Synchronous Data Link Control (SDLC) is based on a synchronous, more-efficient, faster, and
flexible bit-oriented format. SDLC has several derivatives that perform similar functions with
some enhancements: HDLC, LAPB (Link Access Procedure, Balanced), and IEEE 802.2, just to
name a few. HDLC is the default encapsulation type on most Cisco router serial interfaces.
SDLC is used for many link types. Two node types exist within SDLC: primary nodes and
secondary nodes. Primary nodes are responsible for the control of secondary stations and for
link management operations such as link setup and teardown. Secondary nodes talk only to the
primary node when fulfilling two requirements. First, they have permission from the primary
node; second, they have data to transmit. Even if a secondary node has data to send, it cannot
send the data if it does not have permission from the primary node.
Primary and secondary stations can be configured together in four different topologies:
Point-to-point This topology requires only two nodes—a primary and a secondary.
Multipoint This configuration uses one primary station and multiple secondary stations.
Loop This configuration uses one primary and multiple secondary stations. The difference
between the loop and multipoint setups is that in a loop, the primary station is connected between
two secondary stations, which makes two directly connected secondary stations. When more
secondary stations are added, they must connect to the other secondary stations that are currently
in the loop. When one of these stations wants to send information to the primary node, it must
transit to the other secondary stations before it reaches the primary.
Hub go-ahead This configuration also uses one primary and multiple secondary stations, but
it uses a different communication topology. The primary station has an outbound channel. This
channel is used to communicate with each of the secondary stations. An inbound channel is
shared among the secondary stations and has a single connection into the primary station.
Frame Structure
SDLC uses three different frame structures: information, supervisory, and unnumbered. Overall,
the structure of the frames is similar among all three, except for the Control frame. The Control
frame is varied to distinguish the type of SDLC frame that is being used. Figure 36.10 gives
the structure for the different SDLC frames. Pay close attention to the bit values next to the send
sequence number within the Control frame.
First, let’s talk about the frame fields that are common among all three frame types. As
you can see, all three frames depicted in Figure 36.10 start with a Flag field that is followed
by an Address field. The Address field of SDLC frames is different from other frame structures
because only the address of the secondary node is used, rather than a destination and
source address. The secondary address is used because all communication is either originated
or received by the primary node; thus, it is not necessary to specify its address within
the frame.
The Control frame follows the Address field. Information contained within the Control
frame defines the SDLC frame type. The Control frame begins with a receive sequence number.
This sequence number is used to tell the protocol the number of the next frame to
be received.
The P/F or Poll Final number following the receive sequence number is used differently
by primary and secondary nodes. Primary nodes use the information to communicate to the secondary
node that an immediate response is required. The secondary node uses the information
to tell the primary node that the frame is the last one in the current dialog.
The Data field follows the Control frame. As with other frame types, the FCS field comes
next and is used to calculate the CRC. SDLC frames differ again with the last field, which is
another Flag field like one at the beginning of the frame.
Now that we have discussed the frame structure, let’s examine the three different frame
types. Information frames carry exactly that—information destined for the upper layer protocols.
Supervisory frames control SDLC communications; they are responsible for flow control
and error control for I-frame (information). Unnumbered frames provide the initialization of
secondary nodes, as well as other managerial functions. 1109
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Point-to-Point Protocol (PPP)
Point-to-Point Protocol (PPP) is used to transfer data over serial point-to-point links. It accomplishes
this by using a layer 2 serial encapsulation called High-Level Data Link Control (HDLC).
HDLC is used for frame encapsulation on synchronous serial lines. It uses a link control protocol
(LCP) to manage the serial connection. Network control protocols (NCPs) are used to allow PPP
to use other protocols from layer 3, thus enabling PPP to assign IP addresses dynamically.
PPP uses the same frame structure as HDLC. Figure 36.9 gives you a picture of what the
frame looks like. As always, we move from right to left.
FIGURE 3 6 . 9 PPP packet structure
First, we have the Flag field, which uses one byte to specify the beginning or ending of a
frame. Then there is another byte that is used in the Address field to hold a broadcast address
of 11111111.
