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UNIT I
Introduction
Switching
A switch is a mechanism that allows us to interconnect links to form a larger
network. A switch is a multi-input, multi-output device, which transfers packets
from an input to one or more outputs.
(a) Circuit switching. (b) Packet switching
Circuit Switching
• Seeking out and establishing a physical copper path from end-to-end
• Circuit switching implies the need to first set up a dedicated, end-to-end path for the
connection before the information transfer takes place.
• Once the connection is made the only delay is propagation time.
Store-and-Forward Networks (Packet Switching)
• Intermediate processors (IMPS, nodes, routers, gateways, switches) along the path
store the incoming block of data.
• Each block is received in its entirety, inspected for errors, and retransmitted along
the path to the destination. This implies buffering at the router and one transmission
time per hop.
Packet Switched Networks
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Basic technology the same as in the 1970s
One of the few effective technologies for long distance data communications
Frame relay and ATM are variants of packet-switching
Advantages:
– flexibility, resource sharing, robust, responsive
• Disadvantages:
– Time delays in distributed network, overhead penalties
– Need for routing and congestion control
Advantages over Circuit-Switching
• Greater line efficiency
Many packets can go over shared link
• Data rate conversions
Two stations of different data rates can exchange packets.
• Non-blocking under heavy traffic (but increased delays)
Disadvantages relative to Circuit-Switching
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Packets incur additional delay with every node they pass through
Jitter: variation in packet delay
Data overhead in every packet for routing information, etc
Processing overhead for every packet at every node traversed
Switching Technique
• Large messages broken up into smaller packets
• Datagram
– Each packet sent independently of the others
– No call setup
– More reliable (can route around failed nodes or congestion)
• Virtual circuit
– Fixed route established before any packets sent
– No need for routing decision for each packet at each node
X.25
• X.25 is a packet-switching wide area network developed by ITU-T in 1976.
• X.25 defines how a packet-mode terminal can be connected to a packet network for
the exchange of data.
• X.25 is what is known as subscriber network interface (SNI) protocol.
• It defines how the user’s DTE communicates with the network and how packets are
sent over that network using DCEs.
X.25 Devices
• Data Terminal Equipment (DTE)
– Terminals, personal computers, and network hosts
– Located on premises of subscriber
• Data Circuit-terminating Equipment (DCE)
– Modems and packet switches
– Usually located at carrier facility
• Packet Switching Exchange (PSE)
– Switches that make up the carrier network
PSE
X.25
WAN
PSE
Modem
DCE
Terminal
DTE
Personal Computer
DTE
Modem
DCE
PSE
PSE
Modem
DCE
Server
DTE
• X.25 network is a packet switching network that used X.25 protocol.
• X.25 is a standard packet switching protocol that has been widely used in WAN.
• X.25 is a standard for interface between the host system with the packet switching
network in which it defines how DTE is connected and communicates with packet
switching network.
• It uses a virtual circuit approach to packet switching (SVC and PVC) and uses
asynchronous (statistical) TDM to multiplex packets.
X.25 Layers in Relation to the OSI Layers
X.25 Layers
X.25 protocol specifies three layers:
i.
ii.
iii.
Physical Layer (X.21)
Frame Layer (LAPB) (Link level)
Packet Layer (PLP) (Packet Layer Protocol)
X.25 mapping to OSI Model
Application
Presentation
Other Services
Session
Transport
Network
PLP
Data Link
LAPB
Physical
x.21 bis, EIA/TIA-232, EIA/TIA-449,
EIA-530, G.703
X.25
Protocol
Suite
X.25 – Physical Layers
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Specifies the physical interface between the node (computer, terminal) and the
link that connected to X.25 network.
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Specifies a protocol called X.21 or X.21bis (interface).
Similar enough to other PHY layer protocols, such as EIA-232.
X.21 in PHY layer of X.25
• X.21,sometimes referred to as X21, interface is a specification for differential
communications introduced in the mid 1970’s by the ITU-T. X.21 was first
introduced as a means to provide a digital signaling interface for
telecommunications between carriers and customer’s equipment. This includes
specifications for DTE/DCE physical interface elements, alignment of call control
characters and error checking, elements of the call control phase for circuit
switching services, and test loops.
