4. • Let Multiplexer with the two lines sending cells instead of frames.
• Frame X has been segmented into three cells: X, Y, and Z. Only the first cell from line 1 gets
put on the link before the first cell from line 2. The cells from the two lines are interleaved so
that none suffers a long delay.
Multiplexing using cells
A cell network uses the cell as the basic unit of data exchange.
A cell is defined as a small, fixed-size block of information.
Note
5. • ATM uses asynchronous time-division multiplexing (TDM) that is why it is called Asynchronous Transfer Mode- to
multiplex cells corning from different channels.
• It uses fixed-size slots (size of a cell).
• The Fig. shows how cells from three inputs are multiplexed. At the first tick of the clock: channel 2 has no cell
(empty input slot), so the multiplexer fills the slot with a cell from the third channel. When all the cells from all
the channels are multiplexed, the output slots are empty.
ATM multiplexing (Asynchronous TDM)
6. • ATM is a cell-switched network.
• The user access devices, called the endpoints, are connected through a user-to-network
interface (UNI) to the switches inside the network.
• The switches are connected through network-to-network interfaces (NNIs).
Architecture of an ATM
network
7. • Connection between two endpoints is accomplished through transmission paths (TPs), virtual
paths (VPs), and virtual circuits (VCs).
• A transmission path (TP): is the physical connection (wire, cable, satellite, and so on) between
an endpoint and a switch or between two switches.
• A transmission path (TP) : is divided into several virtual paths.
• A virtual path (VP) provides a connection or a set of connections between two switches.
• Cell networks are based on virtual circuits (VCs).
Virtual Connection (TP, VPs, and
VCs)
8. • The designers of ATM created a hierarchical identifier with two levels:
a virtual path identifier (VPI) and a virtual-circuit identifier (VCI).
• The VPI defines the specific VP, and the VCI defines a particular VC inside the VP.
• . Most of the switches in a typical ATM network are routed using VPIs.
• The switches at the boundaries of the network, those that interact directly with the endpoint
devices, use both VPIs and VCIs
Example of VPs and
VCs
9. Note that a virtual connection is defined by a pair
of numbers:
the VPI and the VCI.
Not
e
10. • The VPIs and VCls for a transmission path. The rationale for dividing an identifier into two parts
will become clear when we discuss routing in an ATM network.
Connection identifiers
11. • The lengths of the VPIs for UNIs and NNIs are different.
• In a UNI, the VPI is 8 bits, whereas in an NNI, the VPI is 12 bits. The length of the VCI is the
same in both interfaces (16 bits).
• A virtual connection is identified by 24 bits in a UNI and by 28 bits in an NNI
Virtual connection identifiers in UNIs and
NNIs
12. •The basic data unit in an ATM network is called a cell.
•A cell is only 53 bytes long with 5 bytes allocated to the header and 48 bytes
carrying the payload (user data may be less than 48 bytes).
An ATM
cell
13. • ATM uses switches to route the cell from a source endpoint to the destination endpoint.
• A switch routes the cell using both the VPls and the VCls. The routing requires the whole
identifier.
• Figure shows how a VPC switch routes the cell. A cell with a VPI of 153 and VCI of 67 arrives at switch
interface (port) 1.
• the combination corresponds to output interface 3, VPI 140, and VCI 92. It changes the VPI and VCI in the
header to 140 and 92, respectively, and sends the cell out through interface 3.
Routing with a switch
14. • Virtual-circuit switching is a data link layer technology in which links are shared.
• A virtual-circuit identifier (VCI) identifies a frame between two switches.
• Frame Relay is a relatively high-speed, cost-effective technology that can handle bursty data.
• Both PVC and SVC connections are used in Frame Relay.
• The data link connection identifier (DLCI) identifies a virtual circuit in Frame Relay.
• Asynchronous Transfer Mode (ATM) is a cell relay protocol that, in combination with SONET, allows high-speed connections.
• A cell is a small, fixed-size block of information.
• The ATM data packet is a cell composed of 53 bytes (5 bytes of header and 48 bytes
• of payload).
• ATM eliminates the varying delay times associated with different-size packets.
• ATM can handle real-time transmission.
• A user-to-network interface (UNI) is the interface between a user and an ATM switch.
• A network-to-network interface (NNI) is the interface between two ATM switches.
• In ATM, connection between two endpoints is accomplished Cl through transmission paths (TPs), virtual paths (VPs), and
virtual circuits (VCs).
• In ATM, a combination of a virtual path identifier (VPI) and a virtual-circuit identifier identifies a virtual connection.
SUMMARY
15. • The ATM standard defines three layers:
• a. Application adaptation layer (AAL) accepts transmissions from upper-layer services and maps them into ATM cells.
• b. ATM layer provides routing, traffic management, switching, and multiplexing ser VIces.
• c. Physical layer defines the transmission medium, bit transmission, encoding, and electrical-to-optical transformation.
• The AAL is divided into two sublayers: convergence sublayer (CS) and segmentation and reassembly (SAR).
• o There are four different AALs, each for a specific data type:
• a. AALI for constant-bit-rate stream. b. AAL2 for short packets.
• c. AAL3/4 for conventional packet switching (virtual-circuit approach or datagram approach).
• d. AAL5 for packets requiring no sequencing and no error control mechanism.
• ATM technology can be adopted for use in a LAN (ATM LAN).
• In a pure ATM LAN, an ATM switch connects stations.
• In a legacy ATM LAN, the backbone that connects traditional LANs uses ATM technology.
• A mixed architecture ATM LAN combines features of a pure ATM LAN and a legacy ATM LAN.
• Local-area network emulation (LANE) is a client/server model that allows the use of ATM technology in LANs.
• LANE software includes LAN emulation client (LECS), LAN emulation configuration server (LECS), LAN emulation server (LES),
and broadcast/unknown server (BUS) modules.
16. • 1. There are no sequence numbers in Frame Relay. Why?
• Frame Relay does not use flow or error control, which means it does not use the sliding window protocol. Therefore, there is no need for
sequence numbers.
• 2. Compare an SVC with a PVc.?
• In a PVC, two end systems are connected permanently through a virtual connection.In a SVC, a virtual circuit needs to be established
each time an end system wants to be connected with another end system.
• 3. Why is Frame Relay a better solution for connecting LANs than T-llines?
• T-lines provide point-to-point connections, not many-to-many. In order to connect several LANs together using T-lines, we need a mesh
with many lines. Using Frame Relay we need only one line for each LAN to get connected to the Frame Relay network.
• 4. Discuss the Frame Relay physical layer?
• Frame Relay does not define a specific protocol for the physical layer. Any protocol recognized by ANSI is acceptable
• 5. How does an NNI differ from a UNI?
• A UNI (user network interface) connects a user access device to a switch inside the ATM network, while an NNI (network to network
interface) connects two switches or two ATM networks.
• 6. What is the relationship between TPs, VPs, and VCs?
• A TP (transmission path) is the physical connection between a user and a switch or between two switches. It is divided into several VPs
(virtual paths), which provide a connection or a set of connections between two switches. VPs in turn consist of several VCs (virtual
circuits) that logically connect two points together.
QUIZ
17. 7. How is an ATM virtual connection identified?
• An ATM virtual connection is defined by two numbers: a virtual path identifier (VPI) and a virtual circuit identifier (VCI).
VPI+VCI is 28 bits. This means that we can define virtual circuits in an NNI.
8. Briefly describe the issues involved in using ATM technology in LANs.
• We can briefly summarize the most important issues: a. Traditional LANs are connectionless protocols; ATM is a connection-oriented
protocol.
• b. Traditional LANs define the route of a packet through source and destination addresses; ATM defines the route of a cell through virtual
connection identifiers. c. Traditional LANs can do unicast, multicast, and broadcast transmission; ATM is designed only for unicast
transmission.
9. Name the ATM layers and their functions.
a. Application adaptation layer (AAL) accepts transmissions from upper-layer services and maps them into ATM cells.
b. ATM layer provides routing, traffic management, switching, and multiplexing serVIces. c. Physical layer defines the transmission medium, bit
transmission, encoding, and electrical-to-optical transformation
10. An AALllayer receives data at 2 Mbps. How many cells are created per second by the ATM layer?
• In AAL1, each cell carries only 47 bytes of user data. This means the number of cells sent per second can be calculated as
[(2,000,000/8)/47] ≈ 5319.15.
18. • In Figure a virtual connection is established between A and B. Show the DLCI for each link.
19. In Figure a virtual connection is established between A and B. Show the corresponding entries in the tables
of each
switch.
20. MCQ
• 1. To handle frames arriving from other protocols, Frame Relay uses a device called a --------
A) MUX B) VOFR C) FRAD D) none of the
above Answer: Option C
• 2. In Frame Relay, an address can be bytes.
A)2 to 3 B) 2 to 4 C) only 2 D) none of the
above Answer: Option B
• 3. In ATM, the layer defines the transmission medium, bit
transmission, encoding, and electrical-to-optical
transformation.
A)AAL B) physical C) ATM layer D) none of the
above Answer: Option B
• 4. A(n) is the interface between a user and an ATM switch.
A) NNN B) UNIC) NNID) None of the above
Answer: Option B
• 5. In ATM, a virtual connection is defined by .
A) DLCI B) VPI C) VCI D) a combination of (b) and (c)
21. • 6. In ATM, is for short
packets.
A) AAL5 B) AAL3/4
C) AAL2
Answer: Option C
D) AAL1
• 7. In ATM, the layer accepts transmissions from upper-layer services and maps them into ATM
cells.
A) AAL B) physical C) ATM D) none of the
above Answer: Option A
• 8. The VPI of a UNI is bits in length.
A) 32 B) 16 C) 12
D) 8
Answer: Option D
• 9. The ATM standard defines
layers.
A) five B) four C) three
D) two
Answer: Option C
• 10. technology can be adapted for use in a LAN (ATM
LAN).
A) ATM B) X.25 C) Frame Relay D) none of the
above Answer: Option A
22. • 11. VCIs in Frame Relay are called .
A) SVC B) DLCIs C) PVC D) none of the
above Answer: Option B
• 12. The AAL is divided into sublayers.
A)four B) three C) two D) none of the
above Answer: Option C
• 13. is the interface between two ATM
switches.
A) NNI B) NNN C) UNI D) none of the
above Answer: Option A
• 14. A is defined as a small, fixed-size block of
information.
A) packet B) cell C) frame D) none of the
above Answer: Option B
• 15. In ATM, the layer provides routing, traffic management,
switching, and multiplexing services.
23. • 16. The VPI of an NNI is bits in length.
A) 24 B) 16 C) 12 D) 8
Answer: Option C
• 17. The ATM data packet is a cell composed of
bytes.
A) 53 B) 52 C) 43 D) 42
Answer: Option A
![7. How is an ATM virtual connection identified?
• An ATM virtual connection is defined by two numbers: a virtual path identifier (VPI) and a virtual circuit identifier (VCI).
VPI+VCI is 28 bits. This means that we can define virtual circuits in an NNI.
8. Briefly describe the issues involved in using ATM technology in LANs.
• We can briefly summarize the most important issues: a. Traditional LANs are connectionless protocols; ATM is a connection-oriented
protocol.
• b. Traditional LANs define the route of a packet through source and destination addresses; ATM defines the route of a cell through virtual
connection identifiers. c. Traditional LANs can do unicast, multicast, and broadcast transmission; ATM is designed only for unicast
transmission.
9. Name the ATM layers and their functions.
a. Application adaptation layer (AAL) accepts transmissions from upper-layer services and maps them into ATM cells.
b. ATM layer provides routing, traffic management, switching, and multiplexing serVIces. c. Physical layer defines the transmission medium, bit
transmission, encoding, and electrical-to-optical transformation
10. An AALllayer receives data at 2 Mbps. How many cells are created per second by the ATM layer?
• In AAL1, each cell carries only 47 bytes of user data. This means the number of cells sent per second can be calculated as
[(2,000,000/8)/47] ≈ 5319.15.](https://image.slidesharecdn.com/lecture11atm-250705180600-7a83f5ec/85/lecture-11-lecture-lecture-lecture-ATM-pptx-17-320.jpg)
