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PUBLIC SWITCHED TELEPHONE NETWORKS
PSTN
Networking Key word
Synchronous Transfer Mode (STM)
Time-Division-Multiplexing (TDM)
Circuit switching Routing: Connection Oriented
Asynchronous Transfer Mode (ATM)
Statistical Multiplexing (SM)
Packets Switching
Routing: Connection/Connectionless
Oriented
Time Division Multiplexing
•Assume that we have m communication terminals, T1, T2, .., Tm
sharing a transmission line, how do we schedule the sharing of
communication bandwidth?
T1
T2
Tm
BROADBAND BUS
SCHEDULER
Multiplexing with scheduling
• Assume that the bandwidth is shared by the terminals
transmitting at different times.
• We also assume that a scheduling mechanism is available
so that the transmissions are conflict free, namely, that no
two terminals attempt to transmit at the same time.
• We call this scheduled or arbitrated access communication.
• In the absence of an arbitration mechanism, two
communication terminals may transmit at the same time,
often resulting in unintelligible transmissions.
Two basic approaches to multiplexing:
1. The first approach assumes a common time reference among the
terminals. We call this time reference a frame reference.
The communication bandwidth assigned for each terminal is
termed a circuit. This mode of multiplexing is commonly known
as the Synchronous Transfer Mode (STM).
2. The second approach assumes no frame reference among the
terminals, hence the name Asynchronous Transfer Mode (ATM).
This mode allows more flexible sharing of bandwidth by avoiding
rigid bandwidth assignments.
Bandwidth is seized on demand, and the information transmitted
(together with a proper label) upon a successful seizure is termed
a packet.
The Asynchronous Transfer Mode
• The definition of a frame depends on the bit-rates of the terminals
multiplexed on the transmission link.
• The choice of frame structure is difficult since we have little
knowledge of the traffic mix.
• An alternative approach abandons the concept of a frame reference
altogether. Instead of choosing a basic terminal bit-rate as in
TDM, ATM achieves more flexible bandwidth sharing
allowing the terminals to seize bandwidth when a sufficient number
of bits are generated.
• Without a frame reference, these bits have no implicit ownership,
unlike STM for which each slot is assigned an owner.
• Hence a key feature of ATM is that information from each
terminal must be labeled.
The Asynchronous Transfer Mode
There are many forms of asynchronous multiplexing:
• First, we may have fixed length blocks of information from each
terminal.
•These blocks are termed cells in ATM terminology.
• A cell is labeled block of transmitted information, and usually has
a small information payload (typically from 32 bytes to 128 bytes).
• We shall also refer to them as short fixed length packets.
The Asynchronous Transfer Mode
Cell (or Short fixed length packets):
• Each cell or packet has a fixed size of l bits. The channel is slotted
into fixed intervals of duration l/C, each transporting a cell.
• The terminals are asynchronous in the sense that they have no
common time reference other than the common slot reference.
• A label for each time slot must be provided by the terminal which
transmits in that time slot.
The Asynchronous Transfer Mode
The label identifies the terminal generating the bits delivered in the
time slot. A label is included in the header part of a packet. The
header may serve other functions; such as classifying the
information payload (type and priority), and possible error check
sums for protecting the header from transmission error.
PACKET
INFO
HEADER
t
l BITS SLOTS
Multiplexing of Fixed Length Packets
The Asynchronous Transfer Mode
There are two major factors in determining the proper packet size:
First, headers use up part of the communication capacity of the link. This
overhead is inversely proportional to the packet size l, consequently favoring
long packet.
Second, a packetization delay is needed for the terminal to collect the l bits for a
packet. The delay between signal generation and reception is given by , t = l/b
plus the delay taken for the signal to travel in the network.
For some applications, excessive delay results in perceivable degradation of the
quality of communication.
Consequently, minimizing packetization delay requires choosing short packets.
A compromise has to be chosen between two opposing factors.
