LESSON 5

PACKET SWITCHED NETWORK

Chifundo Polska Mlangeni
Chifundo.mlangeni20@gmail.com
 Packet Switched Services: Basic Architecture

• Packet switched services enable multiple connections to exist simultaneously

between computers.
• With packet switching users buy a connection into the common carrier
network, and connect via a packet assembly/ disassembly device (PAD).
• Packets from separate messages are interleaved with other packets for
transmission.
• Organizations usually connect to a packet network by leasing dedicated circuits
from their offices to the packet switched network’s point-of-presence (POP).

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Packet Switched Services
3
Packet Switching
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 Packet Routing Methods
• There are two methods for routing packets:
• A datagram is a connectionless service which adds a destination
and sequence number to each packet, in addition to information
about the data stream to which the packet belongs. Individual
packets can follow different routes before being reassembled on
the destination host.
• In a virtual circuit the packet switched network establishes an
end-to-end circuit between the sender and receiver. All packets
for that transmission take the same route over the virtual circuit
that has been set up for that transmission.

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 Permanent and Switched Virtual Circuits
• Two types of virtual circuits, permanent (PVCs) and switched (SVC), are
available from common carriers. PVCs are far more common.
• Although established using software, setting up or taking down a PVC
takes days or weeks to do.
• Each PVC has two data rates: a committed information rate (CIR), which
is guaranteed and a maximum allowable rate (MAR), which sends data
only when the extra capacity is available.
• Packets sent at rates exceeding the CIR are marked discard eligible (DE),
and discarded if the network becomes overloaded, in which case they
may need to be retransmitted.

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Packet Switched Service Protocols

• There are five protocols in use for packet switched

services:
• X.25
• Asynchronous Transfer Mode (ATM)
• Frame Relay
• Switched Multimegabit Data Service (SMDS)
• Ethernet/IP packet networks

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 X.25
• The oldest packet switched service is X.25, a standard
developed by ITU-T. X.25 offers datagram, switched virtual
circuit, and permanent virtual circuit services.
• X.25 uses the LAPB and PLP protocols at the data link and
network layers, respectively.
• X.25 is a reliable protocol, meaning it performs error control
and retransmits bad packets.
• Although widely used in Europe, X.25 is not in widespread
use in North America. The primary reason is the low
transmission speed, now 2.048 Mbps (up from 64 Kbps).

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X.25 History and Overview
• Designed to provide a low cost alternative for data
communication over public networks
• Pay only for bandwidth actually used
• Ideal for “bursty” communication over low quality
circuits
• Standard provides error detection and correction for
reliable data transfer
• X.25 standard approved in 1976 by CCITT (now known as
ITU)
• Can support speeds of 9.6 Kbps to 2 Mbps
• Can provide multiplexing of up to 4095 virtual circuits
over on DTE-DCE link
X.25 Devices
• Data Terminal Equipment (DTE)
• Terminals, personal computers, and network hosts
• Located on premises of subscriber
• Data Circuit-terminating Equipment (DCE)
• Modems and packet switches
• Usually located at carrier facility
• Packet Switching Exchange (PSE)
• Switches that make up the carrier network
Sample X.25 Network

X.25
PSE WAN PSE

Modem Personal Computer

DCE DTE

Terminal Modem
DTE DCE

Modem
PSE PSE
DCE

Server
DTE
Packet Assembler/Disassembler (PAD)
• Used for DTE devices that are too simple to
implement X.25 (such as character-mode terminals)
• Acts as intermediary device between DTE and DCE
• Performs three functions
• Buffering to store data until a device is ready to process it
• Packet Assembly
• Packet Disassembly
PAD in Action PSE

