Deepak John,SJCET-Pala,High Speed Switched WANs. Frame Relay:- Back ground, Architecture, Frame Relay Layers, Frame Relay frame – LAPF core, LAPF control. ATM :- design goals, Cell Network, Asynchronous TDM, ATM Architecture, Identifiers, ATM Layers:- ATM layer, ATM Headers, ATM Adaptation Layer:- AAL1, AAL2, AAL3/4, AAL5. Transport Layer: Services, elements of transport protocol, simple transport protocol.

1. Frame Relay, ATM, Transport Layery p y
Deepak John
SJCET-PalaSJCET Pala

2. Frame Relay

3. Frame Relay
i k i h d A l h h h i l d d li k is a packet switched WAN protocol that operates at the physical and data link
layers of the OSI reference model.
 As fibre optic was introduced, the quality of circuits improved and there was no As fibre optic was introduced, the quality of circuits improved and there was no
need for error control.
 was developed in response to a high speed, high performance and greater efficient
i itransmission.
 It puts data in variable-size units called "frames" and provide minimal internal
checkingchecking
 support data transfer rates at
 T-1 (1.544 Mb/s)
 T-3 (45 Mb/s) speeds.

4.  enabling end stations to dynamically share the network medium and the available
bandwidth.
i h d l A f ll i h f ll i l Devices attached to a Frame Relay WAN fall into the following two general
categories:
 1. Data terminal equipment (DTE)q p ( )
 for a specific network and typically are located on the premises of a customer.
 Example of DTE devices are terminals, personal computers, routers, and bridges.p , p p , , g

5.  2. Data circuit-terminating equipment (DCE)
carrier-owned internetworking devices.
Th i t id l ki d it hi i i t k hi hThe purpose is to provide clocking and switching services in a network, which
are the devices that actually transmit data through the WAN.

6. A hiArchitecture
 Frame Relay has 2 layers: physical and data link (LAPF, Link Access Procedure for
Frame Mode Bearer Services))
Physical Layer
 No specific protocol is defined for the physical layer in Frame Relay. Instead, it
is left to the implementer to use whatever is availableis left to the implementer to use whatever is available.
 Frame Relay supports any of the protocols recognized by ANSI.

7.  Data Link Layer
 Link layer uses the services of the physical layer. It, in turn, provides the
f ll i ifollowing services :
 Flag recognition.
 Frame check sequence (FCS) generation and checking Frame check sequence (FCS) generation and checking.
 Recognition of invalid frames.
 Discard incorrect frames.
 Routing.
 Congestion control notification

8. Frame Relay Virtual Circuit
 Virtual circuits provide a bidirectional communication path from one DTE device
t th d i l id tifi d b b ll d d t li k tito another and are uniquely identified by a number called data link connection
identifier (DLCI).
 When a virtual circuit is established by the network, a DLCI number is given to ay , g
DTE in order to access the remote DTE.
 Frame Relay virtual circuits fall into two categories:
 switched virtual circuits (SVCs)
 permanent virtual circuits (PVCs).

9. 1. Switched virtual circuits (SVCs)
 temporary connections, A new virtual circuit connection will be established each
time a DTE wants to make a connection with another DTEtime a DTE wants to make a connection with another DTE.
 A communication session across a SVC consists of the following four
operational states(Call setup ,Data transfer ,Idle and Call termination )
2. Permanent virtual circuits (PVCs)
 permanently established connections by the network provider that are used for
frequent and consistent data transfers between DTE devices across the Framefrequent and consistent data transfers between DTE devices across the Frame
Relay network.
 Always operate in one of the following two operational states(Idle and Data
Transfer)

10. P t l A hit tProtocol Architecture

11. LAPF Core
LAPF Frame
 The address area which is 2 bytes in length is comprised of 10 bits The address area, which is 2 bytes in length, is comprised of 10 bits
representing the actual circuit identifier and 6 bits of fields related to
congestion management.
 DLCI fi ld 10 bit DLCI fi ld t th dd f th f d DLCI field: 10-bit DLCI field represents the address of the frame and
corresponds to a PVC.

12.  Command/response (C/R): Designates whether the frame is a command or
response.
 Extended address (EA): used for expanding the number of possible addresses Extended address (EA): used for expanding the number of possible addresses.
 Forward explicit congestion notification (FECN):can be set by any switch to
indicate that traffic is congested. This bit informs the destination that congestiong g
has occurred.
 Backward explicit congestion notification (BECN):is set (in frames that travel
in the other direction) to indicate a congestion problem in the networkin the other direction) to indicate a congestion problem in the network.
 Discard eligibility (DE): indicates the priority level of the frame. In emergency
situations, switches may have to discard frames to relieve bottlenecks and keepy p
the network from collapsing due to overload.

13.  core functions of LAPF are used for frame Relay:
 Frame delimiting and transparency
 Frame mux and demux using addressing field
 Ensure frame is neither too long nor short
 Detection of transmission errors Detection of transmission errors
 Congestion control functions

14. LAPF-Control
 The user terminals(DTEs) implement full LAPF protocol, which is also called
LAPF-Control Protocol.
 The only difference b/w this protocol and LAPF-core is the inclusion of a control
field.
 Control protocol provides the functions of flow and error control that are missing Control protocol provides the functions of flow and error control that are missing
from core protocol

15. Congestion-Control Mechanisms
 Frame Relay reduces network overhead by implementing simple congestion-
tifi ti h i F R l i l t t ti tifi tinotification mechanisms. Frame Relay implements two congestion-notification
mechanisms:
 Forward-explicit congestion notification (FECN)p g ( )
 Backward-explicit congestion notification (BECN)
 FECN and BECN each is controlled by a single bit contained in the Frame Relay
f h d Th F R l f h d l i Di d Eli ibiliframe header. The Frame Relay frame header also contains a Discard Eligibility
(DE) bit, which is used to identify less important traffic that can be dropped
during periods of congestion.g p g

17. Four Cases of Congestion

18. Asynchronous Transfery
Mode(ATM)

19. Introduction
 ATM is a concept similar to frame relay which take advantages of modern
digital facilities to provide faster packet switching
i i i d hi h d l d l i hi d i i is a connection-oriented, high-speed, low-delay switching and transmission
technology
 allows multiple logical connections to be multiplexed over a single physicala ows u t p e og ca co ect o s to be u t p e ed ove a s g e p ys ca
interface.
 uses fixed sized packets called cells
 Developed to enable simultaneous Voice, Video, and Data traffic on the same
network with minimal error and flow control
 data rates of 25 6Mbps to 622 08Mbps data rates of 25.6Mbps to 622.08Mbps

20. Design Goals
1. Use of high data rate transmission media (i.e fiber optic)
2 I t bilit ith iti t h l i2. Interoperability with exiting technologies
3. Implementation at reasonable cost
4. Support for existing telecommunications hierarchiespp g
5. Reliable and predictable
6. Suitable for real-time and non real-time services

21. Cell Networks
 A cell network uses the cell as the basic unit of data exchange
 ATM carries information on cells ATM carries information on cells
 The length of each cell is 53 Bytes
 First 5 bytes are used as the cell header First 5 bytes are used as the cell header
 Next 48 bytes are used as the payload carrying the data

22. Fixed Length Cell Advantage
 Delay or latency is significantly reduced
 ATM is therefore suited for voice and video transmission ATM is therefore suited for voice and video transmission
 Fixed length cells make it easier to switch data across multiple networks
 ATM networks are built based on switches and not routers
 Fixed length cell is similar to container based road transportation

23. Multiplexing with cells
 The cells from the two lines are interleaved so that none suffers a long delay.
 Hi h d f th li k l d ith th ll i f th ll th t ll High speed of the links coupled with the small size of the cells means that cells
from each line arrive at their respective destinations in a continuous stream.
 A cell network can handle real-time transmissions, such as a phone call, without, p ,
the parties being aware of the segmentation

24. Asynchronous Time-Division Multiplexing
 ATM uses asynchronous time-division multiplexing to multiplex cells coming
from different channelsfrom different channels.
 It uses fixed-size slots, ie cells.
 ATM multiplexers fill a slot with a cell from any input channel that has a cell;
h l i if f h h l h ll dthe slot is empty if none of the channels has a cell to send

25. Architecture
ATM Devices
 ATM networks are built around two categories of devices
 ATM Switch
 ATM end-point
 An ATM switch can be connected to either another ATM switch or and ATM An ATM switch can be connected to either another ATM switch or and ATM
end-point.
 ATM end point
 contain and ATM end-point adapter
 Examples of ATM end-points are Workstations,LAN switches, Routers etc

26.  Two Types of Interfaces that interconnect ATM devices over point to point links:
 User-Network Interface (UNI):connects an ATM end-system (client side) with
an ATM switch (network site).
 Network-Network Interface (NNI) : switches are connected through network-
k i f (NNI )to-network interfaces (NNIs).

27. Virtual Connection
Connection between two endpoints is accomplished through
1. Transmission Paths (TPs):is the physical connection between an endpoint and a
switch or between two switches. A transmission path is divided into several
virtual paths.p
2. Virtual Paths (VPs): provides a connection or a set of connections between two
switches.
3. Virtual Circuits (VCs):SVC or PVC

28.  A virtual connection is defined by a pair of numbers: VPI and VCI
 ATM assigns each Virtual Connection a 24-bit identifier
i l h d ifi ( ) ifi h h h f ll h h h1. Virtual Path Identifier (VPI), specifies the path the VC follows through the
network.8 bits long.
2. Virtual Channel Identifier (VCI), specifies a single VC within the path.16 bits( ), p g p
long.

29.  Cell networks are based on Virtual Connection and all cells belonging to a
single message follow the same virtual circuit

30. Example of VPs and VCsExample of VPs and VCs

31. ATM Protocol Layers
ATM Adaptation Layer ATM Adaptation Layer
ATM Endpoint ATM Endpoint
ATM Switch
ATM Layer
Physical Layer
ATM Layer
Physical Layer
ATM Layer
Physical Layer
Physical Mediumy
 Physical Layer
 It describes the physical transmission media.
 W hi ld d d hi ld d t i t d i i l bl d fib ti We can use shielded and unshielded twisted pair, coaxial cable, and fiber-optic
cable.

32. ATM Layer
 The ATM layer is responsible for establishing connections and passing cells
through the ATM networkthrough the ATM network.
 It provides routing, traffic management, switching, and multiplexing services

33. ATM cell
 5-byte ATM cell header .the header can be in either UNI or NNI format
d di th I t fdepending on the Interface.
 48-byte payload
General Format

34. ATM Cell Header—UNI Format
ATM Cell Header—NNI Format
 General Flaw Control(GFC): Provides local functions, such as flow control from
end point equipment to the ATM switch.
 P l d T (PT) I di t h th th ll t i d t t l d t Payload Type(PT): Indicates whether the cell contains user data or control data.

35.  Cell Loss Priority(CLP): Indicates whether the cell should be removed if it
encounters errors as it moves through the network.
 d C l( C) C i C li d d Ch k (C C) h Header Error Control(HEC): Contains Cyclic Redundancy Check (CRC) on the
cell header.
 Virt al Path Identifier (VPI): Identifies semi permanent connections bet een Virtual Path Identifier (VPI): Identifies semi-permanent connections between
ATM end points.
 Virtual Channel Identifier (VCI): Have only local significance on the link Virtual Channel Identifier (VCI): Have only local significance on the link
between ATM nodes.

36. ATM Adaptation Layer(AAL)
 It converts the submitted information into streams of 48-octet segments and
t t th i th l d fi ld f lti l ATM lltransports these in the payload field of multiple ATM cells.
 Similarly, on receipt of the stream of cells it converts the 48-octet information
field into required form for delivery to the particular higher protocol layer.q y p g p y
 AAL exists only in end systems, not in switches.
 AAL Services
dl i i Handle transmission errors
 Segmentation/reassembly (SAR)
 Handle lost and misinserted cell conditionsa d e ost a d s se ted ce co d t o s
 Flow control and timing control

37.  AAL is classified into four(The classification was made with respect to the
,following parameters:
 Timing relationship between sender and receiver Timing relationship between sender and receiver
 Related
 Not related
 Bit rate
 Constant bit rate
 Variable bit rate
 Connection mode
 Connection oriented Connection-oriented
 Connectionless

38.  AAL is divided into two sub layers:
i. Convergence Sub layer :manages the flow of data to and from
SAR blSAR sublayer.
ii. Segmentation and reassembly sub layer:Packages data from CS
into cells and unpacks at other endp

39. Class A Class B Class C Class DService
Timing
between
Bit Rate Constant Variable
Related Not Related
between
Source and
Destination
Connectionless
Connection
Mode
Examples
Constant Bit
R t Vid
Connection Oriented Connectionless
Variable Bit
Connection-
oriented Connectionless
Data Transfer
Examples
of
Services
Rate Video
and Audio
Variable Bit
Rate Video
and Audio
Data
Transfer
AAL
TYPE AAL 1 AAL 2
AAL 3/4
AAL 5
AAL 5
AAL 3/4TYPE AAL 1 AAL 2 AAL 5 AAL 3/4

40. AAL 1 (Constant Bit Rate)
The CS layer divides the bit stream into 47-byte segments and passes them to
the SAR sub layer belowthe SAR sub layer below
The SAR sub layer adds 1 byte of header and passes the 48-byte segment to the
ATM layer.
The 1 byte header is divided into two 4 bit fieldsThe 1 byte header is divided into two 4-bit fields
Sequence number (SN)
Sequence number protection (SNP)

42. ATM Adaptation Layers: AAL2
 AAL2 was intended to support a variable-data-rate bit stream.
 It is used for low-bit-rate traffic and short-frame traffic such as audio
(compressed or uncompressed), video, or fax
 allows the multiplexing of short frames into one cell allows the multiplexing of short frames into one cell.
 It widely used in wireless applications

44. AAL 3/4
 AAL3 was intended to support connection-oriented data services and AAL4 to
support connectionless servicessupport connectionless services
 Later they have been combined into a single format called AAL3/4
 the convergence sub layer (CS) creates a protocol data unit (PDU) by adding a
b i i h d t th f l th fi ld t il d i bl l th dbeginning header to the frame ,a length field as a trailer and a variable-length pad
 the segmentation and reassembly (SAR) sub layer fragments the PDU and append
a header to it .
 Then, the SAR sub layer appends a CRC-10 trailer to each PDU fragment for
error control
 The completed SAR PDU becomes the Payload field of an ATM cell The completed SAR PDU becomes the Payload field of an ATM cell

46. AAL 5
 is the primary AAL for data and supports both connection-oriented and
ti l d tconnectionless data.
 also known as the Simple and Efficient Adaptation Layer (SEAL)
 The SAR sub layer simply accepts the CS-PDU and segments it into 48-octet The SAR sub layer simply accepts the CS PDU and segments it into 48 octet
SAR-PDUs without adding any additional fields.
 the CS sublayer appends a variable-length pad and an 8-byte trailer to a frame.
The trailer includes the length of the frame and a 32-bit cyclic redundancy check
(CRC)
 the SAR sub layer segments the CS-PDU into 48-byte blocks the SAR sub layer segments the CS PDU into 48 byte blocks .
 the ATM layer places each block into the Payload field of an ATM cell

48. Transport Layer

49. Introduction
 The transport layer is concerned with the provision of host-to-host user
ti f th li bl d t ff ti t f f d tconnections for the reliable and cost effective transfer of user data
 It Isolates upper layers from the network layer
 The transport layer is responsible for process-to-process delivery of a packet The transport layer is responsible for process to process delivery of a packet.
 At the transport layer, we need a transport layer address, called a port number,
to choose among multiple processes running on the destination host.

50. Transport Services
 provide logical communication between application processes running on
different hosts.
Th t t f t t i Th ti i t d t t There are two types of transport service. The connection-oriented transport
service and connection-less transport service.
 transport protocols are used for providing transport services .transport protocolsp p p g p p p
run in end systems
 sender side: breaks messages into segments, passes to network layer
 receiver side: reassembles segments into messages, passes to higher layer
 more than one transport protocol available to apps
 Internet: TCP and UDP Internet: TCP and UDP

51. Elements Of Transport Protocols
 Elements
1. Addressing
2. Connection Establishment
3 Connection Release3. Connection Release
4. Flow Control and Buffering
5. Multiplexing5. Multiplexing
6. Crash Recovery

52. 1. Addressing
 When an application process wishes to set up a connection to a remote application
it t if hi h t t tprocess, it must specify which one to connect to
 The method normally used is to define transport addresses is by using connection
requestsq
 The network layer address identifies a host. The transport layer address identifies
a user process – a service – running on a host
 In the Internet, these endpoints are called ports or TSAP (Transport Services
Access Points).
 The endpoints in the network layer (i e network layer addresses) are called The endpoints in the network layer (i.e., network layer addresses) are called
NSAPs (Network Service Access Points)

53. TSAPs, NSAPs, and transport connections
TSAPs, NSAPs, and transport connections

54. IP addresses versus port numbers

55. 2 C i i2. Connection Establishment
 Just send REQUEST, wait for ACCEPTED.
 The problem occurs when the network can lose and duplicate packets The problem occurs when the network can lose and duplicate packets.
 Main problem is delayed duplicates

56.  Solutions for delayed duplicates
1. Using throw-away transport addresses
I hi h h i dd i d d In this approach, each time a new transport address is needed,
 When a connection is released, the address is discarded and never used again.
2 Give each connection a connection identifier2. Give each connection a connection identifier
 Each connection is associated with a connection identifier. Whenever a
connection request comes ,transport entity update a table with connection.
 After each connection is released, each transport entity could update a table
listing obsolete connections.
 Wh ti t i it ld b h k d i t th t bl Whenever a connection request comes in, it could be checked against the table,
to see if it belonged to a previously-released connection

57. 3.Setting Packet lifetime
 Packet lifetime can be restricted to a known maximum using one of the
following techniques:following techniques:
Restricted subnet design(Any method that prevents packets from looping)
Putting a hop counter in each packet(hop counter incremented every timePutting a hop counter in each packet(hop counter incremented every time
the packet is forwarded)
Time stamping each packet(Each packet caries the time it was created, with
i di d k ld h i i )routers agreeing to discard any packets older than a given time)

58. Three-way handshake protocol
 Used for connection establishment
 Each packet is responded to in sequence Each packet is responded to in sequence
 Duplicates must be rejected
 Three protocol scenarios for three way hand shake
) l ia) Normal operation
b) Old CONNECTION REQUEST appearing out of nowhere.
c) Duplicate CONNECTION REQUEST and duplicate ACK.) p Q p

59. Three way handshake
(a)Normal operation. (b) Old duplicate CONNECTION REQUEST appearing out of nowhere. (c) Duplicate CONNECTION REQUEST
(a) and duplicate ACK.

60. 3. Connection RELEASE
 There are two styles of terminating a
connection:
 asymmetric release
 symmetric release
Asymmetric releaseAsymmetric release
 only 1 peer closes the connection.is abrupt
and may cause data loss
CR: Connection Request
DR: Disconnect Request
Asymmetric releaseAsymmetric release
Abrupt disconnection with loss of data

61. The two-army problem.
The blue army has 4 troops (2 on either side of valley) while the white army has 3
troops. If both blue armies charge at the same time they can vanquish the whitetroops. If both blue armies charge at the same time they can vanquish the white
army. If only one of the blue armies charges it will succumb (3 white troops against
2 blue troops). This means: the blue armies have to synchronize their attack. But in
order to synchronize they need to send a messenger through the valley; of courseorder to synchronize they need to send a messenger through the valley; of course
the messenger can get caught by the white army (‘lost packet’).

62. Approach #1: The blue army #1 sends a messenger to tell blue army #2 to attack
@ 1400.
 Problem: The blue army #1 does not know if the messenger managed to convey
message or if he was caught. Thus blue army #1 will not attack.
Approach #2: The blue army #2 sends back a messenger to acknowledge to blueApproach #2: The blue army #2 sends back a messenger to acknowledge to blue
army #1 that it got the message.
 Problem: The blue army #2 does not know if acknowledge-messenger reached
blue army #1. Thus blue army #2 will not attack.
 This play can be continued.

63. S i lSymmetric release
 each direction is released independently of the other one.
 Four protocol scenarios for releasing a connection: Four protocol scenarios for releasing a connection:
(a) Normal case of a three-way handshake. (b) final ACK lost

64. (c) Response lost. (d)Response lost and subsequent DRs lost.

65. 4. Flow control and Buffering
 The sender process may send at much higher speed than the receiver process can
handle the data thus causing overflow (= packet loss)handle the data thus causing overflow ( packet loss).
 Transport layer segments the data stream

66.  If most Segments are nearly the same size, it is natural to organize the buffers asg y , g
a pool of identically-sized buffers, with one Segment per buffer
 If there is wide variation in Segment size, a pool of fixed-sized buffers presents
problemsproblems.
(a) Chained fixed-size buffers. (b) Chained variable-sized buffers. (c) One large circular buffer per connection.

67.  If the buffer size is chosen equal to the largest possible Segment, space will be
wasted whenever a short Segment arrives.
If h b ff i i h l h h i S i l i l If the buffer size is chosen less than the maximum Segment size, multiple
buffers will be needed for long Segments, with the attendant complexity.
 Another approach to the buffer size problem is to use variable-sized buffers.ot e app oac to t e bu e s e p ob e s to use va ab e s ed bu e s.
 The advantage here is better memory utilization, at the price of more
complicated buffer management.
 A third possibility is to dedicate a single large circular buffer per connection
 This system is simple and elegant and does not depend on segment sizes, but
makes good use of memory only when the connections are heavily loadedmakes good use of memory only when the connections are heavily loaded.

68. 5. Multiplexing & De-multiplexing
 In the transport layer the need for
multiplexing can arise in amultiplexing can arise in a
number of ways.
 There are two types of
l i l imultiplexing
i. Upward
ii. Downward

69. i. Upward multiplexing
 Traffic from a “data stream” is distributed over several transport connections
(TSAPs)(TSAPs).
 For Eg, if only one network address is available on a host, all transport
connections on that machine have to use it.

70. ii. Downward Multiplexing
 Many “data streams” share the same transport connection using multiple NSAPs,
possibly over multiple network interfaces (load balancing)possibly over multiple network interfaces (load balancing).

71. 6. Crash Recovery
 A crash of one host (server) during the transmission leads to a connection loss
which results in data loss Solution for this the client retransmits onlywhich results in data loss. Solution for this, the client retransmits only
unacknowledged packets.
 Does not work in all cases

72.  A crash at layer N can only be handled at layer N+1 (a system crash is a crash at
every layer).
 Th It i l ft t th li ti l t h dl h f th t h t Thus: It is left to the application layer to handle crashes of the remote host
(client or server).
 Generally applications detect that the remote host has died and then simplyy pp p y
restart the connection and retransmit everything.

73.  Each client can be in one of two states
i. S1: 1 unacknowledged packet outstanding
ii S0:No unacknowledged packet outstanding
 Three events are possible
at the server
i sending an ack (A)ii. S0:No unacknowledged packet outstanding
 The server can be programmed in one of two ways
i. First ACK, then write
i. sending an ack (A),
ii. writing to the output
process (W),,
ii. First write, then ACK
 The client can be programmed in one of four ways
iii. crashing (C)
i. always retransmit the last segment,
ii. never retransmit the last segment,
i l i S0iii. retransmit only in state S0,
iv. retransmit only in state S1.

75. A Simple Transport Protocol
i i i i ll ( li i ) Transport Service Primitives allows transport users (e.g., application programs) to
access transport service.
 five primitives: CONNECT, LISTEN, DISCONNECT, SEND, and RECEIVE. five primitives: CONNECT, LISTEN, DISCONNECT, SEND, and RECEIVE.

76.  The parameters for the service primitives and library procedures are as follows:
 connum = LISTEN(local)
 connum = CONNECT(local, remote)
 status = SEND(connum, buffer, bytes)
 status = RECEIVE(connum buffer bytes) status = RECEIVE(connum, buffer, bytes)
 status = DISCONNECT(connum)
 The LISTEN primitive announces the caller's willingness to accept connectionp g p
requests directed at the indicated TSAP.

77.  The CONNECT primitive takes two parameters, a local TSAP (i.e., transport
address), local, and a remote TSAP, remote, and tries to establish a transport
connection between the two If it succeeds it returns in connum a nonnegativeconnection between the two.If it succeeds, it returns in connum a nonnegative
otherwise a negative number
 The SEND primitive transmits the contents of the buffer as a message on the
indicated transport connection, in several units if needed. Possible errors, returned
in status, are no connection, illegal buffer address.
 The RECEIVE primitive indicates the caller's desire to accept data The size of the The RECEIVE primitive indicates the caller s desire to accept data. The size of the
incoming message is placed in bytes. If the remote process has released the
connection or the buffer address is illegal, status is set to an error code indicating
h f h blthe nature of the problem

78.  The DISCONNECT primitive terminates a transport connection. The parameter
connum tells which one.
P ibl b l t th i t lid Possible errors are connum belongs to another process or connum is not a valid
connection identifier.
 The transport layer makes use of the network service primitives to send andp y p
receive TPDUs.
 The hardware and/or software within the transport layer that does the work is
ll d th t t titcalled the transport entity.

79. Network packet Meaning
Call request Sent to establish a connection
Call accepted Response to Call Request
Clear Request Sent to release connectionClear Request Sent to release connection
Clear confirmation Response to Clear request
Data Used to transport data
Credit Control packet to manage window
Transport entity: packet types

80.  Transport entity: states of a connection
State Meaningg
Idle Connection not established
Waiting CONNECT done; Call Request sent
Queued Call Request arrived; no LISTEN yet
Established connection has been established.
Sending Waiting for permission to send a packet
Receiving RECEIVE has been done
Disconnecting DISCONNECT done locally

81. Graphical Representation