The Address field is followed by the one-byte Control field, which requests a transmission
of user data. The two-byte Protocol field follows the Control field. This field indicates the
encapsulated data’s protocol.
The Data field contains the information that will be handed to the upper layer protocols. It is a
variable-length field. After that is the FCS. Like the other protocols, it is used for CRC calculation.
this by using a layer 2 serial encapsulation called High-Level Data Link Control (HDLC).
HDLC is used for frame encapsulation on synchronous serial lines. It uses a link control protocol
(LCP) to manage the serial connection. Network control protocols (NCPs) are used to allow PPP
to use other protocols from layer 3, thus enabling PPP to assign IP addresses dynamically.
PPP uses the same frame structure as HDLC. Figure 36.9 gives you a picture of what the
frame looks like. As always, we move from right to left.
FIGURE 3 6 . 9 PPP packet structure
First, we have the Flag field, which uses one byte to specify the beginning or ending of a
frame. Then there is another byte that is used in the Address field to hold a broadcast address
of 11111111.
The Address field is followed by the one-byte Control field, which requests a transmission
of user data. The two-byte Protocol field follows the Control field. This field indicates the
encapsulated data’s protocol.
The Data field contains the information that will be handed to the upper layer protocols. It is a
variable-length field. After that is the FCS. Like the other protocols, it is used for CRC calculation.
Frame Structure
Frame formats are similar between Ethernet and IEEE 802.3. Figure 36.8 depicts the similarities
and differences between the two. The frame structures are read from right to left. Starting at the
right, you see that both frames begin with a preamble. The
Preamble
is a seven-byte field.
(Notice that we have moved from bits to bytes to specify field lengths.) The preamble consists
of alternating 1s and 0s.
The next field is the
SOF
, the start-of-frame delimiter. It is used to synchronize the framereception
portions of all the machines on the segment. This field is only one byte long.
The two fields following the SOF are six bytes each; they are the
Destination
and Source
MAC addresses of the receiving and sending stations. Each MAC address is unique.
Up to this point, the frames are exactly the same. Starting with the next field, they are different.
The next field is a two-byte field in both frame structures. Ethernet defines the field as a Type field;
IEEE 802.3 defines it as a Length field. Ethernet uses this field to specify which upper layer protocol
will receive the packet. IEEE 802.3 uses the field to define the number of bytes in the payload
(802.2 header and data) field. One easy method of observing the difference between an Ethernet
and 802.3 frame is to look at the Type/Length field. If this value is 1500 (0x05DC) or less, then
it is an IEEE 802.3 frame. If it is greater than 1500, it is an Ethernet frame.
Next is the Data field, in both Ethernet and 802.3 formats. The only difference between the
two versions of this field is that Ethernet uses a variable byte size, between 46 and 1500 bytes,
for data. This data is what will be handed to the upper layer protocols. IEEE 802.3 uses a
46–1500 variable byte size, as well, but the information here contains the 802.2 header and the
encapsulated data that will eventually be passed to an upper layer protocol that is defined
within the Data field.
Finally, the last field is the Frame Check sequence (FCS) field. It is four bytes and stores
information that will be used for calculating the CRC after the data has been sent or received.
and differences between the two. The frame structures are read from right to left. Starting at the
right, you see that both frames begin with a preamble. The
Preamble
is a seven-byte field.
(Notice that we have moved from bits to bytes to specify field lengths.) The preamble consists
of alternating 1s and 0s.
The next field is the
SOF
, the start-of-frame delimiter. It is used to synchronize the framereception
portions of all the machines on the segment. This field is only one byte long.
The two fields following the SOF are six bytes each; they are the
Destination
and Source
MAC addresses of the receiving and sending stations. Each MAC address is unique.
Up to this point, the frames are exactly the same. Starting with the next field, they are different.
