Frame Relay

Frame Relay 

Petr Grygárek 

rek 

© 2005 Petr Grygarek, Advanced Computer Networks Technologies 1

Basic characteristics 

• Used to implement WAN links over shared infrastructure 

• infrastructure provided by independent operator 

• private Frame-Relay infrastructure 

• Layer 2 technology 

• Assumes fast and reliable links between switching elements in FR cloud 

• No hop-by-hop error-control and flow-control 

• upper layers of user devices have to solve it by themselves 

• Fast and efficient switching 

• Independent of layer 3 protocol(s) used 

• Virtual-circuit based 

• implemented using fast frame (L2) switching (“relaying”) 

• Fast VC creation/deletion (compared to leased line) 

• QoS-enabled 

© 2005 Petr Grygarek, Advanced Computer Networks Technologies 

2

Infrastructure and DTEs 

Most-common DTEs 

• Router 

• Frame Relay Access Device (FRAD) 


3

Virtual circuits 

• Set-up over FR infrastructure (cloud) 

• Simulate “real” circuits with predefined parameters 

• Permanent Virtual Circuit (PVC) 

• requires (software based) preconfiguration by infrastructure provider 

• used most often 

• Switched Virtual Circuit (SVC) 

• created on user-device (DTE) demand 

• need for signalling standard and unique DTE addresses 

• Q.933 – similar to ISDN Q.931 


4

VC Principle 

FR Switch 

I 1 

I 2 

I 1 

DLCI in 

I 2 

I in 

FR Switch 

110 

DLCI out I out 

---------------------------------------- 

110 I 2 

17 I 3 

17 I 3 

110 I 2 

I 3 17 

DTE B 

I 1 

I 2 

I 1 

FR Switch 

56 121 

DTE A 

I 3 

DLCI in 


I in 


---------------------------------------- 

56 I 3 

121 I 2 

121 I 2 

56 I 3 

I 2 

FR Switch 

DLCI in 

I in 


---------------------------------------- 

121 I 1 

110 I 2 

110 I 2 

121 I 1 

5

Data Link Connection Identifier 

(DLCI) 

• Identifies individual virtual circuits on local loop 

• and on inter-FR-switch links, if network cloud implemented 

using FR switches 

• 10-bit value 

• Locally unique 

• can be repeated on different links 

• Assigned by FR cloud operator 

• User DLCI: 16 to 1007 

• Frame Relay switches change DLCI along PVC path 


6

Virtual circuit configuration 

(FR switch-based infrastructure) 

• In each FR switch along PVC path, switching 

table entry must be entered 

• 

• Both PVC direction must be configured 

• the same DLCIs used on each link in both directions 

commonly 


7

DLCI local significance 


8

Virtual circuit advantages 

• Requires only one physical access line regardless 

to number of virtual circuits established 

• doesn’t require multiple leased lines to customer site 

• per PVC payment, but smaller fees compared to 

separate leased lines 

• Flexible – new circuits can be quickly added and 

unnecessary ones deleted on as-needed basis 


9

Switched virtual circuits (SVC) 

• Q.933 signalling between user and FR switch 

• subset of Q.931 

• Sent over DLCI 0 (together with LMI messages) 

• Q.922 provides reliable link for Q.933 messages transfer 

• QoS parameters appointed during call setup 

• Connection establishment: 

• SETUP, PROGRESS, ALERTING, CALL PROCEEDING, 

CONNECT, CONNECT ACK 

• Connection Termination: 

• DISCONNECT, RELEASE, RELEASE COMPLETE 


10

Local access loop 

•Subscriber connected by single leased line (or ISDN) 

•Typical access line rates 56kbps to (approx) 54 Mbps 


11

Frame Relay standard 

Frame-relay defines es local access loop, not 

implementation of network cloud 

(infrastructure) 

• may be built upon frame-relay switches, ATM, 

connectionless IP or other technology 

• invisible to Frame Relay user 

• Infrastructure must provide committed 

parameters to provisioned VCs 

• CAR, Bc, Be, … 


12

Frame Relay Standardization Bodies 

1. Gang of Four: 

1. 

• DEC, Cisco, Northern Telecom, StrataCom … 

2. www.frforum.com 

2. 

• Cooperates with ITU-T 

• FRF.x standards 


13

Frame Relay Interfaces 

• User-to-Network Interface (UNI) 

• between customer and FR operator 

• Network-to-Network Interface (UNI) 

• between two FR operators 


14

User-to-Network Interface 

(UNI) 


15

DTE to DCE protocol (Q.922) 

• Frame delimited by flags (bit stuffing used) 

• Frame based on HDLC 

• Variable frame length (max 8/16kB) 

• Multiple frame formats (historical reasons) 

• IETF (RFC1490) 

• (older) CISCO frame: 4B header 


16

Frame header fields (1) 

• DLCI – Data Link Connection Identifier 

• C/R (Command/Response) bit 

• currently unused (maintained for compatibility with 

HDLC) 