X.25 Packet
Data
PAD
Terminal Modem
DTE DCE

PSE

Data

Assembly/ Buffer
Disassembly
X.25 mapping to OSI Model
Application

Presentation
Other Services
Session

Transport

Network PLP
X.25
Data Link LAPB Protocol
Suite
x.21 bis, EIA/TIA-232, EIA/TIA-449,
Physical EIA-530, G.703
X.25 Physical Layer
• Several well-known standards are used for X.25 networks
• X.21bis – supports up to 2 Mbps
• 15-pin connector
• RS-232 (EIA/TIA-232) – supports up to 19.2 Kbps
• 25-pin connector
• RS-449 (EIA/TIA-449) – supports up to 64 Kbps
• 37-pin connector
• V.35 – supports up to 2 Mbps
• 34-pin connector
• Uses serial communications in either asynchronous or synchronous
modes
X.25 Data Link Layer
• Link Access Procedure, Balanced (LAPB) is the protocol used for this layer
• LAPB is a version of High-level Data Link Control (HDLC )
• (Reliable, Full duplex, Flow control, Synchronization)
• HDLC in Asynchronous Balanced Mode (ABM)
• DTE and DCE are peers and can both perform all functions
• LAPB manages communication and packet framing between DTE and DCE devices
• Makes sure that frames are delivered in sequence and error-free
• Uses sliding window of 8 or 128 frames
LAPB Frame Types
• Three types of frames
• I-Frames (Information Frames)
• Carry data as well as Next Send (NS) and Next Receive (NR) counts
• S-Frames (Supervisory Frames)
• Controls flow of data with Receiver Ready (RR), Receiver Not Ready (RNR), and Reject
(REJ) frames
• U-Frames (Unnumbered Frames)
• Establish and maintain communications with Set Asynchronous Balanced Mode (SABM),
Unnumbered Acknowledgment (UA), Disconnect (DISC), Disconnect Mode (DM) and
Frame Reject (FRMR)
X.25 Network Layer
• Packet Layer Protocol (PLP) is the X.25 network layer protocol
• PLP manages calls between a pair DTE devices using a Permanent
Virtual Circuit (PVC) or a Switched Virtual Circuit (SVC)
• PLP handles segmentation, reassembly, bit padding and error and
flow control
• PLP uses X.121 Addressing Scheme to setup a virtual circuit
PLP Operates in Five Modes
• Call Setup
• Used to setup virtual circuit for SVC
• Data Transfer
• Used for transferring data with both SVC and PVC
• Idle
• Used when SVC call has been established but no data is currently being transferred
• Call Clearing
• Used to end communication between DTEs for a SVC
• Restarting
• Used to synchronize DTE and DCE for all virtual circuits that exist between them
X.121 Addressing
• PLP uses X.121 addressing during the call setup phase
to establish a virtual circuit between DTEs
• Only used for SVC calls
• Address consists of up to 14 digits
• 3 digits for Country Code
• 1 digit for Network Number (only 10 per country)
• Up to 10 digits to define the terminal number on the
network
Country PSN National Terminal Number
X.25 Call Setup DTE to DCE
Interface
DCE to DTE
Interface

Call Request
Call Incoming Call
Setup Call Accepted
Phase Call Connected

Data Packet
Data Incoming Data
Transfer Data Packet
Phase Incoming Data

Clear Request
Call
Clear Indication
Clearing
Clear Response
Phase
Clear Confirm
 Asynchronous Transfer Mode (ATM)
• Asynchronous transfer mode (ATM) is one of the fastest growing new
WAN technologies, and is similar to frame relay.
• ATM is an unreliable protocol, meaning no error control is done by
the ATM protocol as data is moves through the network.
• Instead, error control must be handled by another network layer
(typically the transport layer, which handles end-to-end
communications).

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 Asynchronous Transfer Mode (ATM)
• Three important ATM features are:
• ATM uses fixed length, 53 byte ‘cells’ (5 bytes of overhead and 48 bytes of
user data), which is more suitable for real time transmissions.
• ATM provides extensive quality of service information that enables the
setting of very precise priorities among different types of transmissions (i.e.
voice, video & e-mail).
• ATM is scaleable, since basic ATM circuits are easily multiplexed onto much
faster ones.

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 Frame Relay
• Frame relay is a packet switching technology that transmits data
faster than X.25 but slower than ATM.
• Like ATM, Frame relay encapsulates packets, so packets are
delivered unchanged through the network.
• Also like ATM, Frame relay networks are unreliable (although they
are capable of doing error checking, this is not enough to make
Frame relay reliable).
• Common carriers offer frame relay with different transmission
speeds: 56 Kbps to 45 Mbps.

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 Switched Multimegabit Data Service (SMDS)

• Switched multimegabit data service (SMDS) is another

unreliable packet service like ATM and frame relay.
• Most, but not all, Regional Bell Operating Companies
(RBOCs) offer SMDS at a variety of transmission rates,
ranging from 56 Kbps up to 45 Mbps.
• SMDS is not standardized and offers no clear advantages
over frame relay.
• For this reason, it is not a widely accepted protocol and
offers no advantages over frame relay. Its future is uncertain.