• When X.21 is used with V.11, it provides synchronous data transmission at rates
from 100 kbit/s to 10 Mbit/s. There is also a variant of X.21 which is only used in
select legacy applications, “circuit switched X.21”. X.21 normally is found on a 15pin D Sub connector and is capable of running full-duplex data transmissions.
• The Signal Element Timing, or clock, is provided by the carrier (your telephone
company), and is responsible for correct clocking of the data. X.21 was primarily
used in Europe and Japan, for example in the Scandinavian DATEX and German
DATEX-L circuit switched networks during the 1980s.
X.21 hardware interface
X.25 Frame Layer
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Provides a reliable data transfer process through data link control which used link
access procedure, balanced (LAPB) protocol.
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There are 3 categories of frame involved in the LAPB frame format:
I-Frames – encapsulate PLP packets from the network layer and before being passed to
the physical layer
S-Frames – flow and error control in the frame layer
U-Frames - used to set up and disconnect the links between a DTE and a DCE.
In the frame layer, communication between a DTE - DCE involves three
phases:
1: Link Setup ; 2: Packet Transfer ; 3: Link Disconnect
Format of a Frame in X.25
Frame Layer and Packet Layer Domains
Three Phases of the Frame Layer
X.25 Packet layer (PLP)
iii.
Packet Layer Protocol (PLP)
- It is the network layer in X.25
- This layer is responsible for establishing the connection, transferring the data,
and terminating the connection between 2 DTEs.
- It also responsible for creating the virtual circuits and negotiating network
services between two DTEs.
- Virtual circuits in X.25 are created at the network layer (not the data link layers
as in some other wide area networks such as Frame Relay and ATM)
Implementation of X.25
• X.25 protocol is a packet-switched virtual circuit network.
• Virtual Circuit in X.25 created at the network layer. unlike Frame Relay and ATM
which both VC created at Data Link Layer.
• Fig 17.7 shows an X.25 network in which 3 virtual circuits have been created
between DTE A and 3 other DTEs.
Frame Relay
Frame Relay Architecture
• X.25 has 3 layers: physical, link, network
• Frame Relay has 2 layers: physical and data link (or LAPF)
• LAPF core: minimal data link control
– Preservation of order for frames
– Small probability of frame loss
• LAPF control: additional data link or network layer end-to-end functions
LAPF Core
LAPF core protocol installed on all subscriber systems and on all frame relay nodes.
LAPF core provides a minimal set of data link control functions
• Frame delimiting, alignment and transparency
• Frame multiplexing/demultiplexing
• Inspection of frame for length constraints
• Detection of transmission errors
• Congestion control
LAPF-core Formats
• The Flag field is a unique pattern that delimits the start and end of the frame. The
FCS field is used for error detection. On transmission, the FCS checksum is
calculated and stored in the FCS field. On reception, the checksum is again
calculated and compared to the value strode in the incoming FCS field. I there is a
mismatch, then the frame is assumed to be in error and is discarded.
• The Address field has a default length of 2 octets and may be extended to 3 or 4
octets. It carries a data link connection identifier (DLCI) of 10,17, or 24 bits. The
length of the Address field, and hence of the of the DLCI, is determined by the
address field extension (EA) bits. The C/R bit is application specific and not used by
the standard frame relay protocol. The remaining bits in the address field have to do
with congestion control.
User Data Transfer
• No control field, which is normally used for:
– Identify frame type (data or control)
– Sequence numbers
• Implication:
– Connection setup/teardown carried on separate channel
– Cannot do flow and error control
Data transfer involves the following stages
1. Establish a logical connection between two end points, and assign a unique
DLCI to the connection
2. Exchange information in data frames. Each frame includes a DLCI field to
identify the connection
3. Release the logical connection
Frame Relay Call Control
4 message types needed
• SETUP
• CONNECT
• RELEASE
• RELEASE COMPLETE
A frame with DLCI = 0 contain a call control message in the information field.