• EA (Extended Address) 

• if set to 1, current byte is the last addressing byte 

(otherwise another byte follows) 

• Provides capability to extend DLCI in the future 

• Current implementations all use 2B address field 


17

Frame header fields (2) 

Fields for congestion avoidance and resolution 

• FECN – Forward Explicit Congestion 

Notification 

• BECN – Backward Explicit Congestion 

Notification 

• DE – Discard Eligibility 


18

Multiprotocol encapsulation 

Frame header does not carry layer 3 protocol ID 

• FRF.3/RFC 1490 - Multiprotocol Interconnect over Frame Relay 

• L3 protocol given by NLPID 

NLPID (Network Layer Protocol ID) 

• 2 bytes just behind frame header 

• NLPID values defined in ISO/IEC TR 9577 

• NLPID may indicate usage of SNAP header 

• Earlier proprietary L3 protocol ID identification 

Can vary on different VCs 


19

Network to Network Interface 

(NNI) 

• Connects two different FR clouds 

• Supports multi-network PVCs 

• Consists of PVC segments (over individual FR 

clouds) 

• DLCI assignment and QoS parameters setting 

coordination needed on NNI 

• Bridges signalling protocols variations 


20

Local Management Interface 

(LMI) 


21

Local Management Interface (LMI) 

• Operates between DTE and FR access switch 

• FR DTE, FR DCE 

• Multiple variations (historical reasons) 

• ANSI T1.617 (=Annex D), q933a(=Annex A), Gang 

of Four 

• DLCI 0 / 1023 

• VC status messages 

• DTE-DCE line keepalives 

• Obtaining list of VCs from FR switch 


22

LMI frame 

• Carried in normal FR frame 

• reserved protocol number (0x09) 

• Reserved DLCI 

• Call reference – unused 

• Message Type 

• Status-inquiry 

• Status-message (keepalive), Full Status Message (PVC list) 

• differ in IEs included 

• Variable number of information elements (IE) 


23

Frame Relay Congestion 

Management 


24

Congestion avoidance and recovery 

• Congestion avoidance 

• ability to inform stations about emerging congestion 

• Congestion recovery 

• actions taken to recover from congestion and return 

to normal operating state 

• frame discard 

Separate for every VC and direction 


25

Congestion avoidance 

implementation 

• Backward 

Explicit 

Congestion 

Notification (BECN) 

• Set by intermediate switching elements to ask source to slow 

down 

• Cisco: source slows down by 25 % (if not under configured minimal 

imal 

CIR) 

• Forward Explicit 

Congestion 

Notification (FECN( 

FECN) 

• Set by intermediate switching elements to inform destination 

it should ask source to slow down 

• Destination utilizes upper layer protocol to accomplish that 

• (e.g. TCP starts to delay ACKs) 


26

Congestion recovery 

implementation 

• Discard Eligibility (DE) bit 

• Set by access FR switch for nonconforming frames 

• If congestion occurs on some link, frames with DE 

set should be dropped first 

• May be set by source DTE to give priorities to 

traffic entering FR cloud 

• If congestion persists even when DE frames 

discarded, network have to start discarding of 

non-DE frames 


27

Frame Relay QoS parameters, 

their measurement and enforcement 


28

FR virtual circuit QoS parameters 

Parameters take into account bursty nature of traffic 

• Bc – committed burst 

• Maximum amount of data the network commits to deliver over a VC during 

appointed interval Tc 

• Committed information rate (CIR) 

• estimate of normal traffic during a busy period 

• CIR=Bc/Tc 

• Sum of CIRs of all VCs commonly greater than physical access line speed 

• Be - excess burst 

• Maximum amount of data by which a user can exceed Bc during an interval Tc 

• Delivered at lower probability than the data within Bc 

• Excess Information Rate (EIR) 

• EIR=Be/Tc 

• Frame above CIR+EIR (maximum rate) are discarded 


29

Service Level Agreement 

Access rate = physical access line bitrate 


30

Traffic policing and traffic shaping 

• Traffic policing – done at provider’s network 

edge to limit traffic coming from customer 

• Traffic exceeding committed information rate is 

either discarded or marked as eligible for later 

discard 

• Traffic shaping – done at customer boundary 

router to shape traffic to comply with rate 

provider agrees to transfer 

• uses memory buffers to shape bursts 


31

Traffic policing 

• Runs on FR access switch 

• Determines whether to pass incoming frame to 

network cloud, mark it as discard eligible or 

discard it 

• No further traffic flow measurements are taken 

inside network cloud 


32

Traffic shaping /rate limiting 

• Applied at customer boundary router 

• Mechanism to smooth (bursty) traffic flow to 

meet service provider requirements (CIR,Bc,Be) 

• Frames exceeding agreement are buffered and 

sent later 


33

Traffic policing/shaping 

implementation algorithms 

• Leaky bucket 

• imposes a hard limit on the data transmission rate 

• Token bucket 

• allows a certain amount of burstiness while imposing 

a limit on the average data transmission rate 

• sometimes considered an improvement of Leaky 

bucket 

Leaky bucket and token bucket algorithms are 

often mistakenly lumped together 


34

Leaky bucket Algorithm 

Provides a mechanism by which bursty traffic can be shaped into 

steady stream of traffic to the network 

• Arriving frames are placed in a bucket with a hole in the bottom 

• The bucket can queue at most b bytes. If a frame arrives when 

the bucket is full, the frame is discarded. 