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 Ethernet/IP Packet Networks
• Recently, Internet startups began offering Ethernet/IP services over
MAN/WAN networks.
• All other MAN/WAN services; X.25, ATM, Frame Relay and SMDS use
different protocols from Ethernet, so data must be translated or
encapsulated before it is sent over these networks.
• Companies offering Ethernet/IP have set up their own gigabit
Ethernet fiber optic networks in some large cities, bypassing common
carrier networks.
• Ethernet/IP packet network services currently offer CIR speeds from 1
Mbps to 1 Gbps at 1/4 the cost of more traditional services.

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Control Signaling Functions
• Audible communication with subscriber
• Transmission of dialed number
• Call can not be completed indication
• Call ended indication
• Signal to ring phone
• Billing info
• Equipment and trunk status info
• Diagnostic info
• Control of specialist equipment
Control Signal Sequence
• Both phones on hook
• Subscriber lifts receiver (off hook)
• End office switch signaled
• Switch responds with dial tone
• Caller dials number
• If target not busy, send ringer signal to target subscriber
• Feedback to caller
• Ringing tone, engaged tone, unobtainable
• Target accepts call by lifting receiver
• Switch terminates ringing signal and ringing tone
• Switch establishes connection
• Connection release when Source subscriber hangs up
Switch to Switch Signaling
• Subscribers connected to different switches
• Originating switch seizes interswitch trunk
• Send off hook signal on trunk, requesting digit register at target
switch (for address)
• Terminating switch sends off hook followed by on hook (wink) to
show register ready
• Originating switch sends address
Location of Signaling
• Subscriber to network
• Depends on subscriber device and switch
• Within network
• Management of subscriber calls and network
• ore complex
 In Channel Signaling
• Use same channel for signaling and call
• Requires no additional transmission facilities
• Inband
• Uses same frequencies as voice signal
• Can go anywhere a voice signal can
• Impossible to set up a call on a faulty speech path
• Out of band
• Voice signals do not use full 4kHz bandwidth
• Narrow signal band within 4kHz used for control
• Can be sent whether or not voice signals are present
• Need extra electronics
• Slower signal rate (narrow bandwidth)
Drawbacks of In Channel Signaling
• Limited transfer rate
• Delay between entering address (dialing) and connection
• Overcome by use of common channel signaling
Common Channel Signaling
• Control signals carried over paths independent of voice channel
• One control signal channel can carry signals for a number of subscriber channels
• Common control channel for these subscriber lines
• Associated Mode
• Common channel closely tracks interswitch trunks
• Disassociated Mode
• Additional nodes (signal transfer points)
• Effectively two separate networks
Common v. In Channel Signaling
Common
Channel
Signaling
Modes
 Signaling System Number 7
• The protocol that is used in the signaling network is called Signaling
System Seven (SS7). It is very similar to the five-layer Internet model
we saw in Chapter 2, but the layers have different names
• Common channel signaling scheme
• Optimized for 64k digital channel network
• Call control, remote control, management and maintenance
• Reliable means of transfer of info in sequence
• Will operate over analog and below 64k
• Point to point terrestrial and satellite links
• Physical Layer: MTP Level 1
• The physical layer in SS7, called message transport part (MTP) level 1, uses several
physical layer specifications such as T-1 (1.544 Mbps) and DC0 (64 kbps).
• Data-link Layer: MTP Level 2
• The MTP level 2 layer provides typical link-layer services such as packetizing, using
source and destination addresses in the packet header, and CRC for error checking.
• Network Layer: MTP Level 3
• The MTP level 3 layer provides end-to-end connectivity by using the datagram
approach to switching. Routers and switches route the signal packets from the
source to the destination.
• Transport Layer: SCCP
• The signaling connection control point (SCCP) is used for special
services such as 800-call processing.
• Upper Layers: TUP, TCAP, and ISUP
• There are three protocols at the upper layers. Telephone user port (TUP) is
responsible for setting up voice calls. It receives the dialed digits and routes
the calls.
• Transaction capabilities application port (TCAP) provides remote calls
that let an application program on a computer invoke a procedure on
another computer.
• ISDN user port (ISUP) - can replace TUP to provide services similar to
those of an ISDN network.
SS7
Signaling Network Elements
• Signaling point (SP)
• Any point in the network capable of handling SS7 control message
• Signal transfer point (STP)
• A signaling point capable of routing control messages
• Control plane
• Responsible for establishing and managing connections
• Information plane
• Once a connection is set up, info is transferred in the information plane
Transfer
Points
Signaling Network Structures
• STP capacities
• Number of signaling links that can be handled
• Message transfer time
• Throughput capacity
• Network performance
• Number of SPs
• Signaling delays
• Availability and reliability
• Ability of network to provide services in the face of STP failures
THANK YOU