The Asynchronous Transfer Mode
Variable Length Packets:
Instead of short fixed length packets, it is often convenient
(particularly for data communications) to use long (say 128 bytes
or more) variable length packets.
Besides the label for ownership, the packet header should also
contain the information for packet length to mark the end of the
packet, as well as a flag to mark the beginning of the packet.
PACKET
INFO
HEADER
t
l BITS SLOTS
Multiplexing of Fixed Length Packets
PSTN
Local Exchange Carriers (LECs)
 LECs provide local telephone service, usually within the boundaries of a
metropolitan area, state, or province.
 LECs also provide short-haul, long distance service, Centrex, certain enhanced
services such as voice mail, and various data services.
 BOCS (Bell Operating Companies), originally were wholly owned by AT&T,
dominated the ILECs landscape.
Local Access and Transport Area (LATA)
 Effective January 1, 1984, those 22 BOCs were spun off from AT&T as a result of
the Modified Final Judgement (MFJ).
 BOCs were reorganized into seven Regional BOCS (RBOCS).
 BOCs were limited to providing basic voice and data services within defined
geographical areas, known as Local Access and Transport Areas (LATAs).
 Are some 170 areas defined by the MFJ

Collectively span all BOC territories
 In general, each Boc territory comprises several LATAs
PSTN Continue
InterExchange Carriers (IXCs or IECs)
 IXCs are responsible for long-haul, long-distance connections
across LATA boundaries.
 IXC networks are connected to the LECs through a Point of
Presence (POP) which typically is in the form of a tandem
switch.
 A POP is a location where IXC interfaces BOC for exchange
access to IXC services.
 The IXC POP is connected to the LEC access tandem switch
via dedicated trunks leased from the LEC. Alternatively, the
IXC may collocate network termination equipment in the LEC
office, assuming that space is available and that secure
physical separation can be established and maintained.
 IXCs provide inter-lata services.
LEC Domain
IXC Domain
Central
end
office
Tandem
Switch
Central
Tandem
office
POP
Remote
Terminal
(RT)
Distribution
Network
Tandem
Switch
Tandem
Switch
Feeder
Network
Access (Local) Network
Regional Network
Long-distance Network
Basic Architecture of a PSTN
IXC Access Types
Direct Access
Switch
Customer
PBX
Switched Access
LEC
End
Office
Switch
Customers
POP
POP
Switch
Customers
Customer has large enough
volume of traffic accessing
the POP or requiring egress
from it to pay for the direct
connect facility, bypassing
the LEC switching network.
LEC
End
Office
LEC
Access
Tandem
POP
Customer traffic to/from POP doesn’t justify
direct connect.
• The IXC purchase access/egress facilities
from the LEC which uses its switched network
to deliver/receive that traffic.
End user access to an IXC via a
CAP, bypassing the LEC
IXC domain
ATT POP
CAP Fiber Ring
Sprint POP
Office Park
Switch
CAP
MCI POP
Achieving Connectivity
Full Mesh
Shared Medium
Role of Switching
Connectivity, network resource sharing, customer coordination
Sharing Transmission Bandwidth
Dedicated
Line
Time Shared
Synchronous
TDM
Time Shared
Packet, Burst
Circuit Switching
 Circuit” refers to the capability of transmitting one telephone conversation
along one link.
 To set up a call, a set of circuits has to be connected, Joining the two
telephone sets. By modifying the connections, the operators can switch the
circuits.
 Circuit switching occurs at the beginning of a new telephone call. Operators
were later replaced by mechanical switches and, eventually, by electronic
switches.
 An electronic interface in the switch converts the analog signal traveling on
the link from the telephone set to the switch into a digital signal, called a bit
stream. The same interface converts the digital signal that travels between
the switches into an analog signal before sending it from the switch to the
telephone.
 The switches use a dedicated data communication network “Common
channel signaling (CCS)” to exchange control information among
Circuit Switching Continue
In current telephone networks, the bit streams in the trunks (lines
connecting switches) and access links (lines connecting subscriber
telephones to the switch) are organized in the digital signal (DS)
hierarchy.
The DS-1 signal carries 24 DS-0 channels, but its rate is more than 24
times 64 kb/s. The additional bits are used to accommodate DS-0
channels with rates that deviate from the nominal 64 because the signals
are generated using clocks that are not perfectly synchronized.
Since the 1980s the transmission links of the telephone network have
been changing to the SONET or Synchronous Optical Network,
standard.
In circuit switching, the route and bandwidth allocated to the stream
remain constant over the lifetime of the stream.
Circuit Switching Continue
 The capacity of each channel is divided into a number of fixed-rate logical
channels, called circuits. The division is usually accomplished by TDM.
Circuit switching involves three phases:
(1) The source makes a connection or call request to the network, the network
assigns a route and one idle circuit from each link along the route, and the
call is then said to be admitted (if the network is unable to make this
assignment, the call is rejected). This phase is called connection setup.
(2) Data transfer now occurs-the duration of the transfer is called the call
holding time.
(3) When the transfer is complete, the route and the circuits are deallocated.
That phase is called connection teardown.
Rate in Mb/s
Meium
Signal
North America
Europe
DS-1
No. of Voice
Circuits
24
T-1 paired Cable
1.5
2.0
T-1C paired
cable
T-2 paired cable
DS-1C
48
3.1
DS-2
96
6.3
8.4
T-3 coax, radio,
fiber
Coax,
waveguide,
radio, fiber
DS-3
672
45.0
32.0
DS-4
4032
274.0
Digital Signal Hierarchy
Note that the bit rate of a DS-1 signal is greater than 24
times the rate of voice signal (64 Kb/s) because of the
additional framing bit required.
Time Division Multiplexing
Channel 1
Channel 2
Frame 1
1 2
...
Frame 2
N
1 2
...
N
...
Channel N
Circuits / Time Slots





TDM is ideal for constant bit rate traffic.
The capacity of the outgoing channel is divided into N logical channels.
Time on the outgoing channel is divided into fixed-length intervals called frames.
Frames are delimited by a special bit sequence called a framing pattern.
Time in each frame is further subdivided into N fixed-length intervals called
slots/circuits.
 Each frame consists of a sequence of slots: slot 1, slot 2,.., slot N. (A slot is usually
1 bit or 1 byte wide).
 A logical channel occupies every Nth slot. There are thus N logical channels. The
first logical channel occupies slots 1, N + 1, 2N + 1,..; the second occupies slots 2,
N+2, 2N+2,...; and so on.
Synchronous Transfer Mode
PBX
STM
Multiplexer
Router
3
2
1
Workstation
STM Multiplexing is also known as Time Division Multiplexing (TDM)
3
2
1
TDM Continues
 The T1 Frame (or the OSI term, PDU) consists of 24 8-bits slots.
The TDM multiplexer operates as follows:
 The data bits in each incoming channe1 are read into a separate FIFO (first in,
first out) buffer.
 The multiplexer reads this buffer in sequence for an amount of time equal to
the corresponding slot time: buffer 1 is read into slot 1, buffer 2 is read into slot
2, etc.
 If there are not enough bits in a buffer, the corresponding slot remains partially
empty.
 The bit stream of the outgoing channel is easily demultiplexed: the
demultiplexer detects the framing pattern from which it determines the beginning of each frame, and then each slot.
Statistical Multiplexing (SM)
Channel 1
Channel 2
1
N
1
2
...
Channel N
Most effective in the case of bursty input data.
As in TDM, the data bits in each incoming channel are read into separate FIFOs.
The multiplexer reads each buffer in turn until the buffer empties.
The data read in one turn is called a “data packet”.
Asynchronous Transfer
Mode
A
Y
PBX
B
ATM
Multiplexer Z
Y
Z
Z
Z
Y
Router
C
Workstation
Z
SM Continues
 In TDM each FIFO is read for a fixed amount of time-one slot-and
so each incoming channel is allocated a fixed fraction of the
outgoing channel capacity, independent of the data rate on that
channel.
 By contrast, in SM, the capacity allocated to each incoming
channel varies with time, depending on the instantaneous data
rate: the higher the rate, the larger the capacity allocated to it at
that time.
 The size of packets read from each FIFO can vary across channels
and over time within each channel.
 The demultiplexer cannot sort the packets belonging to different
channels merely from their positions within a frame.
SM Continues
 Additional bits, which delimit each packet and identify the corresponding
incoming channel or source, must be added to each packet.
 The resulting overhead is significantly larger than under TDM.
 Multiplexer and demultiplexer implementations are more difficult;
 Multiplexer must now add the packet delimiter and channel or source
identifier.
 Demultiplexer must locate and decode those bit patterns.
 These increases in complexity and overhead must be balanced against high
utilization in the face of bursty data to determine whether SM or TDM is more
efficient.
DATA COMMUNICATIONS
Data
Binary Codes
 Between machines, information is exchanged by binary digits (bits).
Two sets are in common use today:
ASCII: the American Standard Code for Information Interchange
employs a sequence of seven bits. Since each bit may be 0 or 1, ASCII
contains 128 unique patterns.
EBCDIC: the Extended Binary Coded Decimal Interchange Code
employs a sequence of eight bits. It contains 256 unique patterns.
 There are two basic methods of data transmission Asynchronous and
Synchronous.
Asynchronous (Character Framed) Transmission;
 Characters are generated and transmitted singly, one after the other.
 In some terminals, the characters are collected until a complete line of
text is created, or the return key is pressed, causing the line to be sent as a
burst of continuous characters.
Data Continues
 Whether sent one-by-one as they are generated, or sent line-by-line as each line
is completed, each character is framed by a start bit (0) and a stop bit (1)
Synchronous (Message Framed ) Transmission:
 Such transmission is message framed and overcome the inefficiencies of
asynchronous, start-stop transmission for high speed data transmission.
 Rather than surrounding each character with start and stop bits, a
relatively large set data is framed, or blocked with one or more
synchronization bits or bit patterns used to synchronize the receiving
terminal on the rate of transmission of the data.
 The start sequence is called the header – it contains synchronizing,
address, and control information. The stop sequence is called the trailer –
it contains error checking and terminating information.
 The entire data entity is called a “Frame”
In asynchronous transmission,
each character is framed by one
start bit and one or two stop bits.
Stop Bit (1)
Start Bit (0)
Framed characters sent as they are created -- a data
stream typical of keyboard input to a terminal or
communications controller.
Framed characters that are concatenated and sent when a
string is completed -- a datastream typical of a terminal
sending keyboard input line-by-line to a communications
controller
Data Block
Trailer
Header
Frame
Character
Synchronous Transmission Format
Asynchronous Transmission Format
Character
Characters are assembled
into a datablock that is
framed by a header and a
trailer to produce a frame.
The frame is sent when a
command is received from
the controlling unit in the
communication system.
Error Control/Detection
Message
Datastream that includes redundant bits and the
result of the sender’s calculations
Message
Sender
Sender adds redundant bits and
performs calculations to assist the
receiver in error detection
Receiver
Receiver checks redundant bits and
repeats calculations looking for
agreement with sender’s results
Because each character is assigned a unique code, it is extremely
important to be sent without error. For instance, the ASCII code for p
is 1110000. An error in bit # 1produces 1110001
which is the
code for q.
 Error detection is a cooperative activity between the sender and the
receiver in which a sender adds information to the character or frame
to assist the receiver in determining whether an error has occurred in
transmission or reception.
Receiver performs same calculation...
Sender performs calculation...
MK
Gn+1
MK
MK
Gn+1
= integer + Fn
= integer + F’n
If F’n = Fn’ transmission is without error
If F’n  Fn’ transmission is without error
Sender adds
Frame Check
Sequence
(Fn) to frame
Gn+1
Generating
Function
Fn MK
Receiver
re-calculates
Fn
Gn+1
Generating
Function
Cyclic Redundancy Check
MK
Error Correction
Once detected,an error must be corrected. Two basic approaches to
error correction:
1. Automatic-Repeat-Request (ARQ):
Requires the transmitter to re-send the portions of the exchange in
which errors have been detected. ARQ techniques include:
•Stop-and-Wait: The sender sends a frame and waits for
acknowledgement from the receiver. This technique is slow.
•Go-back-n:
2. Forward Error Correction (FEC): FEC techniques employ special
codes that allow the receiver to detect and correct a limited number of
errors without referring to the transmitter. This convenience is bought
at the expense of adding more bits (more overhead)
MODEM
Data Terminal
Equipment
DTE
EIA232
Digital Signals
DTE
EIA232
DCE
Analog (Voice
Grade) Line
Data Circuit
Terminating
Equipment
DSU/CSU
Digital
Line
• The data equivalent of Customer Premise Equipment (CPE) in the
voice world, Data Terminal Equipment (DTE) comprises the computer
transmit and receive equipment; are digital devices that send or receive
data messages.
• Internally, their signals are simple, unipolar pulses; externally, they
may use one the more sophisticated digital signaling schemes.
Data Communication
 Data Circuit Terminating Equipment (DCE): is the equipment that interfaces the DTE
to the network; maps the incoming bits into signals appropriate for the channel, and at
the receiving end, maps the signals back to bits.
 DCEs includes modems, digital service units (DSUs), and channel service units
(CSUs).
 If the transmission channel is an analog line (voice-grade), the DCE is called a
modem. When sending, DCE convert the digital signal received by the DTE to
analog signals to match the bandwidth of the channel.
 If the connections are digital connections, the DCE consists of two parts:
DSU- receives unipolar digital signals from the DTE and converts them to bipolar
signals.
CSU: provides loopback (for testing), limited diagnostic capabilities. When
sending, it converts bipolar signals to AMI.
Data Communication Continues
EIA232 interface
 A DET is connected to a DCE by a cable that conforms to EIA232 standard.
 EIA232 describes a multi-wire cable that terminates in 25-pin connectors.
 The cable supports asynchronous or synchronous operation at speed up to
19.2 kb/s. At 19.2 kb/s, the cable length is limited to 50 feet.
 The EIA232 circuits linking DTE and DCE carry signals that initiate,
maintain, and terminate communication between the two.
Higher Speed Interconnections
EIA449: It permits operation up to 2 Mb/s at distances up to 4000 feet.
Enterprise Systems Connection (ESCON):
an optical fiber connection operating up to 40 kilometers at 17 Mb/s.
Fiber Channel Standard (FCS): Operates up to 10 kilometers at speeds up
to 800 Mb/s. FCS includes error control and switching.
Protocols
Data Link Control (DLC) Protocol
 A set of rules that governs the exchange of messages over a data link.
DLC protocols are divided into two classes:
• Asynchronous Operation:
 Start-Stop DLC protocol
• synchronous Operation:
 Bit-oriented DLC protocol (e.g., SDLC): Introduced in 1972, SDLC
was modified and standardized by ITU-T and ISO as:
 HDLC (High Level Data Link Control Protocol)
 LAP-B (Link Access-Procedure Balanced), for X.25 Standard
 LAP-D ((Link Access-Procedure Channel), for ISDN-D Channel
 LAP-F ((Link Access-Procedure Frame Relay), a version of LAP-D
used in Frame Relay applications.
Different in the detailed meaning of specific control field bits, all of
these protocols share a common structure. In the order that they are
transmitted, they consist of the following fields: Flag, Address,
Control, Text, Frame Check Sequence, and Flag.
Start
Bit
CHARACTER
ASCII ‘a’
Stop
Bit
Line
Idle 0 1 0 0 0 0 1 1 1
State
Timing Mark
Start
Bit
Line
Idle
State
CHARACTER
ASCII ‘b’
Stop
Bit
Line
0 0 1 0 0 0 1 1 1 Idle
State
Timing Mark
Time between characters
Transmission Format for Start-stop (Asynchronous)
Signaling. In idle state, the line is maintained at the 1
level. The start bit (0) reduces the level to zero
signaling the commencement of activity.
SDLC Frame Format
01111110
NR
Receive Sequence Number
01111110
Number (in sequence 000
SDLC FRAME
F
L
Address
A
G
8
bits
24
through 110) of frame
Trailer
Control
Header
(not Supervisory Frames)
F
C
S
F
L
A
G
Nx8
16
8
TEXT
usually 1024 bits
8
expected. 111
acknowledges sequence of
seven frames.
NS
Send Sequence Number
Number (in sequence 000
through 110) of this
Information Format
0
NS
F NR
frame.
Mode
00 = Ready to Receive
10 = Not ready to Receive
Supervisory Format
1 0
Mode
01 = Reject
P NR
NO TEXT
P = 0 = not polled
1 = poll
F = 0 = more frames to come.
Information transfer is not
complete.
1 = last Frame
Host
#1
Host
#2
Gateway
Gateway
Packet Network
Ethernet Frame (LAN A)
MAC
LLC
IP
TCP
Header
Header Header Header
A
User’s
Data
MAC
Trailer
A
Packet Network Frame
IP
TCP
LAPB Packet
Header Header Header Header
User’s
Data
LAPB
Trailer
Token-Ring Frame (LAN B)
MAC
LLC
IP
TCP
Header
Header Header Header
B
PROTOCOL STACKS
User’s
Data
MAC
Trailer
B
Upper
Layers
Upper
Layers
TCP
IP
TCP
IP
IP
LLC
CSMA/
CD
LLC
CSMA/
CD
Physical
IP
Packet
HDLC
Packet
HDLC
LLC
TokenRing
Physical Physical
Physical Physical
Gateway
Gateway
LAN A
Ethernet
Packet
Network
LLC
TokenRing
Physical
LAN B
Token-Ring
PACKET SWITCHING
Packet Switching
 The data stream originating at the source is divided into packets of fixed or
variable size.
 The time interval between consecutive packets may vary, depending on the
burstiness of the stream.
 As the bits in a packet arrive at a switch or router; they are read into a
buffer when the entire packet is stored, the switch routes the packet over
one of its outgoing links.
 The packet remains queued in its buffer until the outgoing link becomes
idle. This store-and-forward technique thereby introduces a random
queuing delay at each link;
 The delay depends on the other traffic sharing the same link. Packets from
different sources sharing the same link are statistically multiplexed.
Packet Switching Continues
The routing decision
Connectionless (datagra
Connectionless
(datagram)
Connection Oriented (virtual circuit)
In datagram packet networks, each packet within a stream is independently routed.
 A routing table stored in the router (switch) specifies the outgoing link for each
destination. The table may be static, or it may be periodically updated.
 Each packet must contain bits denoting the address of the source and destination.
In virtual circuit packet networks, a fixed route is selected before any data is transmitted
in a call setup phase similar to circuit-switched networks.
 However; there is no notion of a fixed-rate circuit or logical channel. All packets
belonging to the same data stream follow this fixed route, called a virtual circuit.
 Packets must now contain a virtual circuit identifier; this bit string is usually shorter than
the source and destination address identifiers needed for datagrams. However; the call
setup phase takes time and creates a delay not present in datagram packet networks.
Connection-Oriented vs Connectionless
Transport
Connection Oriented
Circuits and Virtual
Circuits
Connectionless
Guaranteed
Resource
Shared
Resource
Connection State
Yes
Yes
No
Delay
Constant
Variable
Variable
Bandwidth
Guaranteed
Shared
Shared
Overload
“Busy”
“Share Pain”
“Share Pain”
Packet Sequence
Maintained
Maintained
Could change
Connection Oriented Packet Transport
•
•
•
•
•
Connection Request
Resource Check
Route Selection
Destination Acceptance
Connection begins
Connectionless Transport
•
•
Lower Level Protocol (IP)
“Send and Pray”
Upper Level Protocol
Guaranteed delivery
Frame
Relay
(Switching)
Cell Relay
(Switching)
Hold &
forward
Hold &
forward
Hold &
forward
Copper,
wireless
Copper,
wireless,
optical
Copper,
wireless,
optical
Copper,
wireless,
optical
No such
thing
Variable,
large to
small
Variable,
large to
small
Variable,
large to
small
Fixed, very
small
Very Fast
Slow
Fast
Faster
Very Fast
Delay
Techniques
PDU
Size of
Media
Relay
Circuit
Switching
Message
Switching
Packet
Switching
Direct
Connection
Store &
forward
Copper,
wireless
Switching Technologies
Fast Relay
Frame Relay
Cell Relay
(Variable size
PDU’s--frames)
(Fixed size PDU’s-cells)
PVC
SVC
ATM Based
802.6 Based
(LAPD)
(Q.931)
(For B-ISDN)
(For SMDS)
PVC
SVC
(Q.2931)
Types of relay systems
Typical X.25 Topology
X.75 (NNI)
X.25
X.25
User
User
= Packet switches
X.25 is not a packet switching specification. It’s a packet network
interface specification. X.25 says nothing about operations within
the network.
It Provides for an interface between an end-user device (DTE) and a
network (DCE). Its formal title is “Interface between DTE and DCE
for terminals operating in the packet node on public data networks”
In X.25, the DCE is the “agent” for the packet network to the DTE.
X.25 Continue
 X.25 encompasses the lower three layers of the OSI model
X.25-3 layer (network layer)
Packets are created at the network layer that Establishes, manage,
and teardown the connections between the user and the network.
X.25-2 layer (data link layer)
The packet is encapsulated within the Link Access Procedure, Balanced
(LAPB) protocol as the information field. The LAPB protocol is a subset of HDLC (High Level Data Link Control).
X.25-1 layer (physical layer)
The physical layer is the physical interface between the DTE and the
DCE.
X.25 Continue
 X.25 uses logical channel numbers (LCNs) to identify the DTE connections to the
network. An LCN is really nothing more than a virtual circuit identifier (VCI).
 Octets #1 and Octet #2 of the packet header provide a 12-bit identifier. If all-zeros
possibility is excluded, as many as 4095 logical channels (i.e., user sessions) can be
assigned to a physical channel.
 The LCN serves as an identifier (a label) for each user's packets that are transmitted
through the physical circuit to and from the network.
 Typically, the virtual circuit is identified with two different LCNs-one for the user at the
local side of the network and one for the user at the remote side of the network.
 X.25 provides two mechanisms to establish and maintain communications between the
user devices and the network (and ATM has borrowed these concepts): Permanent
Virtual Circuit (PVC) and Switched Virtual Circuit (SVC).
X.25 Continue
 PVCs may support large users. All packets travel the same path between two computers;
which path is established by routing instructions programmed in the involved nodes.
 The circuits involved in the route are defined on a permanent basis, until such time as
they are permanently redefined, perhaps as the service
 Alternatively, the network may select the most available and appropriate path on a callby-call basis using Switched Virtual Circuits (SVCs);
 Again, all packets in a given session travel the same path.
 SVCs demand a greater level of network intelligence that adds to total network cost; this
translates into higher cost to the end-user organization.
 The establishment of a SVC also involves some level of delay since the network nodes
must examine multiple paths in order to make a proper selection.