Logical Connection established by sending a SETUP message. Upon
receiving the SETUP message, must reply with a CONNECT message if it accepts
the connection; otherwise it responds with a RELEASE COMPLETE message. The
side sending the SETUP message may assign the DLCI by choosing an unused
value and including this value in the SETUP message.
Either side may request to clear a logical connection by sending a RELEASE
message. The other side, upon receipt of this message, must respond with a
RELEASE COMPLETE message.
ATM
• Multi-speed network environment that provides a variety of complex network
services
• Can carry voice, data, video separately or simultaneously
• Can be used in LANs, MANs, or WANs
• Fixed-lenth packets (cells)
• Allows multiple logical connections to be multiplexed
• Minimal error and flow control capabilities
• Connection-oriented virtual channel
Cell Switched ATM
• Similar to frame relay
• Difference?
– Frame relay switches variable length frames within frame relay cloud from
source to destination
– ATM switches fixed-length cells (48 byte information field, 5 byte header)
• Based on packet switching (connection-oriented)
– Cell sequence integrity preserved via virtual channel
– VCC – virtual channel connection – is set up between end users, variable rate,
full duplex
– VCC also used for control
• Information field is carried transparently through the network, with minimal error
control
Protocol Architecture
• User plane : Provides for user information, along with associated controls (e.g. flow
control, error control)
• Control plane : Performs call control and connection control functions
• Management plane : Includes plane management, which performs management
functions related to a system as a whole and provides coordination between all the
planes, and layer management, which performs management functions relating to
resources and parameters residing in its protocol entities
ATM Physical Layer
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Transports cells via a communications channel (either optical or electrical)
LAN support: 25-155 Mbps copper or fiber
WAN support: SONET rates over fiber
Physical Medium Sublayer: bit transfer, bit alignment, and copper/fiber conversions
Transmission Convergence Sublayer: bit/cell conversion at sending and receiving
nodes
ATM Layer
• Handles functions of the network layer:
• Connection-oriented without acknowledgements
• Two possible interfaces:
– UNI – User-Network Interface: Boundary between an ATM network and host
– NNI – Network-Network Interface: Between two ATM switches
UNI/NNI Interface
ATM Adaptation Layer (AAL)
• Maps higher-layer information into ATM cells to be transported over an ATM
network
• Collects information from ATM cells for delivery to higher layers
Virtual Connections
• Virtual Channel Connection (VCC) – Full duplex virtual circuit with logical
connection between source and destination – can be PVC or SVC
• Virtual Path Connection (VPC) – Semi-permanent (or customer controlled or
network controlled) connection that provides a logical collection of virtual channels
that have the same endpoint
• A single virtual path supports multiple virtual channels (analogy – highway = VPC,
lane = VCC)
VCI vs VPI
• VPI – Virtual Path Identifier – identified in cell’s header. Cannot establish a virtual
channel before virtual path
• VCI – Virtual Channel Identifier – only have local significance – different virtual
paths reuse VCIs (but VCIs on same path must be unique)
What is so special about a virtual path?
• ATM is connection-oriented, so circuit must be established before transmission
– As route established, VPIs and VCIs are assigned
• VPI and VCI info suffices for addressing info
• Simplified network architecture (based on VC or VP)
• Increased network performance and reliability (fewer, aggregated entities because of
simplified network architecture)
• Reduces processing and short connection setup time
• User may define closed user group or closed networks of virtual channel bundles
ATM Connection Relationships
Definition of Terms
• COS – Class of Service – sets a priority of data delivery, based upon the class.
Higher priority data get delivered before lower priority data (example – which
should have higher priority – streaming video or email?)
• QOS – Quality of Service – involves establishing certain parameters for a specific
transmission – e.g. amount of bandwidth required for a given priority data
transmission, max. amount of latency tolerated, etc
• Both are required to deliver real-time voice and video traffic
Call Establishment Using VPs
VP/VC Characteristics
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Quality of service based on VCC
Switched and semi-permanent channel connections
Call sequence integrity – packets arrive in order
Traffic parameter negotiation and usage monitoring
• VPC only
– Virtual channel identifier restriction within VPC – some VCCs reserved for
network management
ATM Cells
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Fixed size – 53 bytes
5 octet header
48 octet information field
Small cells reduce queuing delay for high priority cells
Small cells can be switched more efficiently
Easier to implement switching of small cells in hardware
ATM Cell Format
Header Format
• Generic flow control
– Only at user to network interface
– Controls flow only at this point
• Virtual path identifier
• Virtual channel identifier
• Payload type
– e.g. user info or network management
• Cell loss priority
• Header error control
Generic Flow Control (GFC)
• Control traffic flow at user to network interface (UNI) to alleviate short term
overload
• Two sets of procedures
– Uncontrolled transmission
– Controlled transmission
• Every connection either subject to flow control or not
• Flow control is from subscriber to network
– Controlled by network side
ATM Service Categories
• ATM is designed to transfer many different types of traffic simultaneously,
including real-time voice, video, and bursty TCP traffic
• Way in which data flow is handled depends on the characteristics of the traffic flow
and requirements of the application (ex. Real-time video must be delivered within
minimum variation in delay)
• Primary service categories – real time service, non-real time service
ATM Service Categories
• Real time
– Constant bit rate (CBR)
– Real time variable bit rate (rt-VBR)
• Non-real time
– Non-real time variable bit rate (nrt-VBR)
– Available bit rate (ABR)
– Unspecified bit rate (UBR)
– Guaranteed frame rate (GFR)
Real Time Services
• If want to avoid or decrease variation of delay (jitter), use CBR or rt-VBR
• CBR Fixed data rate continuously available
• Commonly used for uncompressed audio and video
– Video conferencing
– Interactive audio
– A/V distribution and retrieval
• rt-VBR Best for time sensitive applications
– Tightly constrained delay and delay variation
• rt-VBR applications transmit at a rate that varies with time
– e.g. compressed video
– Produces varying sized image frames
– Original (uncompressed) frame rate constant (isochronous)
– So compressed data rate varies
• Can statistically multiplex connections
Non-Real Time
• Intended for applications with bursty traffic and limited constraints on delay and
delay variation
• Greater flexibility, greater use of statistical multiplexing
nrt-VBR
• May be able to characterize expected traffic flow
• Improve QoS in loss and delay
• End system specifies:
– Peak cell rate
– Sustainable or average rate
– Measure of how bursty traffic is
• e.g. Airline reservations, banking transactions
UBR
• Unused capacity of CBR and VBR traffic made available to UBR
• For application that can tolerate some cell loss or variable delays
– e.g. TCP based traffic
• Cells forwarded on FIFO basis
• Best efforts service
ABR
• Application specifies peak cell rate (PCR) it will use and minimum cell rate (MCR)
it requires
• Resources allocated to give at least MCR
• Spare capacity shared among ABR and UBR sources
• e.g. LAN interconnection
Guaranteed Frame Rate (GFR)
• Designed to support IP backbone subnetworks
• Purpose: optimize handling of frame based traffic passing from LAN through router
to ATM backbone
– Used by enterprise, carrier and ISP networks
– Consolidation and extension of IP over WAN
• UNI establishes hand shaking between NIC and switch
ATM versus Frame Relay
• Frame relay uses variable length frames
• ATM fixed length cells
• ATM has higher overhead, but faster speed and traffic management (better suited for
video and voice)
Why is ATM so Efficient?
• Minimal error and flow control
– Reduces overhead of processing ATM cells
– Reduces number of required overhead bits
• Fixed size simplified processing at each ATM node (can be switched more
efficiently – more efficient use of router)
• Small cells reduce queuing delay
• Minimal addressing info on each cell
• Efficient traffic management
• Parallelism could be incorporated into the switching system if all packets were of
the same length; multiple switching elements could be working in parallel
performing the same operation on different packets.