X.25
X.25 is an ITUT-T standard that specifies an interface between a host system and a packet
switching network.
In packet-switching networks, the attached stations must organize their data into packets for
transmission. This requires certain level of cooperation between the network and the attached
stations. This cooperation is embodied in an interface standard. X.24 is the standard used for
traditional packet-switching networks. Functionality of X.25 is specified on three levels:
physical level, link level and packet level.
1. Packet level deals with the physical interface between an attached station and the link that
attaches that station to the packet-switching node. Physical level specifications are
defined by X.21 standard and EIA-232 standard.
2. Link level provides for the reliable transfer of data across the physical link, by
transmitting data as a sequence of frames. Link level standard known as LAPB (Link
Access Protocol – Balanced). LAPB is a subset of HDLC.
3. Packet level provides a virtual circuit service. This service enables any subscriber to the
network to set up logical connections, called virtual circuits, to other subscribers.
User data are passed down to X.25 level 3, which appends control information as a header,
creating a packet. The control information identifies by number a particular virtual circuit with
which the data is to be associated. Control information also provides sequence numbers for flow
and error control on a virtual circuit basis. P(S) is sequence no. of packets sent on particular
virtual circuit and P(R) is acknowledgement of packet received on particular virtual circuit.
User data
Layer 3
header
LAPB
header
X.25 packet
LAPB
trailer
LAPB
frame
Frame Relay
Frame relay is designed to provide a more efficient transmission scheme than X.25. It eliminates
much of the overhead that X.25 imposes on end user systems and on the packet-switching
network.
Differences between Frame Relay and X.25:
1. Call control signaling, the information needed to set up and manage a connection, is
carried on a separate logical connection from user data. So, intermediate nodes need not
maintain state tables or process messages relating to call control on an individual perconnection basis.
2. Multiplexing and switching of logical connections takes place at layer 2 instead of layer
3, eliminating one entire layer of processing.
3. There is no hop-by-hop flow control and error control.
The result is lower delay and higher throughput.
The frame relay architecture significantly reduces the amount of work required of the network.
User data are transmitted in frames with virtually no processing by the intermediate network
nodes, other than to check for errors and to route based on connection number. A frame in error
is simply discarded, leaving error recovery to higher layers.
The frame format of frame relay (LAPF core) does not contain control information.
Flag
Address
1 byte
2-4 bytes
a) Frame format of LAPF core
Information
Variable
8
7
6
5
Upper DLCI
Lower DLCI
b) Address field – 2 bytes (default)
FCS
2 byte
4
3
FECN
BECN
Flag
1 byte
2
C/R
DE
1
EA = 0
EA = 1
Frame relay involves the use of logical connections, called data link connections (rather than
virtual circuits). Data Link Connection Identifier (DLCI) identifies each data link connection.
Frames can be of 2 bytes (default), 3 bytes and 4 bytes. If default DLCI with 10 bits is not
sufficient, then the address field is extended so that more bits define DLCI.
C/R
EA
FECN
Command/ Response bit
Address field extension bit
Forward Explicit Congestion
Notification
BECN
DLCI
DE
Backward Explicit Congestion
Notification
Data Link Connection Identifier
Discard Eligibility
Asynchronous Transfer Mode (ATM)
ATM network is designed to be able to transfer many different types of traffic simultaneously,
including real-time flows such as voice, video and bursty TCP flows. Information flow on each
logical connection is organized into fixed sized packets (53 bytes) called cells. Logical
connections in ATM are called Virtual Channel Connections (VCCs). VCC is analogous to
virtual circuit in X.25. A VCC is set up between two end users through the network and a
variable-rate, full-duplex flow of fixed-size cells is exchanged over the connection. VCCs are
also used for user-network exchange (control signaling) and network-network exchange
(network management and routing). A Virtual Path Connection (VPC) is a bundle of VCCs that
have the same end points. All of the cells flowing over all of the VCCs in a single VPC are
switched together.
VCI (Virtual Channel Identifier)
VPI (Virtual Path Identifier)
Physical layer involves the specification of transmission medium and signal encoding scheme.
Data rates of 25.6 Mbps to 622.08 Mbps are possible. ATM layer is common to all services and
provides packet transfer capabilities whereas ATM adaptation layer is service dependent. ATM
layer defines the transmission of data in fixed-size cells and defines the use of logical
connections. The use of ATM creates the need for an adaptation layer to support information
transfer protocols not based on ATM. AAL maps higher-layer information into ATM cells to be
transported over the ATM network, then collects information from ATM cells for delivery to
higher layers.
Higher layer
ATM Adaptation Layer (AAL)
ATM layer
Physical layer
ATM protocol architecture
Protocol reference model involves three separate planes:
1. User Plane provides for user information transfer, along with associated controls (e.g.
flow control, error control).
2. Control Plane performs call control and connection control functions.
3. Management Plane includes plane management and layer management. Plane
management performs management functions related to a system as a whole and provides
coordination between all the planes. Layer management performs management functions
relating to resources and parameters residing in its protocol entities.
The standard size every cell being sent in the ATM technology is 53 bytes. This standard sized
53 byte cell consists of a 5 byte header and a 48 byte payload.
8
7
6
5
Generic Flow
Control
VPI
VCI
VCI
Header Error Control
Info field
48 bytes
4
3
VPI
2
1
VCI
Payload Type
UNI (User – Network Interface)
CLP
8
7
VPI
6
5
4
3
2
VPI
VCI
VCI
VCI
Payload Type
Header Error Control
Info field
48 bytes
NNI (Network – Network Interface)
1
CLP
CLP stands for Cell Loss Priority. CLP of 0 means high priority. CLP of 1 means that frame is of
low priority and hence may be discarded.
ATM took its name from the first letters of Asynchronous Transfer Mode. It forms the
infrastructure of access applications requiring more band width in the wide area. Losing its
popularity day by day consequent to the development of the IP-Ethernet, ATM subsists as a
transportation system. It performs the cell transfer, which can be considered as a type of packet
switching transmission technique, by also benefiting from the advantages of the circuit switching
technology.
It carries the traffics having different structural features such as sound, image and data through
the same platform. In this transmission technique, in which the hardware based cell switching
perfectly suitable for real time data transmission, the carrier cells have a fixed size of 53 bytes.
The frames larger than 53 bytes are converted into the suitable format by the ATM adaptation
level and the transmission is performed.
In the ATM service, the scalable parameters from Mbit level to Gbit level and the QoS (Quality
of Service) parameters are assigned / guaranteed before the data transmission.
Hardware based switching, the existence of fixed and small packets have allowed the ATM
system to be fast and reliable. Virtual paths and channels have brought flexibility to the system.
The ATM system is a connection oriented transmission technique. Therefore, one of the parties
sends a packet to the opposite party in order the connection to be made. This packet saves the
information of the sources it requires in the paths and switches it passes from. These paths are
called virtual paths (Private Virtual Paths). Each connection carries a unique identity and this
information is hidden on these switches in case there would be a permanent connection.
When the connection is made, as the cells forming the transmission also carry the header
information, the ATM switches know from which path they would transmit the incoming cells.
The cells go through these virtual paths follow each other but it is not checked whether there is a
cell loss or not.
The cells are switched through the use of (VPI/VCI) channel indicators and the connection
based, that is before the data transmission is initiated, resource allocation is performed.