The document discusses several topics related to data link layer protocols: 1) It describes the main responsibilities of the data link layer and the two types of data link protocols - character-based and bit-oriented, and the differences between them. 2) It explains the relationships between HDLC, LAPB, and LAPD protocols. 3) It discusses the node-by-node acknowledge scheme used in X.25 networks and the concepts of X.25 PVC and SVC along with their main differences. 4) It briefly describes four types of virtual circuits in X.25 and how the LCI field relates to the virtual circuit type.

1. 1. Discuss the main responsibilities of the data link layer. Describe the two types of data link
protocols, i.e., character-based and bit-oriented, and the main differences between them.?
 Interfacing the physical layer to receive or send data in a bit stream
 Delineating the received bit stream into link layer frames
 Synchronizing the link to ensure that the receiver is in step with the transmitter
 Detecting transmission errors and recovering from such errors
identifying and reporting certain protocol errors to higher layers
 In a balanced mode, each station is responsible for both information transmission and
error recovery using acknowledgments
 In an unbalanced mode, one of the two communicating stations is designated as primary
and the other as secondary.
bit-oriented protocol is a communications protocol that sees the transmitted data as an opaque stream
of bits with no semantic
character oriented a communications protocol in which full bytes are used as control code
2 Describe the relationships between HDLC and LAPB/LAPD.?
LAPB is derived from HDLC and is one of the most commonly used data link protocols
LAPD is derived from LAPB and used for ISDN to transmit data between DTEs through
D channels
3 What are the three types of frames defined in the HDLC frame and what purposes
do each serve?
The Informational Frame This type of frame contains actual user data being transferred
The Supervisory Frame This type of frame allows a receiver to notify a sender the
following status information
Unnumbered Frame This type of frame provides a means for a DTE and a DCE to set
up
4 Discuss the rationale behind the node-by-node acknowledge scheme used at the data link
layer of X.25 network.?
When we connected node-by-node we use address of data link layer that address called physical
address or mac
5 Discuss the X.25 PVC and SVC concepts and the main differences between them.?

2. PVC is a logical association between two DTEs that is permanently held by the network,
regardless of whether or not there is data being passed between the two DTEs.
SVC is a logical connection that is dynamically set up and maintained only for a given time
period between two DTEs. SVCs are closed or taken down when a data transfer session is
completed and there is no more data to send.
PVCs are normally set up manually, while SVCs are set up using a signaling protocol.
6 Briefly describe four different types of virtual circuits and how the LCI field of an X.25 packet is
related to a virtual circuit type.?
Permanent virtual circuits (PVCs). These are set up permanently and are assigned lowest LCI
numbers.
Incoming-only switched virtual circuits (incoming SVCs). These are one-way virtual circuits set
up from a DCE to a DTE, not the other way around
Two-way switched virtual circuits (two-way SVCs). These allow a DTE and DCE to request
connections to each other
Outgoing-only switched virtual circuits (outgoing SVCs). These allow a local DTE to request a
connection to a DCE
Logical Channel Identifier (LCI) The 12-bit logical channel identifier (LCI) identifies a virtual
circuit
7 Describe what the fast select service is and under what circumstances the service might be useful.
One-way logical channel. This is a variant of call barring
Throughput-class negotiation. This service allows a DTE to request a particular throughput class
8 8 In an intermediate X.25 switch, what is the highest layer of X.25 protocol stack
that examines an incoming packet to determine how to switch the packet, the data
link layer, or the packet layer?
physical layer The main responsibility of an intermediate node is switching or routing
frames to the next frame relay devic
9 Discuss one of the main reasons for X.25 to remain the network of choice for many
large corporations in sectors such as finance, insurance, utility, and retail?
 Private X.25 networks are typically deployed within large organizations that have widely
dispersed and communications-intensive operations in fields such as: