Frame Relay and ATM Network Protocols Explained

MODULE 5 MCA-402 Computer Networks ADMN 2012-‘15
Dept. of Computer Science And Applications, SJCET, Palai Page 1
FRAME RELAY
 Is a packet switched WAN protocol that operates at the physical and data link layers of the OSI
reference model.
 As fiber 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 transmission.
 It puts data in variable-size units called "frames" and provide minimal internal checking
 support data transfer rates at
 T-1 (1.544 Mb/s)
 T-3 (45 Mb/s) speeds.
 Enabling end stations to dynamically share the network medium and the available bandwidth.
 Devices attached to a Frame Relay WAN fall into the following two general categories:
1. Data terminal equipment (DTE)
 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.
2. Data circuit-terminating equipment (DCE)
 Carrier-owned internetworking devices.
 The purpose is to provide clocking and switching services in a network, which are
the devices that actually transmit data through the WAN.
Fig 5. 1 frame relay devices
Architecture
 Frame Relay has 2 layers: physical and data link (LAPF, Link Access Procedure for Frame Mode
Bearer Services)

Fig 5.2 frame relay protocol architecture
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 available.
 Frame Relay supports any of the protocols recognized by ANSI
Data Link Layer
 Link layer uses the services of the physical layer. It, in turn, provides the following services :
 Flag recognition.
 Frame check sequence (FCS) generation and checking.
 Recognition of invalid frames.
 Discard incorrect frames.
 Routing.
 Congestion control notification
Frame Relay Virtual Circuit
 Virtual circuits provide a bidirectional communication path from one DTE device to 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 a DTE in order to
access the remote DTE.
 Frame Relay virtual circuits fall into two categories:
 switched virtual circuits (SVCs)
 Permanent virtual circuits (PVCs).
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 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 Frame Relay network.
 Always operate in one of the following two operational states(Idle and Data Transfer)

Protocol Architecture
Fig 5.3 frame relay protocol architecture
LAPF Core
LAPF frame
Fig 5.4 LAPF frame
 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 field: 10-bit DLCI field represents the address of the frame and corresponds to a PVC.
 Command/response (C/R): Designates whether the frame is a command or response.
 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 congestion has occurred.
 Backward explicit congestion notification (BECN):is set (in frames that travel in 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 keep the network from collapsing
due to overload.
 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
 Congestion control functions
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 from core
protocol
CONGESTION-CONTROL MECHANISMS
 Frame Relay reduces network overhead by implementing simple congestion-notification
mechanisms. Frame Relay implements two congestion-notification mechanisms:
 Forward-explicit congestion notification (FECN)
 Backward-explicit congestion notification (BECN)
 FECN and BECN each is controlled by a single bit contained in the Frame Relay frame 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.

Fig 5.5 FECN
Fig 5.6 BECN
Four Cases of Congestion
Fig 5.7 four cases of congestion

ASYNCHRONOUS TRANSFER MODE (ATM)
 ATM is a concept similar to frame relay which take advantages of modern digital facilities to
provide faster packet switching
 is a connection-oriented, high-speed, low-delay switching and transmission technology
 Allows multiple logical connections to be multiplexed over a single physical 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
Design Goals
1. Use of high data rate transmission media (i.e fiber optic)
2. Interoperability with existing technologies
3. Implementation at reasonable cost
4. Support for existing telecommunications hierarchies
5. Reliable and predictable
6. Suitable for real-time and non-real-time services
Cell Networks
 A cell network uses the cell as the basic unit of data exchange
 ATM carries information on cells
 The length of each cell is 53 Bytes
 First 5 bytes are used as the cell header
 Next 48 bytes are used as the payload carrying the data
Fixed Length Cell Advantage
 Delay or latency is significantly reduced
 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
Multiplexing with cells
 The cells from the two lines are interleaved so that none suffers a long delay.
 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 the parties being
aware of the segmentation

Fig 5.8 multiplexing
Asynchronous Time-Division Multiplexing
 ATM uses asynchronous time-division multiplexing to multiplex cells coming from 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; the slot is empty if
none of the channels has a cell to send
Fig 5.9 asynchronous multiplexing
Architecture
ATM Devices
 ATM networks are built around two categories of devices
 ATM Switch
 ATM end-point
 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
 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-to-network
interfaces (NNIs).
Fig 5.10 ATM interfaces
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.
2. Virtual Paths (VPs): provides a connection or a set of connections between two switches.
3. Virtual Circuits (VCs):SVC or PVC
 A virtual connection is defined by a pair of numbers: VPI and VCI
 ATM assigns each Virtual Connection a 24-bit identifier
1. 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 long.
 Cell networks are based on Virtual Connection and all cells belonging to a single message follow
the same virtual circuit
Fig 5.11 ATM Virtual Circuit

Fig 5. 12 example of virtual path and Virtual Circuit
ATM Protocol Layers
Fig 5.13 ATM protocol layers
Physical Layer
 It describes the physical transmission media.
 We can use shielded and unshielded twisted pair, coaxial cable, and fiber-optic cable.
ATM Layer
 The ATM layer is responsible for establishing connections and passing cells through the ATM
network.
 It provides routing, traffic management, switching, and multiplexing.
ATM cell
Fig 5.14 ATM cell

Fig 5.15 a ATM Cell Header—UNI Format Fig 5.15 b ATM Cell Header—NNI Format
 General Flaw Control (GFC): Provides local functions, such as flow control from end point
equipment to the ATM switch.
 Payload Type (PT): Indicates whether the cell contains user data or control data.
 Cell Loss Priority (CLP): Indicates whether the cell should be removed if it encounters errors as it
moves through the network.
 Header Error Control (HEC): Contains Cyclic Redundancy Check (CRC) on the cell header.
 Virtual Path Identifier (VPI): Identifies semi-permanent connections between ATM end points.
 Virtual Channel Identifier (VCI): Have only local significance on the link between ATM nodes.
ATM Adaptation Layer (AAL)
 It converts the submitted information into streams of 48-octet segments and transports 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.
 AAL exists only in end systems, not in switches.
 AAL Services
 Handle transmission errors
 Segmentation/reassembly (SAR)
 Handle lost and misinserted cell conditions
 Flow control and timing control
 AAL is classified into four(The classification was made with respect to the ,following parameters:
 Timing relationship between sender and receiver
 Related

 Not related
 Bit rate
 Constant bit rate
 Variable bit rate
 Connection mode
 Connection-oriented
 Connectionless
 AAL is divided into two sub layers:
 Convergence Sub layer: manages the flow of data to and from SAR sub layer.
 Segmentation and reassembly sub layer:Packages data from CS into cells and
unpacks at other end
Fig 5.16 AAL classes
AAL 1 (Constant Bit Rate)
 The CS layer divides the bit stream into 47-byte segments and passes them to the 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
 Sequence number (SN)
 Sequence number protection (SNP)

Fig 5.17 AAL1 Operation
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.
 It widely used in wireless applications
Fig 5.18 AAL2 operation
AAL 3/4
 AAL3 was intended to support connection-oriented data services and AAL4 to support
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 beginning 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
Fig 5.19 AAL3/4 operation
AAL 5
 Is the primary AAL for data and supports both connection-oriented and connectionless 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 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 ATM layer places each block into the Payload field of an ATM cell
Fig 5.20 AAL 5

TRANSPORT LAYER
Introduction
 The transport layer is concerned with the provision of host-to-host user connections 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.
 At the transport layer, we need a transport layer address, called a port number, to choose among
multiple processes running on the destination host.
Transport Services
 Provide logical communication between application processes running on different hosts.
 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 protocols 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
Elements of Transport Protocols
1. Addressing
2. Connection Establishment
3. Connection Release
4. Flow Control and Buffering
5. Multiplexing
6. Crash Recovery
1. Addressing
 When an application process wishes to set up a connection to a remote application process, it must
specify which one to connect to
 The method normally used is to define transport addresses is by using connection requests
 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 NSAPs (Network
Service Access Points).

Fig 5.21 TSAPs, NSAPs, and transport connections
Fig 5.22 IP addresses versus port numbers
2. Connection Establishment
 Just send REQUEST, wait for ACCEPTED.
 The problem occurs when the network can lose and duplicate packets.
 Main problem is delayed duplicates

Fig 5.23 connection establishment strategy
 Solutions for delayed duplicates
1. Using throw-away transport addresses
 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 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.
 Whenever a connection request comes in, it could be checked against the table, to see if it
belonged to a previously-released connection.
3. Setting Packet lifetime
 Packet lifetime can be restricted to a known maximum using one of the following techniques:
 Restricted subnet design(Any method that prevents packets from looping)
 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 routers
agreeing to discard any packets older than a given time
Three-way handshake protocol
 Used for connection establishment
 Each packet is responded to in sequence
 Duplicates must be rejected
 Three protocol scenarios for three way hand shake
a) Normal operation
b) Old CONNECTION REQUEST appearing out of nowhere.

c) Duplicate CONNECTION REQUEST and duplicate ACK.
Fig 5.24 three way hand shake operation (a) Normal operation. (b) Old duplicate CONNECTION
REQUEST appearing out of nowhere. (c) Duplicate CONNECTION REQUEST
and duplicate ACK.
3. Connection RELEASE
 There are two styles of terminating a connection:
 asymmetric release
 symmetric release
Asymmetric release
 only 1 peer closes the connection.is abrupt and may cause data loss
CR: Connection Request
DR: Disconnect Request
Fig 5.25 Connection release Asymmetric release
The two-army problem

Fig 5. 26 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 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 course the messenger can get caught by the white army (‘lost packet’).
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 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.
Symmetric release
 Each direction is released independently of the other one.
 Four protocol scenarios for releasing a connection:

Fig 5.27 four scenarios for symmetric release (a) Normal case of a three-way handshake.
(b) Final ACK lost (c) Response lost. (d)Response lost and subsequent DRs lost.
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).
 Transport layer segments the data stream
Fig 5.28 flow control and buffering strategy

 If most Segments are nearly the same size, it is natural to organize the buffers as 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 problems.
Fig 5.29 types of buffering (a) Chained fixed-size buffers. (b) Chained variable-sized buffers. (c)
One large circular buffer per connection.
 If the buffer size is chosen equal to the largest possible Segment, space will be wasted whenever a
short Segment arrives.
 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.
 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 loaded.
4. Multiplexing & De-multiplexing
 In the transport layer the need for multiplexing can arise in a number of ways.
 There are two types of multiplexing
i. Upward
ii. Downward

Fig 5.30 multiplexing and multiplexing
i. Upward multiplexing
 Traffic from a “data stream” is distributed over several transport connections (TSAPs).
 For Eg, if only one network address is available on a host, all transport connections on that
machine have to use it.
Fig 5.31 upward multiplexing
ii. Downward Multiplexing
 Many “data streams” share the same transport connection using multiple NSAPs, possibly over
multiple network interfaces (load balancing).
Fig 5.32 downward multiplexing

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 only unacknowledged packets.
 Does not work in all cases
Fig 5.33 normal crash recovery mechanism
 A crash at layer N can only be handled at layer N+1 (a system crash is a crash at every layer).
 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 simply restart the connection
and retransmit everything.
 Each client can be in one of two states
i. S1: 1 unacknowledged packet outstanding
ii. S0:No unacknowledged packet outstanding
 The server can be programmed in one of two ways
i. First ACK, then write
ii. First write, then ACK
 The client can be programmed in one of four ways
i. always retransmit the last segment,
ii. never retransmit the last segment,
iii. retransmit only in state S0,
iv. Retransmit only in state S1.

 Three events are possible at the server
i. sending an ack (A),
ii. writing to the output process (W),
iii. crashing (C)
Fig 5.34 crash recovery states
A SIMPLE TRANSPORT PROTOCOL
 Transport Service Primitives allows transport users (e.g., application programs) to access transport
service. Five primitives: CONNECT, LISTEN, DISCONNECT, SEND and RECEIVE.
Fig 5.35 connection primitives

 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 = DISCONNECT(connum)
 The LISTEN primitive announces the caller's willingness to accept connection requests directed at
the indicated TSAP.
 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 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 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 the nature of the problem
 The DISCONNECT primitive terminates a transport connection. The parameter connum tells
which one.
 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 and receive TPDUs.
 The hardware and/or software within the transport layer that does the work is called the transport
entity.
Fig 5.36 network packets in transport layer

 Transport entity: states of a connection
Fig 5.37 states of transport entity

Frame Relay and ATM Network Protocols Explained

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