzer if one has been installed.

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Basic Networking Technologies

Question 9

How many bits represent the preamble in Ethernet? [Choose the two best
answers]

❑ a. 8 bytes

❑ b. 9 bits

❑ c. 7 bits

❑ d. 64 bits

The correct answers are a and d. In the 802.3 Ethernet implementation, the
preamble is seven bytes and the eighth byte is called the start frame delimiter.
The preamble is 64 bits in length, which is the equivalent of 1 byte. Answers b
and c are incorrect, because neither 9 bits nor 7 bits are a byte. Be on your guard
for similar questions when more than one answer is correct.

Question 10

What is the maximum frame size in 802.5?

❍ a. 4,500 bytes

❍ b. 17,997 bytes

❍ c. 17,800 bits

❍ d. 1,500 bytes

❍ e. 17,800 bytes

The correct answer is b. The maximum frame size in 802.5, or Token Ring, is
17,997 bytes. Answer a is incorrect; it represents the MTU for FDDI. Answer c
is incorrect, because 17,800 bits represents only 2,225 bytes (17800/8), which is
well below the maximum of 17,800 bytes. Answer d is incorrect; it represents the
maximum frame size for Ethernet. Answer e is incorrect because the standard for
Token Ring specifies an MTU of 17,997. Of course, not all devices conform to the
standard, but Cisco routers support an MTU range from 68 through 17,997 bytes.

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Chapter 4

Question 11

What is the international standard for the common channel signaling system?

❍ a. BGP

❍ b. SS7

❍ c. IGRP

❍ d. OSPF

The correct answer is b. SS7 is an international standard. Answers a, c, and d are
incorrect, because they represent IP routing protocols. BGP is widely used in the
Internet, and IGRP is used to route IP in large networks. OSPF is another IP
routing protocol.

Question 12

What does the H.323 standard define?

❍ a. How IP packets are routed on the Internet.

❍ b. How routers can be reloaded.

❍ c. A realtime multimedia communications standard.

❍ d. H.323 is not a standard.

The correct answer is c. H.323 is an international standard that defines how
realtime multimedia communications and conferencing is attained over WAN
technologies, such as ISDN and the plain old telephone service (POTS). Answer
a is incorrect, because the routing of IP packets is not described in H.323. An-
swer b is incorrect, because the H.323 standard does not include reloading rout-
ers, which is more an administrative function by the local router designer or
operator. Answer d is incorrect, because H.323 is an international standard.

○

Basic Networking Technologies

Need to Know More?

Cisco Connection CD. Look under the command references for de-
tailed displays and full explanations to sharpen your IOS command
set for the examination.

www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/
qos_c/qcintro.htm points to Cisco’s Web site’s discussion about Quality
Of Service (QOS). For more information, search for “QOS” on the
Cisco home page (www.cisco.com).

www.comsoc.org/standards.html presents all the IEEE 802 standards
discussed in this chapter. In particular download the IEEE 802.3 and
802.5 standard.

www.cisco.com/univercd/cc/td/doc/product/access/sc/r2/
6011.htm#xtocid204232 provides an excellent overview of SS7 and
Cisco IOS. This URL provides some good design guidelines and ex-
cellent tips for implementation of SS7 across your network.

www.clark.net/pub/rbenn is one of the best Web sites for Cisco hard-
ware and software links. Bookmark this Web site so you can access
information on multiservice design and Cisco end products for imple-
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