Chapter 57. X.25
X.25 is an analog packet switching network and it can be considered Slow Packet Switching. The transfer speeds are typically 56 kbps to 2.08 Mbps. There is a world-wide set of Public X.25 Networks; it's possible for an organization to have its own private X.25 network.
X.25 is an over 20 year old, established technology. There are many multi-vendor solutions: dissimilar technologies in an organization are allowed to access the X.25 network. In Canada, the main X.25 network is called Datapac, a public offering of X.25. You pay either a flat rate or by the packet.
X.25 is used to connect LANs together. Due to its slow transfer speed, it is used for the following:
Host terminal emulations: low data
Client/Server applications such as E-mail: small files, bandwidth
File Server: large amount of data & real-time traffic (doesn't work well)
Databases: usually large databases, but queries are small inbound and medium size outbound.
X.25 has a high protocol overhead compared to other networks. This reduces the transfer speed and bandwidth utilization (i.e. its not as efficient).
Overhead Example
Truck A represents X.25. It has a heavy "empty" weight of 5 tons (overhead). The bridge (medium) only allows 6 tons of weight. This means that Truck A can only carry 1 Ton of cargo (i.e. data). Truck B is a smaller truck, and weighs 3 tons empty. This means that it can carry up to 3 tons of cargo (data) across the bridge (medium). Truck B makes better use of its weight when crossing the bridge (i.e. it utilizes its bandwidth better; it is more efficient).
X.25 OSI Layers
X.25 consists of the items shown below:
X.25.3 - Network Layer:
PLP (Packet Layer Protocol), or
SNDCF (Subnetwork Dependant Convergence Function)
X.25.2 - Datalink:
LAPB (Link Access Procedure Balanced)
HDLC (High Level Data Link Control)
X.25.1 - Physical with 4 different types:
X.21 - Sync Digital Interface 9.6kbps - unbal, 64Kbps - bal
X.21bis - Leased Line Analog Interface
V.24 - RS232 Leased Lines
V.35 - RS232 Duplex operation over Leased Lines
In fact, pure X.25 only defines the DTE-to-DCE connection. However, when we talk about the X.25 Packet Switching Network, we talk about the above-mentioned 3 layers: Network, Datalink, and Physical.
X.25.1 is the Physical layer, and it uses four flavors of medium (similar to the multiple Ethernet flavors: 10BaseT, Thinnet and Thicknet). The X.25 packet is carried on serial data lines.
X.25.2 uses HDLC & LAPB for the Data Link layer; LAPB is considered a subset of HDLC. Both are similar to IEEE-802.2 LLC (Logical Link Control), and provide 2-way communications. The B in LAPB stands for balanced communications (another way of saying Full-Duplex - both sides communicating at the same time). The X.25 packet is carried within the LAPB frame's info field: this is similar to how the LLC packet is carried within the MAC frame's info field.
X.25.3 is known as SNDCF, or Subnetwork Dependant Convergence Function. X.25 uses IP network addresses and it's one of the reasons for the high overhead.
X.25 connects to the network using either a DCE modem or DSU/CSU (Data Service Unit/Channel Service Unit). X.25 allows 4,096 logical channels to be connected on one physical connection. The Packet Assembler/Disassembler (PAD) connects the DSU/CSU to the DTEs (user devices, which can be terminals or LANs).
The X.3 standard governs the operation of the PAD and the X.28 standard governs the operation of the PAD-to-terminal connection. The X.29 standard defines the End-to-End communications, from DTE-to-DTE through the X.25 Network.
X.25 High overhead
X.25 has a high overhead because it provides extensive error checking. Each device in the X.25 network acknowledges every packet that's sent. This slows down the transfer of information, and uses up available bandwidth. When X.25 was first introduced, the quality of the analog phone lines required this extensive error checking. But now--with digital lines available--it's no longer necessary.
There are 2 types of connections used with X.25.
PVC - Permanent Virtual Circuits: These are leased lines and require no call connect/disconnect
VC - Virtual Circuits: These are like dial-up lines (switched circuits). They require a call connect/disconnect procedure, and end to end communication through the network.
VC (virtual circuits) have handshaking that is very similar to how modems connect (as covered earlier). X.25 is often used because an X.25 network is considered one Hop.
Normal Routing (Non X.25)
Between San Francisco and Boston
Dashed Route takes five hops (Salt Lake City, Denver, Chicago, Detroit and Boston)
Dotted Route takes eight hops (Los Angeles, El Paso, Houston, New Orleans, Alanta, Washington, New York and Boston)
With X.25, the complete Network across the United States would look like only one Hop. X.25 Packet Networks take care of the routing path. To connect any point (in our example of the USA) together would take only one hop.
X.25 Packet Formats
There are three X.25 packet formats.
Call Request - Call connection/disconnection
Control Packet - Data control
Data Packet - Information transfer
Call Request Frame
The Call Request Frame is used to initiate and setup the call from one X.25 service to another. After the call is established, the Call Request frame is no longer used.
Logical Channels
There are 4,096 Logical Channels available on a single physical connection to an X.25 network. The Logical Channels are divided into Groups and Channels. There can be 16 groups (4 bits) of 256 channels (8 bits). 16 x 256 = 4,096. The Logical Channel Numbers (LCN) are used to identify the connections to the Network.
Type Field
For Call Request, the Type Field is always equal to 0000 1011. Note that the last bit is called the "Control" bit, and is set to 1.
Length of Calling / Called Address
The Length of Calling / Called Address allows different sizes of addresses for other protocols. The standard protocol is IP, with an address length of 32 bits.
Calling / Called Address
This is only used during Call Connect--until the Virtual Channel is established-- then the LCN (Logical Channel Numbers) are used to identify the connections.
Facilities Length
It indicates the length of the Facilities Field
Facilities Field
This indicates the types of facilities that are available. Facilities depend upon the Network provider, and can include information such as Charges or Call Forwarding.
Once a Call is established, the Calling / Called Addresses and Facilities Fields are not required, and the Control Packet Format and Data Packet are used.
Control Frame
The Control Frame is used to control communications during the call.
The X.25 Type field table for all 3 packet types.
Data Frame
The Data Frame is used to transfer data between destination and source.
Q
Q is optional. It is used to distinguish between data and control information.
D
D controls the type of Acknowledgment (shown below):
0 - Network Control: DTE-to-DCE
1 - End to End Acknowledgement: DTE-to-DTE
Modulo
Modulo is used by the sliding window. The modulo allows multiple unacknowledged packets on the network for faster response. The modulo can either be Modulo 8 or Modulo 128 (127 packets out on the network).
Piggyback
The Piggyback field is used for Acknowledgement with the Modulus (sliding window).
Sequence
The Sequence field holds a unique packet number that identifies the packet.
More
The More field indicates that more data is coming.
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