The next field is a two-byte field in both frame structures. Ethernet defines the field as a Type field;
IEEE 802.3 defines it as a Length field. Ethernet uses this field to specify which upper layer protocol
will receive the packet. IEEE 802.3 uses the field to define the number of bytes in the payload
(802.2 header and data) field. One easy method of observing the difference between an Ethernet
and 802.3 frame is to look at the Type/Length field. If this value is 1500 (0x05DC) or less, then
it is an IEEE 802.3 frame. If it is greater than 1500, it is an Ethernet frame.
Next is the Data field, in both Ethernet and 802.3 formats. The only difference between the
two versions of this field is that Ethernet uses a variable byte size, between 46 and 1500 bytes,
for data. This data is what will be handed to the upper layer protocols. IEEE 802.3 uses a
46–1500 variable byte size, as well, but the information here contains the 802.2 header and the
encapsulated data that will eventually be passed to an upper layer protocol that is defined
within the Data field.
Finally, the last field is the Frame Check sequence (FCS) field. It is four bytes and stores
information that will be used for calculating the CRC after the data has been sent or received.
Ethernet/IEEE 802.3
These two terms actually refer to different things:
Ethernet
is a communication technology and
IEEE802.3
is a variety of Ethernet. Ethernet, in the more specific sense, is a
carrier sense, multiple
access/collision detection (CSMA/CD)
local area network. An Ethernet network uses these
attributes—carrier sense, multiple access, and collision detection—to enhance communication. This
definitely does
not
mean that Ethernet is the only technology that uses these attributes. In today’s
technical jargon, however, the term
Ethernet
is getting closer to meaning
all
CSMA/CD technologies.
Both Ethernet and IEEE 802.3 are broadcast networks. All frames that cross a given segment
can be heard by all machines populating that segment. Because all machines on the segment have
equal access to the physical media, each station tries to wait for a quiet spot before it transmits its
data. If two machines talk at the same time, a collision occurs.
Ethernet services both the Physical and Data Link layers, whereas IEEE 802.3 is more concerned
with the Physical layer and how it talks to the Data Link layer. Several IEEE 802.3 protocols
exist; each one has a distinct name that describes how it is different from other IEEE 802.3
protocols. Table 36.3 summarizes these differences.
TABLE 3 6 . 3
IEEE 802.3 Characteristics
802.3
Values
Data Rate
(Mbps)
Signaling
Method
Maximum
Segment
Length (m) Media Topology
10Base5 10 Baseband 500 50 Ohm coax Bus
10Base2 10 Baseband 185 50 Ohm coax Bus
1Base5 1 Baseband 185 Unshielded twisted pair Star
10BaseT 10 Baseband 100 Unshielded twisted pair Star
100BaseT 100 Baseband 100 Unshielded twisted pair Star
10Broad36 10 Broadband 1800 75 Ohm coax Bus
1000BaseT 1000 Baseband 100 Unshielded twisted pair Star
Table 36.3 is an excerpt from Cisco documentation; for the full document,
please see
www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/
ethernet.htm
.
In Table 36.3, you will notice that the terms baseband and broadband are used to describe
the signaling type. In a baseband transmission, only a single frequency is used for sending data,
and therefore only a single signal can be sent over the same media. A broadband signal multiplexes
multiple signals of different frequencies together on the same physical media.
Though not specifically called out in the table, there are four different IP encapsulation types
supported by Cisco for Ethernet: ARPA, SNAP, Novell-Ether, and SAP. Of these, ARPA is the
default encapsulation type used.
Ethernet
is a communication technology and
IEEE802.3
is a variety of Ethernet. Ethernet, in the more specific sense, is a
carrier sense, multiple
access/collision detection (CSMA/CD)
local area network. An Ethernet network uses these
attributes—carrier sense, multiple access, and collision detection—to enhance communication. This
definitely does
not
mean that Ethernet is the only technology that uses these attributes. In today’s
technical jargon, however, the term
Ethernet
is getting closer to meaning
all
CSMA/CD technologies.