• Frames s drain through the hole in the bucket, into the network, at 

a constant rate of r bytes per second, thus smoothing traffic 

bursts. 

• In practise, we need to sent not exactly r bytes, but whole frame 

Ineffective, do not allow to pass traffic bursts even if network 

bandwidth is available 


35

Token Bucket Algorithm 

• Once per time interval (Tc), tokens are added to the 

bucket 

• Number corresponds to CIR in Bytes/second 

• Bucket has limited capacity (Bc+Be), overleaking tokens are 

discarded 

• If frame arrives, token bucket is checked whether there 

is at least as many tokens as data byte count in 

incoming frame 

• If there is enough tokens, frame is passed and appropriate 

number of tokens removed from the bucket 

• Otherwise, the frame is stored into waiting queue (buffered) / 

discarded 

• Be e allows to “save” some tokens from previous Tc 

interval(s) to the current one 


36

Token Bucket Implementation 

• FR access switch maintains a cumulative amount 

of data sent by user (counter C) 

• C decremented by Bc every Tc interval 

• C is always greater than 0 

• When frame arrives 

• If C Bc+Be: Discard frame 


37

Typical rate limiting usage 

Often physical local loop rate at central site 

outperforms physical local loop rate at branch 

site 

• Because we want to communicate with multiple 

branch sites 


38

TCP/IP over Frame Relay 


39

Scenerio 


40

Typical Frame Relay usage 

topologies 


41

Typical usage 


42

Relationship between IP subnets and 

FR virtual circuits 

• One physical router interface may correspond to 

one VC 

• One physical router interface may accommodate 

multiple VCs (more( 

obvious) 

• logical subinterface assigned to every VC 

• bound to right VC by DLCI configuration 

• behaves like physical interface from routing 

protocol’s point of view 


43

Subinterfaces 


44

Frame Relay (sub)interface types 

• Point-to-point 

• separate IP subnet for every subinterface 

• Point-to-multipoint 

• common IP subnet on all VCs 

• similar to broadcast network, but without broadcast 

capability 

• router interface “routes” between connected VCs 


45

IP to DLCI mapping 

• Point-to-point interface 

• Single IP address and DLCI configured 

• Point-to-multipoint interface 

• IP address of local side of (all) VCs configured 

• Static maps configured 

• for every possible peer IP address we configure DLCI of 

VC connected to that peer 


46

Inverse ARP (INARP) 

INARP allows automatic discovery of peer IP address to 

DLCI mapping 

• If no DLCI or static map is configured on 

(sub)interface, router sends INARP FR Frame to every 

known VC 

• List of available DLCIs can be obtained from FR switch 

using LMI 

• Peer responds with L3 address configured on it’s 

interface 

• DLCI-to-peer-address mapping dynamic entry can be 

created 


47

Frame-relay routing problems 


48

DV routing updates problems 

• Distance vector routing protocols use broadcast 

to send routing updates 

• Cisco implementation: VCs do not pass 

broadcasts if not explicitly configured to do so 


49

Split Horizon problem 

Split-horizon rule must be abandoned on physical 

interface accommodating multiple VCs 


50

OSPF 

• Frame-relay cloud represented with NBMA network 

node type in OSPF topology graph 

• Non-Broadcast Multi Access = multiple neighbors accessible 

via single physical interface, but no single broadcast frame 

can be sent to all of them 

• Be careful when configuring DR priorities on NBMA 

network with other than full-mesh topology 

• DR should be directly connected to all other routers using 

VC 

• Different OSPF timers used on NBMA networks 


51

FR Additional features 


52

FR multicast support 

• All multicast traffic of a multicast group sent to 

entity called multicast server, which forwards 

data to multicast group members 

• DLCI 1019-1022 reserved for that purpose 

• One-way and two-way multicast 

• Additional LMI messages defined to notify user 

about addition, deletion and presence of 

multicast group 

Not supported by most operators 


53

Interesting FR references 

• http://www.cisco 

cisco.com/univercd/cc/td/doc/cisintwk/ 

ito_doc 

doc/frame.htm 

• Overview and basic concepts 

• http://www.protocols.com/pbook/frame.htm 

• Detailed overview of FR standards 

• http://www.cse.wustl.edu/~jain/cis788-95/ftp/frame_relay 

• Advanced FR features 

• http://qbone.internet2.edu/bb/Bucket.doc 

• http://en.wikipedia.org/wiki/Token_bucket 

• Token Bucket/Leaky Bucket algorithm 


54

Frame Relay

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