USER-NETWORK INTERFACE
X.25
PACKET NETWORK
User’s Data
User Stack
Transport
Packet Header
Packet
Data
Packet
X.25-3
LAPB
X.25-2
X.21
LAPB
Header
Network
LAPB
Trailer
LAPB
Data Link
X.21
Physical
Data
X.25-1
DTE
DCE
USER’S INFORMATION
I.e. message data and/or headers from upper layers
User’s Data
Segment
User’s Data
Segment
User’s Data
Segment
User’s Data
Segment
HDLC FRAME
Header
F
L
A
G
A
d
d
r
e
s
s
Packet
C
o
n
Packet
t
r Headers
o
l
User’s Data
Segment 1024
bits
Trailer
FCS
F
L
A
G
Logical Grp # 1 0 D Q
Logical Channel Number
0 P(S)
M
P(R)
X.25 Packet and Frame Format
COMPUTER NETWORKS
RS-232-C (1969)
2.4 – 38 Kbps
01101011_11011010_
The RS-232-C standard for the serial line
specifies the transfer of one 8-bit character at a
time, separated by time intervals. The speed
and distance of the serial line are limited.
The Synchronous Data Link Control and related
standards transmit long packets of bits. The header (H)
contains the preamble that starts the receiver clock,
which is kept in phase by the self-synchronizing
encoding of the bits. The receiver uses the cyclic
redundancy check (CRC) bits to verify that the packets is
correctly received.
B
A
C
E
D
Store-and-forward transmissions proceed by sending the packet
successively along links from the source to the destination. The
packet header specifies the source and destination addresses (A
and E, for example) of the packet. When it receives a packet, a
computer checks a routing table to find out on which link it
should next send the packet.
A
B
E
C
D
Ethernet. In this network, computers are attached to a
common coaxial cable. The computers read every transmitted
packet and discard those not addressed to them.
A
B
4 or 16 Mbps
E
C
D
Token ring. The computers share a ring. Access is regulated
by a token-passing protocol.
A
B
100 Mbps
E
C
D
Fiber Distributed Data Interface (FDDI). A token-passing
protocol is used to share the ring. The computers time their
holding of the token. This network guarantees that every
computer gets to transmit within an agreed-on time.
155-622 Mbps
A
C
B
E
D
Asynchronous Transfer Mode (ATM) network. The network
transports information in 53-byte cells. Total throughput of
this network is much larger than that of FDDI or of a 100-Mbps
Ethernet.
LAYERING APPROACH
CPU
CPU
DISK
Keyboard,
mouse, etc.
CACHE
VRAM
Display
RAM
User
System
RAM
NIC
NIC
Message Transfers
The left panel gives a simple architecture of a host computer and its
connection to the network. The right panel shows the four copies
that may be involved across the CPU bus to run an application,
reducing the host throughput.
Computer
OSI Hierarchy
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Link
1
Physical
• Physical
– SONET, T1, T3
• Link
– Ethernet, FDDI
– Circuit, ATM, FR
switches
• Network
– Routing, Call control
– IP internetworking
OSI Hierarchy
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Link
1
Physical
• Transport
– Error and congestion
control
– TCP, UDP
• Session, Presentation,
Application
– Data, voice encodings
– Authentication
– web/http, ftp, telnet
Data Transfer Over Frame-based
Networks
File
TCP
IP
Frame
(Ethernet,
FR, PPP)
Data Transfer Over Cell-based
Networks
File
TCP
IP
Adaptation
ATM Cells
Internet Protocol Architecture
Ping
FTP
TELNET
SMTP
ICMP
HTTP
DNS
RTP
BGP
SNMP
RIP
UDP
TCP
OSPF
IP
LANs
10/100BaseT
ATM
FR
Dedicated B/W:
DSx, SONET, ...
PPP
Circuit-Switched B/W:
POTS, SDS, ISDN, ...
CDPD
Wireless
Why a Synchronous Network
“Visibility” of each byte at the line rate
• Simplification of the multiplexing and
switching process
•
Simple access to overhead bytes
Asynchronous
“Stuffing” Bits
OH
OH
Synchronous
Overhead functions – framing, monitoring,
fault location, protection switching,
management communications.