Both Ethernet and IEEE 802.3 are broadcast networks. All frames that cross a given segment
can be heard by all machines populating that segment. Because all machines on the segment have
equal access to the physical media, each station tries to wait for a quiet spot before it transmits its
data. If two machines talk at the same time, a collision occurs.
Ethernet services both the Physical and Data Link layers, whereas IEEE 802.3 is more concerned
with the Physical layer and how it talks to the Data Link layer. Several IEEE 802.3 protocols
exist; each one has a distinct name that describes how it is different from other IEEE 802.3
protocols. Table 36.3 summarizes these differences.
TABLE 3 6 . 3
IEEE 802.3 Characteristics
802.3
Values
Data Rate
(Mbps)
Signaling
Method
Maximum
Segment
Length (m) Media Topology
10Base5 10 Baseband 500 50 Ohm coax Bus
10Base2 10 Baseband 185 50 Ohm coax Bus
1Base5 1 Baseband 185 Unshielded twisted pair Star
10BaseT 10 Baseband 100 Unshielded twisted pair Star
100BaseT 100 Baseband 100 Unshielded twisted pair Star
10Broad36 10 Broadband 1800 75 Ohm coax Bus
1000BaseT 1000 Baseband 100 Unshielded twisted pair Star
Table 36.3 is an excerpt from Cisco documentation; for the full document,
please see
www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/
ethernet.htm
.
In Table 36.3, you will notice that the terms baseband and broadband are used to describe
the signaling type. In a baseband transmission, only a single frequency is used for sending data,
and therefore only a single signal can be sent over the same media. A broadband signal multiplexes
multiple signals of different frequencies together on the same physical media.
Though not specifically called out in the table, there are four different IP encapsulation types
supported by Cisco for Ethernet: ARPA, SNAP, Novell-Ether, and SAP. Of these, ARPA is the
default encapsulation type used.
Layer 2: Data Link Layer
Protocols and Applications
This section is dedicated to layer 2 protocols and applications. It is a very important section
because it provides specific information on how the layer 2 protocols work. What better
way to be able to troubleshoot a problem than by understanding the intricacies of the protocol
in question?
This section covers the following layer 2 protocols:
Ethernet/IEEE 802.3
PPP
SDLC
Frame Relay
ISDN
This section is dedicated to layer 2 protocols and applications. It is a very important section
because it provides specific information on how the layer 2 protocols work. What better
way to be able to troubleshoot a problem than by understanding the intricacies of the protocol
in question?
This section covers the following layer 2 protocols:
Ethernet/IEEE 802.3
PPP
SDLC
Frame Relay
ISDN
Connectionless Protocols
Now that connection-oriented protocols have been discussed, we’ll move on to connectionless
protocols.
Connectionless protocols
differ from connection-oriented protocols because they do
not provide for flow control.
Figure 36.7 shows you how connectionless protocols work. This figure looks somewhat
like Figure 36.3, except that there are no steps that involve a connection setup or
termination. It is also missing the flow control and error control information sent by the
receiving system.
Connectionless protocols do not send data relative to any other data units. The data
included in the PDU must contain enough information for the PDU to get to its destination
and for the receiving system to properly process it. Because there is no established connection,
flow and error control cannot be implemented. Without flow and error control,
the originating system has no way of knowing whether all of the transmitted data was
received by the destination system without errors. Table 36.2 shows examples of connectionless
protocols.
protocols.
Connectionless protocols
differ from connection-oriented protocols because they do
not provide for flow control.
Figure 36.7 shows you how connectionless protocols work. This figure looks somewhat
like Figure 36.3, except that there are no steps that involve a connection setup or
termination. It is also missing the flow control and error control information sent by the
receiving system.
Connectionless protocols do not send data relative to any other data units. The data
included in the PDU must contain enough information for the PDU to get to its destination
and for the receiving system to properly process it. Because there is no established connection,
flow and error control cannot be implemented. Without flow and error control,
the originating system has no way of knowing whether all of the transmitted data was
received by the destination system without errors. Table 36.2 shows examples of connectionless
protocols.
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